Programming with R

Analyzing Patient Data

Overview

Teaching: 45 min
Exercises: 0 min
Questions
  • How do I read data into R?

  • How do I assign variables?

  • What is a data frame?

  • How do I access subsets of a data frame?

  • How do I calculate simple statistics like mean and median?

  • Where can I get help?

  • How can I plot my data?

Objectives
  • Read tabular data from a file into a program.

  • Assign values to variables.

  • Select individual values and subsections from data.

  • Perform operations on a data frame of data.

  • Display simple graphs.

We are studying inflammation in patients who have been given a new treatment for arthritis, and need to analyze the first dozen data sets. The data sets are stored in comma-separated values (CSV) format. Each row holds the observations for just one patient. Each column holds the inflammation measured in a day, so we have a set of values in successive days. The first few rows of our first file look like this:

0,0,1,3,1,2,4,7,8,3,3,3,10,5,7,4,7,7,12,18,6,13,11,11,7,7,4,6,8,8,4,4,5,7,3,4,2,3,0,0
0,1,2,1,2,1,3,2,2,6,10,11,5,9,4,4,7,16,8,6,18,4,12,5,12,7,11,5,11,3,3,5,4,4,5,5,1,1,0,1
0,1,1,3,3,2,6,2,5,9,5,7,4,5,4,15,5,11,9,10,19,14,12,17,7,12,11,7,4,2,10,5,4,2,2,3,2,2,1,1
0,0,2,0,4,2,2,1,6,7,10,7,9,13,8,8,15,10,10,7,17,4,4,7,6,15,6,4,9,11,3,5,6,3,3,4,2,3,2,1
0,1,1,3,3,1,3,5,2,4,4,7,6,5,3,10,8,10,6,17,9,14,9,7,13,9,12,6,7,7,9,6,3,2,2,4,2,0,1,1

We want to:

To do all that, we’ll have to learn a little bit about programming.

Getting started

Since we want to import the file called inflammation-01.csv into our R environment, we need to be able to tell our computer where the file is. To do this, we will create a “Project” with RStudio that contains the data we want to work with. The “Projects” interface in RStudio not only creates a working directory for you, but also remembers its location (allowing you to quickly navigate to it). The interface also (optionally) preserves custom settings and open files to make it easier to resume work after a break.

Create a new project

Loading Data

Now that we are set up with an Rstudio project, we are sure that the and scripts we are using are all in our working directory. The data file should be located in the directory data inside the working directory. Now we can load the data into R using read.csv:

read.csv(file = "data/inflammation-01.csv", header = FALSE)

The expression read.csv(...) is a function call that asks R to run the function read.csv.

read.csv has two arguments: the name of the file we want to read, and whether the first line of the file contains names for the columns of data. The filename needs to be a character string (or string for short), so we put it in quotes. Assigning the second argument, header, to be FALSE indicates that the data file does not have column headers. We’ll talk more about the value FALSE, and its converse TRUE, in lesson 04. In case of our inflammation-01.csv example, R auto-generates column names in the sequence V1 (for “variable 1”), V2, and so on, until V40.

Other Options for Reading CSV Files

read.csv actually has many more arguments that you may find useful when importing your own data in the future. You can learn more about these options in this supplementary lesson.

Loading Data with Headers

What happens if you forget to put header = FALSE? The default value is header = TRUE, which you can check with ?read.csv or help(read.csv). What do you expect will happen if you leave the default value? Before you run any code, think about what will happen to the first few rows of your data frame, and its overall size. Then run the following code and see if your expectations agree:

read.csv(file = "data/inflammation-01.csv")

Solution

R will construct column headers from values in your first row of data, resulting in X0 X0.1 X1 X3 X1.1 X2 ....

Note that the X is prepended just a number would not be a valid variable name. Because that’s what column headers are, the same rules apply. Appending .1, .2 etc. is necessary to avoid duplicate column headers.

Reading Different Decimal Point Formats

Depending on the country you live in, your standard can use the dot or the comma as decimal mark. Also, different devices or software can generate data with different decimal points. Take a look at ?read.csv and write the code to load a file called commadec.txt that has numeric values with commas as decimal mark, separated by semicolons.

Solution

read.csv(file = "data/commadec.txt", sep = ";", dec = ",")

or the built-in shortcut:

read.csv2(file = "data/commadec.txt")

A function will perform its given action on whatever value is passed to the argument(s). For example, in this case if we provided the name of a different file to the argument file, read.csv would read that instead. We’ll learn more about the details of functions and their arguments in the next lesson.

Since we didn’t tell it to do anything else with the function’s output, the console will display the full contents of the file inflammation-01.csv. Try it out.

read.csv reads the file, but we can’t use the data unless we assign it to a variable. We can think of a variable as a container with a name, such as x, current_temperature, or subject_id that contains one or more values. We can create a new variable and assign a value to it using <-.

weight_kg <- 55

Once a variable is created, we can use the variable name to refer to the value it was assigned. The variable name now acts as a tag. Whenever R reads that tag (weight_kg), it substitutes the value (55).

Variables as Tags

To see the value of a variable, we can print it by typing the name of the variable and hitting Return (or Enter). In general, R will print to the console any object returned by a function or operation unless we assign it to a variable.

weight_kg
[1] 55

We can treat our variable like a regular number, and do arithmetic with it:

# weight in pounds:
2.2 * weight_kg
[1] 121

Variables as Tags

Commenting

We can add comments to our code using the # character. It is useful to document our code in this way so that others (and us the next time we read it) have an easier time following what the code is doing.

We can also change a variable’s value by assigning it a new value:

weight_kg <- 57.5
# weight in kilograms is now
weight_kg
[1] 57.5

Variable Naming Conventions

Historically, R programmers have used a variety of conventions for naming variables. The . character in R can be a valid part of a variable name; thus the above assignment could have easily been weight.kg <- 57.5. This is often confusing to R newcomers who have programmed in languages where . has a more significant meaning. Today, most R programmers 1) start variable names with lower case letters, 2) separate words in variable names with underscores, and 3) use only lowercase letters, underscores, and numbers in variable names. The book R Packages includes a chapter on this and other style considerations.

Reassigning Variables

Assigning a new value to a variable breaks the connection with the old value; R forgets that number and applies the variable name to the new value.

When you assign a value to a variable, R only stores the value, not the calculation you used to create it. This is an important point if you’re used to the way a spreadsheet program automatically updates linked cells. Let’s look at an example.

First, we’ll convert weight_kg into pounds, and store the new value in the variable weight_lb:

weight_lb <- 2.2 * weight_kg
# weight in kg...
weight_kg
[1] 57.5
# ...and in pounds
weight_lb
[1] 126.5

In words, we’re asking R to look up the value we tagged weight_kg, multiply it by 2.2, and tag the result with the name weight_lb:

Creating Another Variable

If we now change the value of weight_kg:

weight_kg <- 100.0
# weight in kg now...
weight_kg
[1] 100
# ...and weight in pounds still
weight_lb
[1] 126.5

Updating a Variable

Since weight_lb doesn’t “remember” where its value came from, it isn’t automatically updated when weight_kg changes. This is different from the way spreadsheets work.

Printing with Parentheses

An alternative way to print the value of a variable is to use ( parentheses ) around the assignment statement. As an example: (total_weight <- weight_kg*2.2 + weight_lb) adds the values of weight_kg*2.2 and weight_lb, assigns the result to the total_weight, and finally prints the assigned value of the variable total_weight.

Now that we know how to assign things to variables, let’s re-run read.csv and save its result into a variable called ‘dat’:

dat <- read.csv(file = "data/inflammation-01.csv", header = FALSE)

This statement doesn’t produce any output because the assignment doesn’t display anything. If we want to check if our data has been loaded, we can print the variable’s value by typing the name of the variable dat. However, for large data sets it is convenient to use the function head to display only the first few rows of data.

head(dat)
  V1 V2 V3 V4 V5 V6 V7 V8 V9 V10 V11 V12 V13 V14 V15 V16 V17 V18 V19 V20 V21
1  0  0  1  3  1  2  4  7  8   3   3   3  10   5   7   4   7   7  12  18   6
2  0  1  2  1  2  1  3  2  2   6  10  11   5   9   4   4   7  16   8   6  18
3  0  1  1  3  3  2  6  2  5   9   5   7   4   5   4  15   5  11   9  10  19
4  0  0  2  0  4  2  2  1  6   7  10   7   9  13   8   8  15  10  10   7  17
5  0  1  1  3  3  1  3  5  2   4   4   7   6   5   3  10   8  10   6  17   9
6  0  0  1  2  2  4  2  1  6   4   7   6   6   9   9  15   4  16  18  12  12
  V22 V23 V24 V25 V26 V27 V28 V29 V30 V31 V32 V33 V34 V35 V36 V37 V38 V39 V40
1  13  11  11   7   7   4   6   8   8   4   4   5   7   3   4   2   3   0   0
2   4  12   5  12   7  11   5  11   3   3   5   4   4   5   5   1   1   0   1
3  14  12  17   7  12  11   7   4   2  10   5   4   2   2   3   2   2   1   1
4   4   4   7   6  15   6   4   9  11   3   5   6   3   3   4   2   3   2   1
5  14   9   7  13   9  12   6   7   7   9   6   3   2   2   4   2   0   1   1
6   5  18   9   5   3  10   3  12   7   8   4   7   3   5   4   4   3   2   1

Assigning Values to Variables

Draw diagrams showing what variables refer to what values after each statement in the following program:

mass <- 47.5
age <- 122
mass <- mass * 2.0
age <- age - 20

Solution

mass <- 47.5
age <- 122

Assigning Variables

mass <- mass * 2.0
age <- age - 20

Assigning Variables

Manipulating Data

Now that our data are loaded into R, we can start doing things with them. First, let’s ask what type of thing dat is:

class(dat)
[1] "data.frame"

The output tells us that it’s a data frame. Think of this structure as a spreadsheet in MS Excel that many of us are familiar with. Data frames are very useful for storing data and you will use them frequently when programming in R. A typical data frame of experimental data contains individual observations in rows and variables in columns.

We can see the shape, or dimensions, of the data frame with the function dim:

dim(dat)
[1] 60 40

This tells us that our data frame, dat, has 60 rows and 40 columns.

If we want to get a single value from the data frame, we can provide an index in square brackets. The first number specifies the row and the second the column:

# first value in dat, row 1, column 1
dat[1, 1]
[1] 0
# middle value in dat, row 30, column 20
dat[30, 20]
[1] 16

The first value in a data frame index is the row, the second value is the column. If we want to select more than one row or column, we can use the function c, which stands for combine. For example, to pick columns 10 and 20 from rows 1, 3, and 5, we can do this:

dat[c(1, 3, 5), c(10, 20)]
  V10 V20
1   3  18
3   9  10
5   4  17

We frequently want to select contiguous rows or columns, such as the first ten rows, or columns 3 through 7. You can use c for this, but it’s more convenient to use the : operator. This special function generates sequences of numbers:

1:5
[1] 1 2 3 4 5
3:12
 [1]  3  4  5  6  7  8  9 10 11 12

For example, we can select the first ten columns of values for the first four rows like this:

dat[1:4, 1:10]
  V1 V2 V3 V4 V5 V6 V7 V8 V9 V10
1  0  0  1  3  1  2  4  7  8   3
2  0  1  2  1  2  1  3  2  2   6
3  0  1  1  3  3  2  6  2  5   9
4  0  0  2  0  4  2  2  1  6   7

or the first ten columns of rows 5 to 10 like this:

dat[5:10, 1:10]
   V1 V2 V3 V4 V5 V6 V7 V8 V9 V10
5   0  1  1  3  3  1  3  5  2   4
6   0  0  1  2  2  4  2  1  6   4
7   0  0  2  2  4  2  2  5  5   8
8   0  0  1  2  3  1  2  3  5   3
9   0  0  0  3  1  5  6  5  5   8
10  0  1  1  2  1  3  5  3  5   8

If you want to select all rows or all columns, leave that index value empty.

# All columns from row 5
dat[5, ]
  V1 V2 V3 V4 V5 V6 V7 V8 V9 V10 V11 V12 V13 V14 V15 V16 V17 V18 V19 V20 V21
5  0  1  1  3  3  1  3  5  2   4   4   7   6   5   3  10   8  10   6  17   9
  V22 V23 V24 V25 V26 V27 V28 V29 V30 V31 V32 V33 V34 V35 V36 V37 V38 V39 V40
5  14   9   7  13   9  12   6   7   7   9   6   3   2   2   4   2   0   1   1
# All rows from column 16-18
dat[, 16:18]
   V16 V17 V18
1    4   7   7
2    4   7  16
3   15   5  11
4    8  15  10
5   10   8  10
6   15   4  16
7   13   5  12
8    9  15  11
9   11   9  10
10   6  13   8
11   3   7  13
12   8  14  11
13  12   4  17
14   3  10  13
15   5   7  17
16  10   7   8
17  11  12   5
18   4  14   7
19  11  15  17
20  13   6   5
21  15  13   6
22   5  12  12
23  14   5   5
24  13   7  14
25   4  12   9
26   9   5  16
27  13   4  13
28   6  15   6
29   7   6  11
30   6   8   7
31  14  12   8
32   3   8  10
33  15  15  10
34   4  12   9
35  15   9  17
36  11   5   7
37   7   4   7
38  10   6   7
39  15  12  13
40   6   8  15
41   5   7   5
42   6  10  13
43  15  11  12
44  11   6  10
45  15  12  15
46   6   7  11
47  11  16  12
48  15   5  15
49  14   4   6
50   4   7   9
51  10  13   6
52  15  15  12
53  11  15  13
54   6  11  12
55  13   8   9
56   8   8  16
57   4  16  11
58  13  13   9
59  12  15   5
60   9  14  11

If you leave both index values empty (i.e., dat[,]), you get the entire data frame.

Addressing Columns by Name

Columns can also be addressed by name, with either the $ operator (ie. dat$V16) or square brackets (ie. dat[, 'V16']). You can learn more about subsetting by column name in this supplementary lesson.

Now let’s perform some common mathematical operations to learn more about our inflammation data. When analyzing data we often want to look at partial statistics, such as the maximum value per patient or the average value per day. One way to do this is to select the data we want to create a new temporary data frame, and then perform the calculation on this subset:

# first row, all of the columns
patient_1 <- dat[1, ]
# max inflammation for patient 1
max(patient_1)
[1] 18

We don’t actually need to store the row in a variable of its own. Instead, we can combine the selection and the function call:

# max inflammation for patient 2
max(dat[2, ])
[1] 18

R also has functions for other common calculations, e.g. finding the minimum, mean, median, and standard deviation of the data:

# minimum inflammation on day 7
min(dat[, 7])
[1] 1
# mean inflammation on day 7
mean(dat[, 7])
[1] 3.8
# median inflammation on day 7
median(dat[, 7])
[1] 4
# standard deviation of inflammation on day 7
sd(dat[, 7])
[1] 1.725187

Forcing Conversion

Note that R may return an error when you attempt to perform similar calculations on subsetted rows of data frames. This is because some functions in R automatically convert the object type to a numeric vector, while others do not (e.g. max(dat[1, ]) works as expected, while mean(dat[1, ]) returns NA and a warning). You get the expected output by including an explicit call to as.numeric(), e.g. mean(as.numeric(dat[1, ])). By contrast, calculations on subsetted columns always work as expected, since columns of data frames are already defined as vectors.

R also has a function that summaries the previous common calculations:

# Summarize function
summary(dat[, 1:4])
       V1          V2             V3              V4      
 Min.   :0   Min.   :0.00   Min.   :0.000   Min.   :0.00  
 1st Qu.:0   1st Qu.:0.00   1st Qu.:1.000   1st Qu.:1.00  
 Median :0   Median :0.00   Median :1.000   Median :2.00  
 Mean   :0   Mean   :0.45   Mean   :1.117   Mean   :1.75  
 3rd Qu.:0   3rd Qu.:1.00   3rd Qu.:2.000   3rd Qu.:3.00  
 Max.   :0   Max.   :1.00   Max.   :2.000   Max.   :3.00  

For every column in the data frame, the function “summary” calculates: the minimun value, the first quartile, the median, the mean, the third quartile and the max value, giving helpful details about the sample distribution.

What if we need the maximum inflammation for all patients, or the average for each day? As the diagram below shows, we want to perform the operation across a margin of the data frame:

Operations Across Margins

To support this, we can use the apply function.

Getting Help

To learn about a function in R, e.g. apply, we can read its help documention by running help(apply) or ?apply.

apply allows us to repeat a function on all of the rows (MARGIN = 1) or columns (MARGIN = 2) of a data frame.

Thus, to obtain the average inflammation of each patient we will need to calculate the mean of all of the rows (MARGIN = 1) of the data frame.

avg_patient_inflammation <- apply(dat, 1, mean)

And to obtain the average inflammation of each day we will need to calculate the mean of all of the columns (MARGIN = 2) of the data frame.

avg_day_inflammation <- apply(dat, 2, mean)

Since the second argument to apply is MARGIN, the above command is equivalent to apply(dat, MARGIN = 2, mean). We’ll learn why this is so in the next lesson.

Efficient Alternatives

Some common operations have more efficient alternatives. For example, you can calculate the row-wise or column-wise means with rowMeans and colMeans, respectively.

Subsetting Data

We can take subsets of character vectors as well:

animal <- c("m", "o", "n", "k", "e", "y")
# first three characters
animal[1:3]
[1] "m" "o" "n"
# last three characters
animal[4:6]
[1] "k" "e" "y"
  1. If the first four characters are selected using the subset animal[1:4], how can we obtain the first four characters in reverse order?

  2. What is animal[-1]? What is animal[-4]? Given those answers, explain what animal[-1:-4] does.

  3. Use a subset of animal to create a new character vector that spells the word “eon”, i.e. c("e", "o", "n").

    Solutions

    1. animal[4:1]

    2. "o" "n" "k" "e" "y" and "m" "o" "n" "e" "y", which means that a single - removes the element at the given index position. animal[-1:-4] remove the subset, returning "e" "y", which is equivalent to animal[5:6].

    3. animal[c(5,2,3)] combines indexing with the combine function.

Subsetting More Data

Suppose you want to determine the maximum inflammation for patient 5 across days three to seven. To do this you would extract the relevant subset from the data frame and calculate the maximum value. Which of the following lines of R code gives the correct answer?

  1. max(dat[5, ])
  2. max(dat[3:7, 5])
  3. max(dat[5, 3:7])
  4. max(dat[5, 3, 7])

Solution

Answer: 3

Explanation: You want to extract the part of the dataframe representing data for patient 5 from days three to seven. In this dataframe, patient data is organised in rows and the days are represented by the columns. Subscripting in R follows the [i, j] principle, where i = rows and j = columns. Thus, answer 3 is correct since the patient is represented by the value for i (5) and the days are represented by the values in j, which is a subset spanning day 3 to 7.

Subsetting and Re-Assignment

Using the inflammation data frame dat from above: Let’s pretend there was something wrong with the instrument on the first five days for every second patient (#2, 4, 6, etc.), which resulted in the measurements being twice as large as they should be.

  1. Write a vector containing each affected patient (hint: ?seq)
  2. Create a new data frame in which you halve the first five days’ values in only those patients
  3. Print out the corrected data frame to check that your code has fixed the problem

Solution

whichPatients <- seq(2, 60, 2) # i.e., which rows
whichDays <- seq(1, 5)         # i.e., which columns
dat2 <- dat
# check the size of your subset: returns `30 5`, that is 30 [rows=patients] by 5 [columns=days]
dim(dat2[whichPatients, whichDays])
dat2[whichPatients, whichDays] <- dat2[whichPatients, whichDays] / 2
dat2

Using the Apply Function on Patient Data

Challenge: the apply function can be used to summarize datasets and subsets of data across rows and columns using the MARGIN argument. Suppose you want to calculate the mean inflammation for specific days and patients in the patient dataset (i.e. 60 patients across 40 days).

Please use a combination of the apply function and indexing to:

  1. calculate the mean inflammation for patients 1 to 5 over the whole 40 days
  2. calculate the mean inflammation for days 1 to 10 (across all patients).
  3. calculate the mean inflammation for every second day (across all patients).

Think about the number of rows and columns you would expect as the result before each apply call and check your intuition by applying the mean function.

Solution

# 1.
apply(dat[1:5, ], 1, mean)
# 2.
apply(dat[, 1:10], 2, mean)
# 3.
apply(dat[, seq(1, 40, by = 2)], 2, mean)

Plotting

The mathematician Richard Hamming once said, “The purpose of computing is insight, not numbers,” and the best way to develop insight is often to visualize data. Visualization deserves an entire lecture (or course) of its own, but we can explore a few of R’s plotting features.

Let’s take a look at the average inflammation over time. Recall that we already calculated these values above using apply(dat, 2, mean) and saved them in the variable avg_day_inflammation. Plotting the values is done with the function plot.

plot(avg_day_inflammation)

Scatter plot of average inflammation versus time demonstrating the result of using the plot function

Above, we gave the function plot a vector of numbers corresponding to the average inflammation per day across all patients. plot created a scatter plot where the y-axis is the average inflammation level and the x-axis is the order, or index, of the values in the vector, which in this case correspond to the 40 days of treatment. The result is roughly a linear rise and fall, which is suspicious: based on other studies, we expect a sharper rise and slower fall. Let’s have a look at two other statistics: the maximum and minimum inflammation per day.

max_day_inflammation <- apply(dat, 2, max)
plot(max_day_inflammation)

Scatter plot of maximum inflammation versus time demonstrating the result of using the plot function

min_day_inflammation <- apply(dat, 2, min)
plot(min_day_inflammation)

Scatter plot of minimum inflammation versus time demonstrating the result of using the plot function

The maximum value rises and falls perfectly smoothly, while the minimum seems to be a step function. Neither result seems particularly likely, so either there’s a mistake in our calculations or something is wrong with our data.

Plotting Data

Create a plot showing the standard deviation of the inflammation data for each day across all patients.

Solution

sd_day_inflammation <- apply(dat, 2, sd)
plot(sd_day_inflammation)

Key Points

  • Use variable <- value to assign a value to a variable in order to record it in memory.

  • Objects are created on demand whenever a value is assigned to them.

  • The function dim gives the dimensions of a data frame.

  • Use object[x, y] to select a single element from a data frame.

  • Use from:to to specify a sequence that includes the indices from from to to.

  • All the indexing and subsetting that works on data frames also works on vectors.

  • Use # to add comments to programs.

  • Use mean, max, min and sd to calculate simple statistics.

  • Use apply to calculate statistics across the rows or columns of a data frame.

  • Use plot to create simple visualizations.


Creating Functions

Overview

Teaching: 30 min
Exercises: 0 min
Questions
  • How do I make a function?

  • How can I test my functions?

  • How should I document my code?

Objectives
  • Define a function that takes arguments.

  • Return a value from a function.

  • Test a function.

  • Set default values for function arguments.

  • Explain why we should divide programs into small, single-purpose functions.

If we only had one data set to analyze, it would probably be faster to load the file into a spreadsheet and use that to plot some simple statistics. But we have twelve files to check, and may have more in the future. In this lesson, we’ll learn how to write a function so that we can repeat several operations with a single command.

Defining a Function

Let’s start by defining a function fahrenheit_to_celsius that converts temperatures from Fahrenheit to Celsius:

fahrenheit_to_celsius <- function(temp_F) {
  temp_C <- (temp_F - 32) * 5 / 9
  return(temp_C)
}

We define fahrenheit_to_celsius by assigning it to the output of function. The list of argument names are contained within parentheses. Next, the body of the function–the statements that are executed when it runs–is contained within curly braces ({}). The statements in the body are indented by two spaces, which makes the code easier to read but does not affect how the code operates.

When we call the function, the values we pass to it are assigned to those variables so that we can use them inside the function. Inside the function, we use a return statement to send a result back to whoever asked for it.

Automatic Returns

In R, it is not necessary to include the return statement. R automatically returns whichever variable is on the last line of the body of the function. While in the learning phase, we will explicitly define the return statement.

Let’s try running our function. Calling our own function is no different from calling any other function:

# freezing point of water
fahrenheit_to_celsius(32)
[1] 0
# boiling point of water
fahrenheit_to_celsius(212)
[1] 100

We’ve successfully called the function that we defined, and we have access to the value that we returned.

Composing Functions

Now that we’ve seen how to turn Fahrenheit into Celsius, it’s easy to turn Celsius into Kelvin:

celsius_to_kelvin <- function(temp_C) {
  temp_K <- temp_C + 273.15
  return(temp_K)
}

# freezing point of water in Kelvin
celsius_to_kelvin(0)
[1] 273.15

What about converting Fahrenheit to Kelvin? We could write out the formula, but we don’t need to. Instead, we can compose the two functions we have already created:

fahrenheit_to_kelvin <- function(temp_F) {
  temp_C <- fahrenheit_to_celsius(temp_F)
  temp_K <- celsius_to_kelvin(temp_C)
  return(temp_K)
}

# freezing point of water in Kelvin
fahrenheit_to_kelvin(32.0)
[1] 273.15

This is our first taste of how larger programs are built: we define basic operations, then combine them in ever-larger chunks to get the effect we want. Real-life functions will usually be larger than the ones shown here–typically half a dozen to a few dozen lines–but they shouldn’t ever be much longer than that, or the next person who reads it won’t be able to understand what’s going on.

Nesting Function Calls

This example showed the output of fahrenheit_to_celsius assigned to temp_C, which is then passed to celsius_to_kelvin to get the final result. It is also possible to perform this calculation in one line of code, by “nesting” one function call inside another, like so:

# freezing point of water in Fahrenheit
celsius_to_kelvin(fahrenheit_to_celsius(32.0))
[1] 273.15

Here, we call fahrenheit_to_celsius to convert 32.0 from Fahrenheit to Celsius, and immediately pass the value returned from fahrenheit_to_celsius to celsius_to_kelvin to convert from Celsius to Kelvin. Our conversion from Fahrenheit to Kelvin is done, all in one go!

This is convenient, but you should be careful not to nest too many function calls at once - it can become confusing and difficult to read!

Create a Function

In the last lesson, we learned to combine elements into a vector using the c function, e.g. x <- c("A", "B", "C") creates a vector x with three elements. Furthermore, we can extend that vector again using c, e.g. y <- c(x, "D") creates a vector y with four elements. Write a function called highlight that takes two vectors as arguments, called content and wrapper, and returns a new vector that has the wrapper vector at the beginning and end of the content:

best_practice <- c("Write", "programs", "for", "people", "not", "computers")
asterisk <- "***"  # R interprets a variable with a single value as a vector
                   # with one element.
highlight(best_practice, asterisk)
[1] "***"       "Write"     "programs"  "for"       "people"    "not"      
[7] "computers" "***"      

Solution

highlight <- function(content, wrapper) {
  answer <- c(wrapper, content, wrapper)
  return(answer)
}

If the variable v refers to a vector, then v[1] is the vector’s first element and v[length(v)] is its last (the function length returns the number of elements in a vector). Write a function called edges that returns a vector made up of just the first and last elements of its input:

dry_principle <- c("Don't", "repeat", "yourself", "or", "others")
edges(dry_principle)
[1] "Don't"  "others"

Solution

edges <- function(v) {
   first <- v[1]
   last <- v[length(v)]
   answer <- c(first, last)
   return(answer)
}

The Call Stack

For a deeper understanding of how functions work, you’ll need to learn how they create their own environments and call other functions. Function calls are managed via the call stack. For more details on the call stack, have a look at the supplementary material.

Named Variables and the Scope of Variables

Functions can accept arguments explicitly assigned to a variable name in the function call functionName(variable = value), as well as arguments by order:

input_1 <- 20
mySum <- function(input_1, input_2 = 10) {
  output <- input_1 + input_2
  return(output)
}
  1. Given the above code was run, which value does mySum(input_1 = 1, 3) produce?
    1. 4
    2. 11
    3. 23
    4. 30
  2. If mySum(3) returns 13, why does mySum(input_2 = 3) return an error?

Solution

  1. The solution is 1..

  2. Read the error message: argument "input_1" is missing, with no default means that no value for input_1 is provided in the function call, and neither in the function’s defintion. Thus, the addition in the function body can not be completed.

Testing, Error Handling, and Documenting

Once we start putting things in functions so that we can re-use them, we need to start testing that those functions are working correctly. To see how to do this, let’s write a function to center a dataset around a particular midpoint:

center <- function(data, midpoint) {
  new_data <- (data - mean(data)) + midpoint
  return(new_data)
}

We could test this on our actual data, but since we don’t know what the values ought to be, it will be hard to tell if the result was correct. Instead, let’s create a vector of 0s and then center that around 3. This will make it simple to see if our function is working as expected:

z <- c(0, 0, 0, 0)
z
[1] 0 0 0 0
center(z, 3)
[1] 3 3 3 3

That looks right, so let’s try center on our real data. We’ll center the inflammation data from day 4 around 0:

dat <- read.csv(file = "data/inflammation-01.csv", header = FALSE)
centered <- center(dat[, 4], 0)
head(centered)
[1]  1.25 -0.75  1.25 -1.75  1.25  0.25

It’s hard to tell from the default output whether the result is correct, but there are a few simple tests that will reassure us:

# original mean
mean(dat[, 4])
[1] 1.75
# centered mean
mean(centered)
[1] 0

That seems right: the original mean was about 1.75 and the mean of the centered data is 0. We can even go further and check that the standard deviation hasn’t changed:

# original standard deviation
sd(dat[, 4])
[1] 1.067628
# centered standard deviation
sd(centered)
[1] 1.067628

Those values look the same, but we probably wouldn’t notice if they were different in the sixth decimal place. Let’s do this instead:

# difference in standard deviations before and after
sd(dat[, 4]) - sd(centered)
[1] 0

Sometimes, a very small difference can be detected due to rounding at very low decimal places. R has a useful function for comparing two objects allowing for rounding errors, all.equal:

all.equal(sd(dat[, 4]), sd(centered))
[1] TRUE

It’s still possible that our function is wrong, but it seems unlikely enough that we should probably get back to doing our analysis. However, there are two other important tasks to consider: 1) we should ensure our function can provide informative errors when needed, and 2) we should write some documentation for our function to remind ourselves later what it’s for and how to use it.

Error Handling

What happens if we have missing data (NA values) in the data argument we provide to center?

# new data object and set one value in column 4 to NA
datNA <- dat
datNA[10,4] <- NA

# returns all NA values
center(datNA[,4], 0)
 [1] NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA
[26] NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA
[51] NA NA NA NA NA NA NA NA NA NA

This is likely not the behavior we want, and is caused by the mean function returning NA when the na.rm=TRUE is not provided. We may wish to not consider NA values in our center function. We can provide the na.rm=TRUE argument and solve this issue.

center <- function(data, midpoint) {
  new_data <- (data - mean(data, na.rm=TRUE)) + midpoint
  return(new_data)
}

center(datNA[,4], 0)
 [1]  1.2542373 -0.7457627  1.2542373 -1.7457627  1.2542373  0.2542373
 [7]  0.2542373  0.2542373  1.2542373         NA -1.7457627 -1.7457627
[13] -0.7457627 -1.7457627 -0.7457627 -1.7457627 -1.7457627 -0.7457627
[19] -0.7457627 -1.7457627  1.2542373  1.2542373  1.2542373 -0.7457627
[25] -0.7457627 -0.7457627  0.2542373 -0.7457627  0.2542373 -0.7457627
[31] -1.7457627  1.2542373  0.2542373 -0.7457627  0.2542373  1.2542373
[37]  0.2542373  0.2542373  1.2542373  1.2542373  0.2542373  1.2542373
[43]  1.2542373  1.2542373  1.2542373  0.2542373  1.2542373  1.2542373
[49]  1.2542373  0.2542373 -0.7457627  0.2542373  0.2542373 -0.7457627
[55] -0.7457627  1.2542373  0.2542373 -0.7457627 -0.7457627 -1.7457627

However, what happens if the user were to accidentally hand this function a factor or character vector?

datNA[,1] <- as.factor(datNA[,1])
datNA[,2] <- as.character(datNA[,2])

center(datNA[,1], 0)
Warning in mean.default(data, na.rm = TRUE): argument is not numeric or logical:
returning NA
Warning in Ops.factor(data, mean(data, na.rm = TRUE)): '-' not meaningful for
factors
 [1] NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA
[26] NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA NA
[51] NA NA NA NA NA NA NA NA NA NA
center(datNA[,2], 0)
Warning in mean.default(data, na.rm = TRUE): argument is not numeric or logical:
returning NA
Error in data - mean(data, na.rm = TRUE): non-numeric argument to binary operator

Both of these attempts result in errors. Luckily, the errors are quite informative. In other cases, we may need to add in error handling using the warning and stop functions.

For instance, the center function only works on numeric vectors. Recognizing this and adding warnings and errors provides feedback to the user and makes sure the output of the function is what the user wanted.

Documentation

A common way to put documentation in software is to add comments like this:

center <- function(data, midpoint) {
  # return a new vector containing the original data centered around the
  # midpoint.
  # Example: center(c(1, 2, 3), 0) => c(-1, 0, 1)
  new_data <- (data - mean(data)) + midpoint
  return(new_data)
}

Writing Documentation

Formal documentation for R functions is written in separate .Rd using a markup language similar to LaTeX. You see the result of this documentation when you look at the help file for a given function, e.g. ?read.csv. The roxygen2 package allows R coders to write documentation alongside the function code and then process it into the appropriate .Rd files. You will want to switch to this more formal method of writing documentation when you start writing more complicated R projects.

Functions to Create Graphs

Write a function called analyze that takes a filename as an argument and displays the three graphs produced in the previous lesson (average, min and max inflammation over time). analyze("data/inflammation-01.csv") should produce the graphs already shown, while analyze("data/inflammation-02.csv") should produce corresponding graphs for the second data set. Be sure to document your function with comments.

Solution

analyze <- function(filename) {
  # Plots the average, min, and max inflammation over time.
  # Input is character string of a csv file.
  dat <- read.csv(file = filename, header = FALSE)
  avg_day_inflammation <- apply(dat, 2, mean)
  plot(avg_day_inflammation)
  max_day_inflammation <- apply(dat, 2, max)
  plot(max_day_inflammation)
  min_day_inflammation <- apply(dat, 2, min)
  plot(min_day_inflammation)
}

Rescaling

Write a function rescale that takes a vector as input and returns a corresponding vector of values scaled to lie in the range 0 to 1. (If L and H are the lowest and highest values in the original vector, then the replacement for a value v should be (v-L) / (H-L).) Be sure to document your function with comments.

Test that your rescale function is working properly using min, max, and plot.

Solution

rescale <- function(v) {
  # Rescales a vector, v, to lie in the range 0 to 1.
  L <- min(v)
  H <- max(v)
  result <- (v - L) / (H - L)
  return(result)
}

Defining Defaults

We have passed arguments to functions in two ways: directly, as in dim(dat), and by name, as in read.csv(file = "data/inflammation-01.csv", header = FALSE). In fact, we can pass the arguments to read.csv without naming them:

dat <- read.csv("data/inflammation-01.csv", FALSE)

However, the position of the arguments matters if they are not named.

dat <- read.csv(header = FALSE, file = "data/inflammation-01.csv")
dat <- read.csv(FALSE, "data/inflammation-01.csv")
Error in read.table(file = file, header = header, sep = sep, quote = quote, : 'file' must be a character string or connection

To understand what’s going on, and make our own functions easier to use, let’s re-define our center function like this:

center <- function(data, midpoint = 0) {
  # return a new vector containing the original data centered around the
  # midpoint (0 by default).
  # Example: center(c(1, 2, 3), 0) => c(-1, 0, 1)
  new_data <- (data - mean(data)) + midpoint
  return(new_data)
}

The key change is that the second argument is now written midpoint = 0 instead of just midpoint. If we call the function with two arguments, it works as it did before:

test_data <- c(0, 0, 0, 0)
center(test_data, 3)
[1] 3 3 3 3

But we can also now call center() with just one argument, in which case midpoint is automatically assigned the default value of 0:

more_data <- 5 + test_data
more_data
[1] 5 5 5 5
center(more_data)
[1] 0 0 0 0

This is handy: if we usually want a function to work one way, but occasionally need it to do something else, we can allow people to pass an argument when they need to but provide a default to make the normal case easier.

The example below shows how R matches values to arguments

display <- function(a = 1, b = 2, c = 3) {
  result <- c(a, b, c)
  names(result) <- c("a", "b", "c")  # This names each element of the vector
  return(result)
}

# no arguments
display()
a b c 
1 2 3 
# one argument
display(55)
 a  b  c 
55  2  3 
# two arguments
display(55, 66)
 a  b  c 
55 66  3 
# three arguments
display(55, 66, 77)
 a  b  c 
55 66 77 

As this example shows, arguments are matched from left to right, and any that haven’t been given a value explicitly get their default value. We can override this behavior by naming the value as we pass it in:

# only setting the value of c
display(c = 77)
 a  b  c 
 1  2 77 

Matching Arguments

To be precise, R has three ways that arguments supplied by you are matched to the formal arguments of the function definition:

  1. by complete name,
  2. by partial name (matching on initial n characters of the argument name), and
  3. by position.

Arguments are matched in the manner outlined above in that order: by complete name, then by partial matching of names, and finally by position.

With that in hand, let’s look at the help for read.csv():

?read.csv

There’s a lot of information there, but the most important part is the first couple of lines:

read.csv(file, header = TRUE, sep = ",", quote = "\"",
         dec = ".", fill = TRUE, comment.char = "", ...)

This tells us that read.csv() has one argument, file, that doesn’t have a default value, and six others that do. Now we understand why the following gives an error:

dat <- read.csv(FALSE, "data/inflammation-01.csv")
Error in read.table(file = file, header = header, sep = sep, quote = quote, : 'file' must be a character string or connection

It fails because FALSE is assigned to file and the filename is assigned to the argument header.

A Function with Default Argument Values

Rewrite the rescale function so that it scales a vector to lie between 0 and 1 by default, but will allow the caller to specify lower and upper bounds if they want. Compare your implementation to your neighbor’s: Do your two implementations produce the same results when both are given the same input vector and parameters?

Solution

rescale <- function(v, lower = 0, upper = 1) {
  # Rescales a vector, v, to lie in the range lower to upper.
  L <- min(v)
  H <- max(v)
  result <- (v - L) / (H - L) * (upper - lower) + lower
  return(result)
}

Key Points

  • Define a function using name <- function(...args...) {...body...}.

  • Call a function using name(...values...).

  • R looks for variables in the current stack frame before looking for them at the top level.

  • Use help(thing) to view help for something.

  • Put comments at the beginning of functions to provide help for that function.

  • Annotate your code!

  • Specify default values for arguments when defining a function using name = value in the argument list.

  • Arguments can be passed by matching based on name, by position, or by omitting them (in which case the default value is used).


Analyzing Multiple Data Sets

Overview

Teaching: 30 min
Exercises: 0 min
Questions
  • How can I do the same thing to multiple data sets?

  • How do I write a for loop?

Objectives
  • Explain what a for loop does.

  • Correctly write for loops to repeat simple calculations.

  • Trace changes to a loop variable as the loop runs.

  • Trace changes to other variables as they are updated by a for loop.

  • Use a function to get a list of filenames that match a simple pattern.

  • Use a for loop to process multiple files.

We have created a function called analyze that creates graphs of the minimum, average, and maximum daily inflammation rates for a single data set:

analyze <- function(filename) {
  # Plots the average, min, and max inflammation over time.
  # Input is character string of a csv file.
  dat <- read.csv(file = filename, header = FALSE)
  avg_day_inflammation <- apply(dat, 2, mean)
  plot(avg_day_inflammation)
  max_day_inflammation <- apply(dat, 2, max)
  plot(max_day_inflammation)
  min_day_inflammation <- apply(dat, 2, min)
  plot(min_day_inflammation)
}

analyze("data/inflammation-01.csv")

plot of chunk inflammation-01plot of chunk inflammation-01plot of chunk inflammation-01

We can use it to analyze other data sets one by one:

analyze("data/inflammation-02.csv")

plot of chunk inflammation-02plot of chunk inflammation-02plot of chunk inflammation-02

but we have a dozen data sets right now and more on the way. We want to create plots for all our data sets with a single statement. To do that, we’ll have to teach the computer how to repeat things.

For Loops

Suppose we want to print each word in a sentence. One way is to use six print statements:

best_practice <- c("Let", "the", "computer", "do", "the", "work")
print_words <- function(sentence) {
  print(sentence[1])
  print(sentence[2])
  print(sentence[3])
  print(sentence[4])
  print(sentence[5])
  print(sentence[6])
}

print_words(best_practice)
[1] "Let"
[1] "the"
[1] "computer"
[1] "do"
[1] "the"
[1] "work"

but that’s a bad approach for two reasons:

  1. It doesn’t scale: if we want to print the elements in a vector that’s hundreds long, we’d be better off just typing them in.

  2. It’s fragile: if we give it a longer vector, it only prints part of the data, and if we give it a shorter input, it returns NA values because we’re asking for elements that don’t exist!

best_practice[-6]
[1] "Let"      "the"      "computer" "do"       "the"     
print_words(best_practice[-6])
[1] "Let"
[1] "the"
[1] "computer"
[1] "do"
[1] "the"
[1] NA

Not Available

R has has a special variable, NA, for designating missing values that are Not Available in a data set. See ?NA and An Introduction to R for more details.

Here’s a better approach:

print_words <- function(sentence) {
  for (word in sentence) {
    print(word)
  }
}

print_words(best_practice)
[1] "Let"
[1] "the"
[1] "computer"
[1] "do"
[1] "the"
[1] "work"

This is shorter - certainly shorter than something that prints every character in a hundred-letter string - and more robust as well:

print_words(best_practice[-6])
[1] "Let"
[1] "the"
[1] "computer"
[1] "do"
[1] "the"

The improved version of print_words uses a for loop to repeat an operation—in this case, printing—once for each thing in a collection. The general form of a loop is:

for (variable in collection) {
  do things with variable
}

We can name the loop variable anything we like (with a few restrictions, e.g. the name of the variable cannot start with a digit). in is part of the for syntax. Note that the condition (variable in collection) is enclosed in parentheses, and the body of the loop is enclosed in curly braces { }. For a single-line loop body, as here, the braces aren’t needed, but it is good practice to include them as we did.

Here’s another loop that repeatedly updates a variable:

len <- 0
vowels <- c("a", "e", "i", "o", "u")
for (v in vowels) {
  len <- len + 1
}
# Number of vowels
len
[1] 5

It’s worth tracing the execution of this little program step by step. Since there are five elements in the vector vowels, the statement inside the loop will be executed five times. The first time around, len is zero (the value assigned to it on line 1) and v is "a". The statement adds 1 to the old value of len, producing 1, and updates len to refer to that new value. The next time around, v is "e" and len is 1, so len is updated to be 2. After three more updates, len is 5; since there is nothing left in the vector vowels for R to process, the loop finishes.

Note that a loop variable is just a variable that’s being used to record progress in a loop. It still exists after the loop is over, and we can re-use variables previously defined as loop variables as well:

letter <- "z"
for (letter in c("a", "b", "c")) {
  print(letter)
}
[1] "a"
[1] "b"
[1] "c"
# after the loop, letter is
letter
[1] "c"

Note also that finding the length of a vector is such a common operation that R actually has a built-in function to do it called length:

length(vowels)
[1] 5

length is much faster than any R function we could write ourselves, and much easier to read than a two-line loop; it will also give us the length of many other things that we haven’t met yet, so we should always use it when we can (see this lesson to learn more about the different ways to store data in R).

Printing Numbers

R has a built-in function called seq that creates a list of numbers:

seq(3)
[1] 1 2 3

Using seq, write a function that prints the first N natural numbers, one per line:

print_N(3)
[1] 1
[1] 2
[1] 3

Solution

print_N <- function(N) {
  nseq <- seq(N)
  for (num in nseq) {
    print(num)
  }
}

Summing Values

Write a function called total that calculates the sum of the values in a vector. (R has a built-in function called sum that does this for you. Please don’t use it for this exercise.)

ex_vec <- c(4, 8, 15, 16, 23, 42)
total(ex_vec)
[1] 108

Solution

total <- function(vec) {
  # calculates the sum of the values in a vector
  vec_sum <- 0
  for (num in vec) {
    vec_sum <- vec_sum + num
  }
  return(vec_sum)
}

Exponentiation

Exponentiation is built into R:

2^4
[1] 16

Write a function called expo that uses a loop to calculate the same result.

expo(2, 4)
[1] 16

Solution

expo <- function(base, power) {
  result <- 1
  for (i in seq(power)) {
    result <- result * base
  }
  return(result)
}

Processing Multiple Files

We now have almost everything we need to process all our data files. The only thing that’s missing is a function that finds files whose names match a pattern. We do not need to write it ourselves because R already has a function to do this called list.files.

If we run the function without any arguments, list.files(), it returns every file in the current working directory. We can understand this result by reading the help file (?list.files). The first argument, path, is the path to the directory to be searched, and it has the default value of "." (recall from the lesson on the Unix Shell that "." is shorthand for the current working directory). The second argument, pattern, is the pattern being searched, and it has the default value of NULL. Since no pattern is specified to filter the files, all files are returned.

So to list all the csv files, we could run either of the following:

list.files(path = "data", pattern = "csv")
 [1] "car-speeds-cleaned.csv" "car-speeds.csv"         "inflammation-01.csv"   
 [4] "inflammation-02.csv"    "inflammation-03.csv"    "inflammation-04.csv"   
 [7] "inflammation-05.csv"    "inflammation-06.csv"    "inflammation-07.csv"   
[10] "inflammation-08.csv"    "inflammation-09.csv"    "inflammation-10.csv"   
[13] "inflammation-11.csv"    "inflammation-12.csv"    "sample.csv"            
[16] "small-01.csv"           "small-02.csv"           "small-03.csv"          
list.files(path = "data", pattern = "inflammation")
 [1] "inflammation-01.csv" "inflammation-02.csv" "inflammation-03.csv"
 [4] "inflammation-04.csv" "inflammation-05.csv" "inflammation-06.csv"
 [7] "inflammation-07.csv" "inflammation-08.csv" "inflammation-09.csv"
[10] "inflammation-10.csv" "inflammation-11.csv" "inflammation-12.csv"

Organizing Larger Projects

For larger projects, it is recommended to organize separate parts of the analysis into multiple subdirectories, e.g. one subdirectory for the raw data, one for the code, and one for the results like figures. We have done that here to some extent, putting all of our data files into the subdirectory “data”. For more advice on this topic, you can read A quick guide to organizing computational biology projects by William Stafford Noble.

As these examples show, list.files result is a vector of strings, which means we can loop over it to do something with each filename in turn. In our case, the “something” we want is our analyze function.

Because we have put our data in a separate subdirectory, if we want to access these files using the output of list.files we also need to include the “path” portion of the file name. We can do that by using the argument full.names = TRUE.

list.files(path = "data", pattern = "csv", full.names = TRUE)
 [1] "data/car-speeds-cleaned.csv" "data/car-speeds.csv"        
 [3] "data/inflammation-01.csv"    "data/inflammation-02.csv"   
 [5] "data/inflammation-03.csv"    "data/inflammation-04.csv"   
 [7] "data/inflammation-05.csv"    "data/inflammation-06.csv"   
 [9] "data/inflammation-07.csv"    "data/inflammation-08.csv"   
[11] "data/inflammation-09.csv"    "data/inflammation-10.csv"   
[13] "data/inflammation-11.csv"    "data/inflammation-12.csv"   
[15] "data/sample.csv"             "data/small-01.csv"          
[17] "data/small-02.csv"           "data/small-03.csv"          
list.files(path = "data", pattern = "inflammation", full.names = TRUE)
 [1] "data/inflammation-01.csv" "data/inflammation-02.csv"
 [3] "data/inflammation-03.csv" "data/inflammation-04.csv"
 [5] "data/inflammation-05.csv" "data/inflammation-06.csv"
 [7] "data/inflammation-07.csv" "data/inflammation-08.csv"
 [9] "data/inflammation-09.csv" "data/inflammation-10.csv"
[11] "data/inflammation-11.csv" "data/inflammation-12.csv"

Let’s test out running our analyze function by using it on the first three files in the vector returned by list.files:

filenames <- list.files(path = "data",  
                        # Now follows a regular expression that matches:
                        pattern = "inflammation-[0-9]{2}.csv",
                        #          |            |        the standard file extension of comma-separated values
                        #          |            the variable parts (two digits, each between 0 and 9)
                        #          the static part of the filenames
                        full.names = TRUE)
filenames <- filenames[1:3]
for (f in filenames) {
  print(f)
  analyze(f)
}
[1] "data/inflammation-01.csv"

plot of chunk loop-analyzeplot of chunk loop-analyzeplot of chunk loop-analyze

[1] "data/inflammation-02.csv"

plot of chunk loop-analyzeplot of chunk loop-analyzeplot of chunk loop-analyze

[1] "data/inflammation-03.csv"

plot of chunk loop-analyzeplot of chunk loop-analyzeplot of chunk loop-analyze

Sure enough, the maxima of these data sets show exactly the same ramp as the first, and their minima show the same staircase structure.

Other Ways to Do It

In this lesson we saw how to use a simple for loop to repeat an operation. As you progress with R, you will learn that there are multiple ways to accomplish this. Sometimes the choice of one method over another is more a matter of personal style, but other times it can have consequences for the speed of your code. For instruction on best practices, see this supplementary lesson that demonstrates how to properly repeat operations in R.

Using Loops to Analyze Multiple Files

Write a function called analyze_all that takes a folder path and a filename pattern as its arguments and runs analyze for each file whose name matches the pattern.

Solution

analyze_all <- function(folder = "data", pattern) {
  # Runs the function analyze for each file in the given folder
  # that contains the given pattern.
  filenames <- list.files(path = folder, pattern = pattern, full.names = TRUE)
  for (f in filenames) {
    analyze(f)
  }
}

Key Points

  • Use for (variable in collection) to process the elements of a collection one at a time.

  • The body of a for loop is surrounded by curly braces ({}).

  • Use length(thing) to determine the length of something that contains other values.

  • Use list.files(path = "path", pattern = "pattern", full.names = TRUE) to create a list of files whose names match a pattern.


Making Choices

Overview

Teaching: 30 min
Exercises: 0 min
Questions
  • How do I make choices using if and else statements?

  • How do I compare values?

  • How do I save my plots to a PDF file?

Objectives
  • Save plot(s) in a PDF file.

  • Write conditional statements with if and else.

  • Correctly evaluate expressions containing && (‘and’) and || (‘or’).

Our previous lessons have shown us how to manipulate data, define our own functions, and repeat things. However, the programs we have written so far always do the same things, regardless of what data they’re given. We want programs to make choices based on the values they are manipulating.

Saving Plots to a File

So far, we have built a function analyze to plot summary statistics of the inflammation data:

analyze <- function(filename) {
  # Plots the average, min, and max inflammation over time.
  # Input is character string of a csv file.
  dat <- read.csv(file = filename, header = FALSE)
  avg_day_inflammation <- apply(dat, 2, mean)
  plot(avg_day_inflammation)
  max_day_inflammation <- apply(dat, 2, max)
  plot(max_day_inflammation)
  min_day_inflammation <- apply(dat, 2, min)
  plot(min_day_inflammation)
}

And also built the function analyze_all to automate the processing of each data file:

analyze_all <- function(folder = "data", pattern) {
  # Runs the function analyze for each file in the given folder
  # that contains the given pattern.
  filenames <- list.files(path = folder, pattern = pattern, full.names = TRUE)
  for (f in filenames) {
    analyze(f)
  }
}

While these are useful in an interactive R session, what if we want to send our results to our collaborators? Since we currently have 12 data sets, running analyze_all creates 36 plots. Saving each of these individually would be tedious and error-prone. And in the likely situation that we want to change how the data is processed or the look of the plots, we would have to once again save all 36 before sharing the updated results with our collaborators.

Here’s how we can save all three plots of the first inflammation data set in a pdf file:

pdf("inflammation-01.pdf")
analyze("data/inflammation-01.csv")
dev.off()

The function pdf redirects all the plots generated by R into a pdf file, which in this case we have named “inflammation-01.pdf”. After we are done generating the plots to be saved in the pdf file, we stop R from redirecting plots with the function dev.off.

Overwriting Plots

If you run pdf multiple times without running dev.off, you will save plots to the most recently opened file. However, you won’t be able to open the previous pdf files because the connections were not closed. In order to get out of this situation, you’ll need to run dev.off until all the pdf connections are closed. You can check your current status using the function dev.cur. If it says “pdf”, all your plots are being saved in the last pdf specified. If it says “null device” or “RStudioGD”, the plots will be visualized normally.

We can update the analyze function so that it always saves the plots in a pdf. But that would make it more difficult to interactively test out new changes. It would be ideal if analyze would either save or not save the plots based on its input.

Conditionals

In order to update our function to decide between saving or not, we need to write code that automatically decides between multiple options. The computer can make these deicisons through logical comparisons.

num <- 37
num > 100
[1] FALSE

As 37 is not greater than 100, this returns a FALSE object. And as you likely guessed, the opposite of FALSE is TRUE.

num < 100
[1] TRUE

We pair these logical comparison tools with what R calls a conditional statement, and it looks like this:

num <- 37
if (num > 100) {
  print("greater")
} else {
  print("not greater")
}
print("done")
[1] "not greater"
[1] "done"

The second line of this code uses an if statement to tell R that we want to make a choice. If the following test is TRUE, the body of the if (i.e., the lines in the curly braces underneath it) are executed. If the test is FALSE, the body of the else is executed instead. Only one or the other is ever executed:

Executing a Conditional

In the example above, the test num > 100 returns the value FALSE, which is why the code inside the if block was skipped and the code inside the else statement was run instead.

num > 100
[1] FALSE

And as you likely guessed, the opposite of FALSE is TRUE.

num < 100
[1] TRUE

Conditional statements don’t have to include an else. If there isn’t one, R simply does nothing if the test is false:

num <- 53
if (num > 100) {
  print("num is greater than 100")
}

We can also chain several tests together when there are more than two options. This makes it simple to write a function that returns the sign of a number:

sign <- function(num) {
  if (num > 0) {
    return(1)
  } else if (num == 0) {
    return(0)
  } else {
    return(-1)
  }
}

sign(-3)
[1] -1
sign(0)
[1] 0
sign(2/3)
[1] 1

Note that when combining else and if in an else if statement, the if portion still requires a direct input condition. This is never the case for the else statement alone, which is only executed if all other conditions go unsatisfied. Note that the test for equality uses two equal signs, ==.

Other Comparisons

Other tests include greater than or equal to (>=), less than or equal to (<=), and not equal to (!=).

We can also combine tests. Two ampersands, &&, symbolize “and”. Two vertical bars, ||, symbolize “or”. && is only true if both parts are true:

if (1 > 0 && -1 > 0) {
    print("both parts are true")
} else {
  print("at least one part is not true")
}
[1] "at least one part is not true"

while || is true if either part is true:

if (1 > 0 || -1 > 0) {
    print("at least one part is true")
} else {
  print("neither part is true")
}
[1] "at least one part is true"

In this case, “either” means “either or both”, not “either one or the other but not both”.

Special case of the NA variable

You may remember from the previous lesson that R has a special variable, NA, for designating missing values. Because it represents the absence of a value, we will not be able to test its equality or inequality with another value. Such tests always return NA:

a <- NA
a == 1
[1] NA
a == NA
[1] NA

We need to be particularly careful in tests because trying to compare a NA value will result in an error:

if (a == NA) {
  print("Hi!")
}
Error in if (a == NA) {: missing value where TRUE/FALSE needed

To solve this issue, we need to use the dedicated is.na() function:

is.na(a)
[1] TRUE
if (is.na(a)) {
  print("Hi!")
}
[1] "Hi!"

Choosing Plots Based on Data

Write a function plot_dist that plots a boxplot if the length of the vector is greater than a specified threshold and a stripchart otherwise. To do this you’ll use the R functions boxplot and stripchart.

dat <- read.csv("data/inflammation-01.csv", header = FALSE)
plot_dist(dat[, 10], threshold = 10)     # day (column) 10

plot of chunk using-conditions-01

plot_dist(dat[1:5, 10], threshold = 10)  # samples (rows) 1-5 on day (column) 10

plot of chunk using-conditions-01

Solution

plot_dist <- function(x, threshold) {
  if (length(x) > threshold) {
    boxplot(x)
  } else {
    stripchart(x)
  }
}

Histograms Instead

One of your collaborators prefers to see the distributions of the larger vectors as a histogram instead of as a boxplot. In order to choose between a histogram and a boxplot we will edit the function plot_dist and add an additional argument use_boxplot. By default we will set use_boxplot to TRUE which will create a boxplot when the vector is longer than threshold. When use_boxplot is set to FALSE, plot_dist will instead plot a histogram for the larger vectors. As before, if the length of the vector is shorter than threshold, plot_dist will create a stripchart. A histogram is made with the hist command in R.

dat <- read.csv("data/inflammation-01.csv", header = FALSE)
plot_dist(dat[, 10], threshold = 10, use_boxplot = TRUE)   # day (column) 10 - create boxplot

plot of chunk conditional-challenge-hist

plot_dist(dat[, 10], threshold = 10, use_boxplot = FALSE)  # day (column) 10 - create histogram

plot of chunk conditional-challenge-hist

plot_dist(dat[1:5, 10], threshold = 10)                    # samples (rows) 1-5 on day (column) 10

plot of chunk conditional-challenge-hist

Solution

plot_dist <- function(x, threshold, use_boxplot = TRUE) {
   if (length(x) > threshold && use_boxplot) {
       boxplot(x)
   } else if (length(x) > threshold && !use_boxplot) {
       hist(x)
   } else {
       stripchart(x)
   }
}

Find the Maximum Inflammation Score

Find the file containing the patient with the highest average inflammation score. Print the file name, the patient number (row number) and the value of the maximum average inflammation score.

Tips:

  1. Use variables to store the maximum average and update it as you go through files and patients.
  2. You can use nested loops (one loop is inside the other) to go through the files as well as through the patients in each file (every row).

Complete the code below:

filenames <- list.files(path = "data", pattern = "inflammation-[0-9]{2}.csv", full.names = TRUE)
filename_max <- "" # filename where the maximum average inflammation patient is found
patient_max <- 0 # index (row number) for this patient in this file
average_inf_max <- 0 # value of the average inflammation score for this patient
for (f in filenames) {
  dat <- read.csv(file = f, header = FALSE)
  dat.means <- apply(dat, 1, mean)
  for (patient_index in 1:length(dat.means)){
    patient_average_inf <- dat.means[patient_index]
    # Add your code here ...
  }
}
print(filename_max)
print(patient_max)
print(average_inf_max)

Solution

# Add your code here ...
if (patient_average_inf > average_inf_max) {
  average_inf_max <- patient_average_inf
  filename_max <- f
  patient_max <- patient_index
}

Saving Automatically Generated Figures

Now that we know how to have R make decisions based on input values, let’s update analyze:

analyze <- function(filename, output = NULL) {
  # Plots the average, min, and max inflammation over time.
  # Input:
  #    filename: character string of a csv file
  #    output: character string of pdf file for saving
  if (!is.null(output)) {
    pdf(output)
  }
  dat <- read.csv(file = filename, header = FALSE)
  avg_day_inflammation <- apply(dat, 2, mean)
  plot(avg_day_inflammation)
  max_day_inflammation <- apply(dat, 2, max)
  plot(max_day_inflammation)
  min_day_inflammation <- apply(dat, 2, min)
  plot(min_day_inflammation)
  if (!is.null(output)) {
    dev.off()
  }
}

We added an argument, output, that by default is set to NULL. An if statement at the beginning checks the argument output to decide whether or not to save the plots to a pdf. Let’s break it down. The function is.null returns TRUE if a variable is NULL and FALSE otherwise. The exclamation mark, !, stands for “not”. Therefore the line in the if block is only executed if output is “not null”.

output <- NULL
is.null(output)
[1] TRUE
!is.null(output)
[1] FALSE

Now we can use analyze interactively, as before,

analyze("data/inflammation-01.csv")

plot of chunk inflammation-01plot of chunk inflammation-01plot of chunk inflammation-01

but also use it to save plots,

analyze("data/inflammation-01.csv", output = "inflammation-01.pdf")

Before going further, we will create a directory results for saving our plots. It is good practice in data analysis projects to save all output to a directory separate from the data and analysis code. You can create this directory using the shell command mkdir, or the R function dir.create()

dir.create("results")

Now run analyze and save the plot in the results directory,

analyze("data/inflammation-01.csv", output = "results/inflammation-01.pdf")

This now works well when we want to process one data file at a time, but how can we specify the output file in analyze_all? We need to do two things:

  1. Substitute the filename ending “csv” with “pdf”.
  2. Save the plot to the results directory.

To change the extension to “pdf”, we will use the function sub,

f <- "inflammation-01.csv"
sub("csv", "pdf", f)
[1] "inflammation-01.pdf"

To add the “results” directory to the filename use the function file.path,

file.path("results", sub("csv", "pdf", f))
[1] "results/inflammation-01.pdf"

Now let’s update analyze_all:

analyze_all <- function(pattern) {
  # Directory name containing the data
  data_dir <- "data"
  # Directory name for results
  results_dir <- "results"
  # Runs the function analyze for each file in the current working directory
  # that contains the given pattern.
  filenames <- list.files(path = data_dir, pattern = pattern)
  for (f in filenames) {
    pdf_name <- file.path(results_dir, sub("csv", "pdf", f))
    analyze(file.path(data_dir, f), output = pdf_name)
  }
}

Now we can save all of the results with just one line of code:

analyze_all("inflammation.*csv")

Now if we need to make any changes to our analysis, we can edit the analyze function and quickly regenerate all the figures with analyze_all.

Changing the Behavior of the Plot Command

One of your collaborators asks if you can recreate the figures with lines instead of points. Find the relevant argument to plot by reading the documentation (?plot), update analyze, and then recreate all the figures with analyze_all.

Solution

analyze <- function(filename, output = NULL) {
  # Plots the average, min, and max inflammation over time.
  # Input:
  #    filename: character string of a csv file
  #    output: character string of pdf file for saving
  if (!is.null(output)) {
    pdf(output)
  }
  dat <- read.csv(file = filename, header = FALSE)
  avg_day_inflammation <- apply(dat, 2, mean)
  plot(avg_day_inflammation, type = "l")
  max_day_inflammation <- apply(dat, 2, max)
  plot(max_day_inflammation, type = "l")
  min_day_inflammation <- apply(dat, 2, min)
  plot(min_day_inflammation, type = "l")
  if (!is.null(output)) {
    dev.off()
  }
}

Key Points

  • Save a plot in a pdf file using pdf("name.pdf") and stop writing to the pdf file with dev.off().

  • Use if (condition) to start a conditional statement, else if (condition) to provide additional tests, and else to provide a default.

  • The bodies of conditional statements must be surrounded by curly braces { }.

  • Use == to test for equality.

  • X && Y is only true if both X and Y are true.

  • X || Y is true if either X or Y, or both, are true.


Command-Line Programs

Overview

Teaching: 30 min
Exercises: 0 min
Questions
  • How do I write a command-line script?

  • How do I read in arguments from the command-line?

Objectives
  • Use the values of command-line arguments in a program.

  • Handle flags and files separately in a command-line program.

  • Read data from standard input in a program so that it can be used in a pipeline.

The R Console and other interactive tools like RStudio are great for prototyping code and exploring data, but sooner or later we will want to use our program in a pipeline or run it in a shell script to process thousands of data files. In order to do that, we need to make our programs work like other Unix command-line tools. For example, we may want a program that reads a data set and prints the average inflammation per patient:

$ Rscript readings.R --mean data/inflammation-01.csv
5.45
5.425
6.1
...
6.4
7.05
5.9

but we might also want to look at the minimum of the first four lines

$ head -4 data/inflammation-01.csv | Rscript readings.R --min

or the maximum inflammations in several files one after another:

$ Rscript readings.R --max data/inflammation-*.csv

Our overall requirements are:

  1. If no filename is given on the command line, read data from standard input.
  2. If one or more filenames are given, read data from them and report statistics for each file separately.
  3. Use the --min, --mean, or --max flag to determine what statistic to print.

To make this work, we need to know how to handle command-line arguments in a program, and how to get at standard input. We’ll tackle these questions in turn below.

Command-Line Arguments

Using the text editor of your choice, save the following line of code in a text file called session-info.R:

sessionInfo()

The function, sessionInfo, outputs the version of R you are running as well as the type of computer you are using (as well as the versions of the packages that have been loaded). This is very useful information to include when asking others for help with your R code.

Now we can run the code in the file we created from the Unix Shell using Rscript:

Rscript session-info.R
R version 4.0.5 (2021-03-31)
Platform: x86_64-pc-linux-gnu (64-bit)
Running under: Ubuntu 20.04.2 LTS

Matrix products: default
BLAS:   /usr/lib/x86_64-linux-gnu/blas/libblas.so.3.9.0
LAPACK: /usr/lib/x86_64-linux-gnu/lapack/liblapack.so.3.9.0

locale:
 [1] LC_CTYPE=C.UTF-8       LC_NUMERIC=C           LC_TIME=C.UTF-8       
 [4] LC_COLLATE=C.UTF-8     LC_MONETARY=C.UTF-8    LC_MESSAGES=C.UTF-8   
 [7] LC_PAPER=C.UTF-8       LC_NAME=C              LC_ADDRESS=C          
[10] LC_TELEPHONE=C         LC_MEASUREMENT=C.UTF-8 LC_IDENTIFICATION=C   

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

loaded via a namespace (and not attached):
[1] compiler_4.0.5

The Right Directory

If that did not work, you might have seen an error message indicating that the file session-info.R does not exist. Remember that you must be in the correct directory, the one in which you created your script file. You can determine which directory you are currently in using pwd and change to a different directory using cd. For a review, see this lesson.

Now let’s create another script that does something more interesting. Write the following lines in a file named print-args.R:

args <- commandArgs()
cat(args, sep = "\n")

The function commandArgs extracts all the command line arguments and returns them as a vector. The function cat, similar to the cat of the Unix Shell, outputs the contents of the variable. Since we did not specify a filename for writing, cat sends the output to standard output, which we can then pipe to other Unix functions. Because we set the argument sep to "\n", which is the symbol to start a new line, each element of the vector is printed on its own line. Let’s see what happens when we run this program in the Unix Shell:

Rscript print-args.R
/opt/R/4.0.5/lib/R/bin/exec/R
--no-echo
--no-restore
--file=print-args.R

From this output, we learn that Rscript is just a convenience command for running R scripts. The first argument in the vector is the path to the R executable. The following are all command-line arguments that affect the behavior of R. From the R help file:

Thus running a file with Rscript is an easier way to run the following:

R --slave --no-restore --file=print-args.R --args
/opt/R/4.0.5/lib/R/bin/exec/R
--slave
--no-restore
--file=print-args.R
--args

If we run it with a few arguments, however:

Rscript print-args.R first second third
/opt/R/4.0.5/lib/R/bin/exec/R
--no-echo
--no-restore
--file=print-args.R
--args
first
second
third

then commandArgs adds each of those arguments to the vector it returns. Since the first elements of the vector are always the same, we can tell commandArgs to only return the arguments that come after --args. Let’s update print-args.R and save it as print-args-trailing.R:

args <- commandArgs(trailingOnly = TRUE)
cat(args, sep = "\n")

And then run print-args-trailing from the Unix Shell:

Rscript print-args-trailing.R first second third
first
second
third

Now commandArgs returns only the arguments that we listed after print-args-trailing.R.

With this in hand, let’s build a version of readings.R that always prints the per-patient (per-row) mean of a single data file. The first step is to write a function that outlines our implementation, and a placeholder for the function that does the actual work. By convention this function is usually called main, though we can call it whatever we want. Write the following code in a file called readings-01.R:

main <- function() {
  args <- commandArgs(trailingOnly = TRUE)
  filename <- args[1]
  dat <- read.csv(file = filename, header = FALSE)
  mean_per_patient <- apply(dat, 1, mean)
  cat(mean_per_patient, sep = "\n")
}

This function gets the name of the file to process from the first element returned by commandArgs. Here’s a simple test to run from the Unix Shell:

Rscript readings-01.R data/inflammation-01.csv

There is no output because we have defined a function, but haven’t actually called it. Let’s add a call to main and save it as readings-02.R:

main <- function() {
  args <- commandArgs(trailingOnly = TRUE)
  filename <- args[1]
  dat <- read.csv(file = filename, header = FALSE)
  mean_per_patient <- apply(dat, 1, mean)
  cat(mean_per_patient, sep = "\n")
}

main()
Rscript readings-02.R data/inflammation-01.csv
5.45
5.425
6.1
5.9
5.55
6.225
5.975
6.65
6.625
6.525
6.775
5.8
6.225
5.75
5.225
6.3
6.55
5.7
5.85
6.55
5.775
5.825
6.175
6.1
5.8
6.425
6.05
6.025
6.175
6.55
6.175
6.35
6.725
6.125
7.075
5.725
5.925
6.15
6.075
5.75
5.975
5.725
6.3
5.9
6.75
5.925
7.225
6.15
5.95
6.275
5.7
6.1
6.825
5.975
6.725
5.7
6.25
6.4
7.05
5.9

A Simple Command-Line Program

  1. Write a command-line program that does addition and subtraction of two numbers.

    Hint: Everything argument read from the command-line is interpreted as a character string. You can convert from a string to a number using the function as.numeric.

Rscript arith.R 1 + 2
3
Rscript arith.R 3 - 4
-1

Solution

cat arith.R
main <- function() {
  # Performs addition or subtraction from the command line.
  #
  # Takes three arguments:
  # The first and third are the numbers.
  # The second is either + for addition or - for subtraction.
  #
  # Ex. usage:
  #   Rscript arith.R 1 + 2
  #   Rscript arith.R 3 - 4
  #
  args <- commandArgs(trailingOnly = TRUE)
  num1 <- as.numeric(args[1])
  operation <- args[2]
  num2 <- as.numeric(args[3])
  if (operation == "+") {
    answer <- num1 + num2
    cat(answer)
  } else if (operation == "-") {
    answer <- num1 - num2
    cat(answer)
  } else {
    stop("Invalid input. Use + for addition or - for subtraction.")
  }
}

main()
  1. What goes wrong if you try to add multiplication using * to the program?

Solution

An error message is returned due to “invalid input.” This is likely because ‘*’ has a special meaning in the shell, as a wildcard.

  1. Using the function list.files introduced in a previous lesson, write a command-line program called find-pattern.R that lists all the files in the current directory that contain a specific pattern:
# For example, searching for the pattern "print-args" returns the two scripts we wrote earlier
Rscript find-pattern.R print-args
print-args-trailing.R
print-args.R

Solution

cat find-pattern.R
main <- function() {
  # Finds all files in the current directory that contain a given pattern.
  #
  # Takes one argument: the pattern to be searched.
  #
  # Ex. usage:
  #   Rscript find-pattern.R csv
  #
  args <- commandArgs(trailingOnly = TRUE)
  pattern <- args[1]
  files <- list.files(pattern = pattern)
  cat(files, sep = "\n")
}

main()

Handling Multiple Files

The next step is to teach our program how to handle multiple files. Since 60 lines of output per file is a lot to page through, we’ll start by using three smaller files, each of which has three days of data for two patients. Let’s investigate them from the Unix Shell:

ls data/small-*.csv
data/small-01.csv
data/small-02.csv
data/small-03.csv
cat data/small-01.csv
0,0,1
0,1,2
Rscript readings-02.R data/small-01.csv
0.3333333
1

Using small data files as input also allows us to check our results more easily: here, for example, we can see that our program is calculating the mean correctly for each line, whereas we were really taking it on faith before. This is yet another rule of programming: test the simple things first.

We want our program to process each file separately, so we need a loop that executes once for each filename. If we specify the files on the command line, the filenames will be returned by commandArgs(trailingOnly = TRUE). We’ll need to handle an unknown number of filenames, since our program could be run for any number of files.

The solution is to loop over the vector returned by commandArgs(trailingOnly = TRUE). Here’s our changed program, which we’ll save as readings-03.R:

main <- function() {
  args <- commandArgs(trailingOnly = TRUE)
  for (filename in args) {
    dat <- read.csv(file = filename, header = FALSE)
    mean_per_patient <- apply(dat, 1, mean)
    cat(mean_per_patient, sep = "\n")
  }
}

main()

and here it is in action:

Rscript readings-03.R data/small-01.csv data/small-02.csv
0.3333333
1
13.66667
11

Note: at this point, we have created three versions of our script called readings-01.R, readings-02.R, and readings-03.R. We wouldn’t do this in real life: instead, we would have one file called readings.R that we committed to version control every time we got an enhancement working. For teaching, though, we need all the successive versions side by side.

A Command Line Program with Arguments

Write a program called check.R that takes the names of one or more inflammation data files as arguments and checks that all the files have the same number of rows and columns. What is the best way to test your program?

Solution

cat check.R
main <- function() {
  # Checks that all csv files have the same number of rows and columns.
  #
  # Takes multiple arguments: the names of the files to be checked.
  #
  # Ex. usage:
  #   Rscript check.R inflammation-*
  #
  args <- commandArgs(trailingOnly = TRUE)
  first_file <- read.csv(args[1], header = FALSE)
  first_dim <- dim(first_file)
#   num_rows <- dim(first_file)[1]  # nrow(first_file)
#   num_cols <- dim(first_file)[2]  # ncol(first_file)
  
  
  for (filename in args[-1]) {
    new_file <- read.csv(filename, header = FALSE)
    new_dim <- dim(new_file)
    if (new_dim[1] != first_dim[1] | new_dim[2] != first_dim[2]) {
      cat("Not all the data files have the same dimensions.")
    }
  }
}

main()

Handling Command-Line Flags

The next step is to teach our program to pay attention to the --min, --mean, and --max flags. These always appear before the names of the files, so let’s save the following in readings-04.R:

main <- function() {
  args <- commandArgs(trailingOnly = TRUE)
  action <- args[1]
  filenames <- args[-1]

  for (f in filenames) {
    dat <- read.csv(file = f, header = FALSE)

    if (action == "--min") {
      values <- apply(dat, 1, min)
    } else if (action == "--mean") {
      values <- apply(dat, 1, mean)
    } else if (action == "--max") {
      values <- apply(dat, 1, max)
    }
    cat(values, sep = "\n")
  }
}

main()

And we can confirm this works by running it from the Unix Shell:

Rscript readings-04.R --max data/small-01.csv
1
2

but there are several things wrong with it:

  1. main is too large to read comfortably.

  2. If action isn’t one of the three recognized flags, the program loads each file but does nothing with it (because none of the branches in the conditional match). Silent failures like this are always hard to debug.

This version pulls the processing of each file out of the loop into a function of its own. It also uses stopifnot to check that action is one of the allowed flags before doing any processing, so that the program fails fast. We’ll save it as readings-05.R:

main <- function() {
  args <- commandArgs(trailingOnly = TRUE)
  action <- args[1]
  filenames <- args[-1]
  stopifnot(action %in% c("--min", "--mean", "--max"))

  for (f in filenames) {
    process(f, action)
  }
}

process <- function(filename, action) {
  dat <- read.csv(file = filename, header = FALSE)

  if (action == "--min") {
    values <- apply(dat, 1, min)
  } else if (action == "--mean") {
    values <- apply(dat, 1, mean)
  } else if (action == "--max") {
    values <- apply(dat, 1, max)
  }
  cat(values, sep = "\n")
}

main()

This is four lines longer than its predecessor, but broken into more digestible chunks of 8 and 12 lines.

Parsing Command-Line Flags

R has a package named argparse that helps handle complex command-line flags (it utilizes a Python module of the same name). We will not cover this package in this lesson but when you start writing programs with multiple parameters you’ll want to read through the package’s vignette.

Shorter Command Line Arguments

Rewrite this program so that it uses -n, -m, and -x instead of --min, --mean, and --max respectively. Is the code easier to read? Is the program easier to understand?

Separately, modify the program so that if no action is specified (or an incorrect action is given), it prints a message explaining how it should be used.

Solution

cat readings-short.R
main <- function() {
  args <- commandArgs(trailingOnly = TRUE)
  action <- args[1]
  filenames <- args[-1]
  stopifnot(action %in% c("-n", "-m", "-x"))

  for (f in filenames) {
    process(f, action)
  }
}

process <- function(filename, action) {
  dat <- read.csv(file = filename, header = FALSE)

  if (action == "-n") {
    values <- apply(dat, 1, min)
  } else if (action == "-m") {
    values <- apply(dat, 1, mean)
  } else if (action == "-x") {
    values <- apply(dat, 1, max)
  }
  cat(values, sep = "\n")
}

main()

The program is neither easier to read nor easier to understand due to the ambiguity of the argument names.

cat readings-usage.R
main <- function() {
  args <- commandArgs(trailingOnly = TRUE)
  action <- args[1]
  filenames <- args[-1]
  if (!(action %in% c("--min", "--mean", "--max"))) {
    usage()
  } else if (length(filenames) == 0) {
    process(file("stdin"), action)
  } else {
    for (f in filenames) {
      process(f, action)
    }
  }
}

process <- function(filename, action) {
  dat <- read.csv(file = filename, header = FALSE)

  if (action == "--min") {
    values <- apply(dat, 1, min)
  } else if (action == "--mean") {
    values <- apply(dat, 1, mean)
  } else if (action == "--max") {
    values <- apply(dat, 1, max)
  }
  cat(values, sep = "\n")
}

usage <- function() {
  cat("usage: Rscript readings-usage.R [--min, --mean, --max] filenames", sep = "\n")
}

main()

Handling Standard Input

The next thing our program has to do is read data from standard input if no filenames are given so that we can put it in a pipeline, redirect input to it, and so on. Let’s experiment in another script, which we’ll save as count-stdin.R:

lines <- readLines(con = file("stdin"))
count <- length(lines)
cat("lines in standard input: ")
cat(count, sep = "\n")

This little program reads lines from the program’s standard input using file("stdin"). This allows us to do almost anything with it that we could do to a regular file. In this example, we passed it as an argument to the function readLines, which stores each line as an element in a vector. Let’s try running it from the Unix Shell as if it were a regular command-line program:

Rscript count-stdin.R < data/small-01.csv
lines in standard input: 2

Note that because we did not specify sep = "\n" when calling cat, the output is written on the same line.

A common mistake is to try to run something that reads from standard input like this:

Rscript count-stdin.R data/small-01.csv

i.e., to forget the < character that redirect the file to standard input. In this case, there’s nothing in standard input, so the program waits at the start of the loop for someone to type something on the keyboard. We can type some input, but R keeps running because it doesn’t know when the standard input has ended. If you ran this, you can pause R by typing Ctrl+Z (technically it is still paused in the background; if you want to fully kill the process type kill %; see bash manual for more information).

We now need to rewrite the program so that it loads data from file("stdin") if no filenames are provided. Luckily, read.csv can handle either a filename or an open file as its first parameter, so we don’t actually need to change process. That leaves main, which we’ll update and save as readings-06.R:

main <- function() {
  args <- commandArgs(trailingOnly = TRUE)
  action <- args[1]
  filenames <- args[-1]
  stopifnot(action %in% c("--min", "--mean", "--max"))
  
  if (length(filenames) == 0) {
    process(file("stdin"), action)
  } else {
    for (f in filenames) {
      process(f, action)
    }
  }
}

process <- function(filename, action) {
  dat <- read.csv(file = filename, header = FALSE)

  if (action == "--min") {
    values <- apply(dat, 1, min)
  } else if (action == "--mean") {
    values <- apply(dat, 1, mean)
  } else if (action == "--max") {
    values <- apply(dat, 1, max)
  }
  cat(values, sep = "\n")
}

main()

Let’s try it out. Instead of calculating the mean inflammation of every patient, we’ll only calculate the mean for the first 10 patients (rows):

head data/inflammation-01.csv | Rscript readings-06.R --mean
5.45
5.425
6.1
5.9
5.55
6.225
5.975
6.65
6.625
6.525

And now we’re done: the program now does everything we set out to do.

Implementing wc in R

Write a program called line-count.R that works like the Unix wc command:

  • If no filenames are given, it reports the number of lines in standard input.
  • If one or more filenames are given, it reports the number of lines in each, followed by the total number of lines.

Solution

cat line-count.R
main <- function() {
  args <- commandArgs(trailingOnly = TRUE)
  if (length(args) > 0) {
    total_lines <- 0
    for (filename in args) {
      input <- readLines(filename)
      num_lines <- length(input)
      cat(filename)
      cat(" ")
      cat(num_lines, sep = "\n")
      total_lines <- total_lines + num_lines
    }
    if (length(args) > 1) {
      cat("Total ")
      cat(total_lines, sep = "\n")
    }
  } else {
    input <- readLines(file("stdin"))
    num_lines <- length(input)
    cat(num_lines, sep = "\n")
  }
}

main()

Key Points

  • Use commandArgs(trailingOnly = TRUE) to obtain a vector of the command-line arguments that a program was run with.

  • Avoid silent failures.

  • Use file("stdin") to connect to a program’s standard input.

  • Use cat(vec, sep = " ") to write the elements of vec to standard output, one per line.


Best Practices for Writing R Code

Overview

Teaching: 10 min
Exercises: 0 min
Questions
  • How can I write R code that other people can understand and use?

Objectives
  • Describe changes made to code for future reference.

  • Understand the importance about requirements and dependencies for your code.

  • Know when to use setwd().

  • To identify and segregate distinct components in your code using # or #-.

  • Additional best practice recommendations.

Keep track of who wrote your code and its intended purpose

Starting your code with an annotated description of what the code does when it is run will help you when you have to look at or change it in the future. Just one or two lines at the beginning of the file can save you or someone else a lot of time and effort when trying to understand what a particular script does.

# This is code to replicate the analyses and figures from my 2014 Science
# paper. Code developed by Sarah Supp, Tracy Teal, and Jon Borelli

Also consider version-controlling your code as for example taught by Software Carpentry’s git-novice lessons. Frequent commits with explanatory, synoptic commit messages are an elegant way to comment code. RStudio enables this with a Git integration.

Be explicit about the requirements and dependencies of your code

Loading all of the packages that will be necessary to run your code (using library) is a nice way of indicating which packages are necessary to run your code. It can be frustrating to make it two-thirds of the way through a long-running script only to find out that a dependency hasn’t been installed.

library(ggplot2)
library(reshape)
library(vegan)

Another way you can be explicit about the requirements of your code and improve it’s reproducibility is to limit the “hard-coding” of the input and output files for your script. If your code will read in data from a file, define a variable early in your code that stores the path to that file. For example

input_file <- "data/data.csv" 
output_file <- "data/results.csv"

# read input
input_data <- read.csv(input_file)
# get number of samples in data
sample_number <- nrow(input_data)
# generate results
results <- some_other_function(input_file, sample_number)
# write results
write.table(results, output_file)

is preferable to

# check
input_data <- read.csv("data/data.csv")
# get number of samples in data
sample_number <- nrow(input_data)
# generate results
results <- some_other_function("data/data.csv", sample_number)
# write results
write.table("data/results.csv", output_file)

It is also worth considering what the working directory is. If the working directory must change, it is best to do that at the beginning of the script.

Be careful when using setwd()

One should exercise caution when using setwd(). Changing directories in a script file can limit reproducibility:

  • setwd() will return an error if the directory to which you’re trying to change doesn’t exist or if the user doesn’t have the correct permissions to access that directory. This becomes a problem when sharing scripts between users who have organized their directories differently.
  • If/when your script terminates with an error, you might leave the user in a different directory than the one they started in, and if they then call the script again, this will cause further problems. If you must use setwd(), it is best to put it at the top of the script to avoid these problems. The following error message indicates that R has failed to set the working directory you specified:
    Error in setwd("~/path/to/working/directory") : cannot change working directory
    

    It is best practice to have the user running the script begin in a consistent directory on their machine and then use relative file paths from that directory to access files (see below).

Identify and segregate distinct components in your code

It’s easy to annotate and mark your code using # or #- to set off sections of your code and to make finding specific parts of your code easier. For example, it’s often helpful when writing code to separate the function definitions. If you create only one or a few custom functions in your script, put them toward the top of your code. If you have written many functions, put them all in their own .R file and then source those files. source will define all of these functions so that your code can make use of them as needed.

source("my_genius_fxns.R")

Other ideas

  1. Use a consistent style within your code. For example, name all matrices something ending in _mat. Consistency makes code easier to read and problems easier to spot.

  2. Keep your code in bite-sized chunks. If a single function or loop gets too long, consider looking for ways to break it into smaller pieces.

  3. Don’t repeat yourself–automate! If you are repeating the same code over and over, use a loop or a function to repeat that code for you. Needless repetition doesn’t just waste time–it also increases the likelihood you’ll make a costly mistake!

  4. Keep all of your source files for a project in the same directory, then use relative paths as necessary to access them. For example, use

dat <- read.csv(file = "files/dataset-2013-01.csv", header = TRUE)

rather than:

dat <- read.csv(file = "/Users/Karthik/Documents/sannic-project/files/dataset-2013-01.csv", header = TRUE)
  1. R can run into memory issues. It is a common problem to run out of memory after running R scripts for a long time. To inspect the objects in your current R environment, you can list the objects, search current packages, and remove objects that are currently not in use. A good practice when running long lines of computationally intensive code is to remove temporary objects after they have served their purpose. However, sometimes, R will not clean up unused memory for a while after you delete objects. You can force R to tidy up its memory by using gc().
# Sample dataset of 1000 rows
interim_object <- data.frame(rep(1:100, 10),
                             rep(101:200, 10),
                             rep(201:300, 10))
object.size(interim_object) # Reports the memory size allocated to the object
rm("interim_object") # Removes only the object itself and not necessarily the memory allotted to it
gc() # Force R to release memory it is no longer using
ls() # Lists all the objects in your current workspace
rm(list = ls()) # If you want to delete all the objects in the workspace and start with a clean slate
  1. Don’t save a session history (the default option in R, when it asks if you want an RData file). Instead, start in a clean environment so that older objects don’t remain in your environment any longer than they need to. If that happens, it can lead to unexpected results.

  2. Wherever possible, keep track of sessionInfo() somewhere in your project folder. Session information is invaluable because it captures all of the packages used in the current project. If a newer version of a package changes the way a function behaves, you can always go back and reinstall the version that worked (Note: At least on CRAN, all older versions of packages are permanently archived).

  3. Collaborate. Grab a buddy and practice “code review”. Review is used for preparing experiments and manuscripts; why not use it for code as well? Our code is also a major scientific achievement and the product of lots of hard work! Reviews are built into GitHub’s Pull request feature

Best Practice

  1. What other suggestions do you have for coding best practices?
  2. What are some specific ways we could restructure the code we worked on today to make it easier for a new user to read? Discuss with your neighbor.
  3. Make two new R scripts called inflammation.R and inflammation_fxns.R. Copy and paste code into each script so that inflammation.R “does stuff” and inflammation_fxns.R holds all of your functions. Hint: you will need to add source to one of the files.

Solution

cat inflammation.R
# This code runs the inflammation data analysis.

source("inflammation_fxns.R")
analyze_all("inflammation.*csv")
cat inflammation_fxns.R
# This is code for functions used in our inflammation data analysis.

analyze <- function(filename, output = NULL) {
  # Plots the average, min, and max inflammation over time.
  # Input:
  #    filename: character string of a csv file
  #    output: character string of pdf file for saving
  if (!is.null(output)) {
    pdf(output)
  }
  dat <- read.csv(file = filename, header = FALSE)
  avg_day_inflammation <- apply(dat, 2, mean)
  plot(avg_day_inflammation)
  max_day_inflammation <- apply(dat, 2, max)
  plot(max_day_inflammation)
  min_day_inflammation <- apply(dat, 2, min)
  plot(min_day_inflammation)
  if (!is.null(output)) {
    dev.off()
  }
}

analyze_all <- function(pattern) {
  # Directory name containing the data
  data_dir <- "data"
  # Directory name for results
  results_dir <- "results"
  # Runs the function analyze for each file in the current working directory
  # that contains the given pattern.
  filenames <- list.files(path = data_dir, pattern = pattern)
  for (f in filenames) {
    pdf_name <- file.path(results_dir, sub("csv", "pdf", f))
    analyze(file.path(data_dir, f), output = pdf_name)
  }
}

Key Points

  • Start each program with a description of what it does.

  • Then load all required packages.

  • Consider what working directory you are in when sourcing a script.

  • Use comments to mark off sections of code.

  • Put function definitions at the top of your file, or in a separate file if there are many.

  • Name and style code consistently.

  • Break code into small, discrete pieces.

  • Factor out common operations rather than repeating them.

  • Keep all of the source files for a project in one directory and use relative paths to access them.

  • Keep track of the memory used by your program.

  • Always start with a clean environment instead of saving the workspace.

  • Keep track of session information in your project folder.

  • Have someone else review your code.

  • Use version control.


Dynamic Reports with knitr

Overview

Teaching: 20 min
Exercises: 0 min
Questions
  • How can I put my text, code, and results all in one document?

  • How do I use knitr?

  • How do I write in Markdown?

Objectives
  • Understand the value of knitr for generating dynamic documents that include text, code, and results.

  • Control basic formatting using markdown syntax.

  • Be able to create, edit, and compile an .Rmd document including code chunks and inline code.

knitr is an R package that allows you to organize your notes, code, and results in a single document. It’s a great tool for “literate programming” – the idea that your code should be readable by humans as well as computers! It also keeps your writing and results together, so if you collect some new data or change how you clean the data, you just have to re-compile the document and you’re up to date!

You write knitr documents in a simple plain text-like format called markdown, which allows you to format text using intuitive notation, so that you can focus on the content you’re writing and generating a well-formatted document when needed. In fact, you can turn your plain text (and R code and results) into an HTML file or, if you have an installation of LaTeX and Pandoc on your machine, a PDF, or even a Word document (if you must!).

To get started, install the knitr package.

install.packages("knitr")

When you click on File -> New File, there is an option for “R Markdown…”. Choose this and accept the default options in the dialog box that follows (but note that you can also create presentations this way). Save the file and click on the “Knit HTML” button at the top of the script. Compare the output to the source.

Formatting Text in Markdown

Visit https://rmarkdown.rstudio.com/authoring_basics.html and briefly check out some of the formatting options.

In the example document add

  • Headers using #
  • Emphasis using asterisk: *italics* and **bold**
  • Lists using * and numbered lists using 1., 2., etc.
  • Bonus: Create a table

Markdown also supports LaTeX equation editing. We can display pretty equations by enclosing them in $, e.g., $\alpha = \dfrac{1}{(1 - \beta)^2}$ renders as: Rendered LaTeX equation

The top of the source (.Rmd) file has some header material in YAML format (enclosed by triple dashes). Some of this gets displayed in the output header, other of it provides formatting information to the conversion engine.

To distinguish R code from text, RMarkdown uses three back-ticks followed by {r} to distinguish a “code chunk”. In RStudio, the keyboard shortcut to create a code chunk is Command+Option+I or Ctrl+Alt+I.

A code chunk will set off the code and its results in the output document, but you can also print the results of code within a text block by enclosing code like so: `r code-here`.

Use knitr to Produce a Report

  1. Open a new .Rmd script and save it as inflammation_report.Rmd
  2. Copy code from earlier into code chunks to read the inflammation data and plot average inflammation.
  3. Add a few notes describing what the code does and what the main findings are. Include an in-line calculation of the median inflammation level.
  4. knit the document and view the html result.

Key Points

  • Use knitr to generate reports that combine text, code, and results.

  • Use Markdown to format text.

  • Put code in blocks delimited by triple back quotes followed by {r}.


Making Packages in R

Overview

Teaching: 30 min
Exercises: 0 min
Questions
  • How do I collect my code together so I can reuse it and share it?

  • How do I make my own packages?

Objectives
  • Describe the required structure of R packages.

  • Create the required structure of a simple R package.

  • Write documentation comments that can be automatically compiled to R’s native help and documentation format.

Why should you make your own R packages?

Reproducible research!

An R package is the basic unit of reusable code. If you want to reuse code later or want others to be able to use your code, you should put it in a package.

An R package requires four components:

*There are other optional components. rOpenSci community has written a science-focused guidebook for package development, while the “R packages” book contains all the fundamental information.

DESCRIPTION file

Package: Package name
Title: Brief package description
Description: Longer package description
Version: Version number(major.minor.patch)
Author: Name and email of package creator
Maintainer: Name and email of package maintainer (who to contact with issues)
License: Abbreviation for an open source license

The package name can only contain letters and numbers and has to start with a letter.

.R files

Functions don’t all have to be in one file or each in separate files. How you organize them is up to you. Suggestion: organize in a logical manner so that you know which file holds which functions.

Making your first R package

Let’s turn our temperature conversion functions into an R package.

fahrenheit_to_celsius <- function(temp_F) {
  # Converts Fahrenheit to Celsius
  temp_C <- (temp_F - 32) * 5 / 9
  return(temp_C)
}
celsius_to_kelvin <- function(temp_C) {
  # Converts Celsius to Kelvin
  temp_K <- temp_C + 273.15
  return(temp_K)
}
fahrenheit_to_kelvin <- function(temp_F) {
  # Converts Fahrenheit to Kelvin using fahrenheit_to_celsius() and celsius_to_kelvin()
  temp_C <- fahrenheit_to_celsius(temp_F)
  temp_K <- celsius_to_kelvin(temp_C)
  return(temp_K)
}

We will use the devtools and roxygen2 packages, which make creating packages in R relatively simple. Both can be installed from CRAN like this:

install.packages(c("devtools", "roxygen2"))  # installations can be `c`ombined
library("devtools")
library("roxygen2")

Set your working directory, and then use the create function to start making your package. Keep the name simple and unique.

setwd(parentDirectory)
create_package("tempConvert")

Add our functions to the R directory. Place each function into a separate R script and add documentation like this:

#' Converts Fahrenheit to Celsius
#'
#' This function converts input temperatures in Fahrenheit to Celsius.
#' @param temp_F The temperature in Fahrenheit.
#' @return The temperature in Celsius.
#' @export
#' @examples
#' fahrenheit_to_kelvin(32)

fahrenheit_to_celsius <- function(temp_F) {
  temp_C <- (temp_F - 32) * 5 / 9
  return(temp_C)
}

The roxygen2 package reads lines that begin with #' as comments to create the documentation for your package. Descriptive tags are preceded with the @ symbol. For example, @param has information about the input parameters for the function. Now, we will use roxygen2 to convert our documentation to the standard R format.

setwd("./tempConvert")
document()

Take a look at the package directory now. The /man directory has a .Rd file for each .R file with properly formatted documentation.

Overall, your package directory should look something like this:

tempConvert directory containing 4 items: Namespace file, Description file, man directory with documentation in .Rd files, R directory with functions in .R files

Now, let’s load the package and take a look at the documentation.

setwd("..")
install("tempConvert")

?fahrenheit_to_kelvin

Notice there is now a tempConvert environment that is the parent environment to the global environment.

search()

Now that our package is loaded, let’s try out some of the functions.

fahrenheit_to_celsius(32)
[1] 0
celsius_to_kelvin(-273.15)
[1] 0
fahrenheit_to_kelvin(-459.67)
[1] 0

Creating a Package for Distribution

  1. Create some new functions for your tempConvert package to convert from Celsius to Fahrenheit or from Kelvin to Celsius or Fahrenheit.
  2. Create a package for our analyze function so that it will be easy to load when more data arrives.

Solution

#' Converts Kelvin to Celsius
#'
#' This function converts input temperatures in Kelvin to Celsius.
#' @param temp_K The temperature in Kelvin.
#' @return The temperature in Celsius.
#' @export
#' @examples
#' kelvin_to_celsius(273.15)

kelvin_to_celsius <- function(temp_K) {
  temp_C <- temp_K - 273.15
  temp_C
}
#' Converts Celsius to Fahrenheit
#'
#' This function converts input temperatures in Celsius to Fahrenheit.
#' @param temp_C The temperature in Celsius.
#' @return The temperature in Fahrenheit.
#' @export
#' @examples
#' celsius_to_fahrenheit(0)

celsius_to_fahrenheit <- function(temp_C) {
  temp_F <- (temp_C * 9/5) + 32
  temp_F
}
#' Converts Kelvin to Fahrenheit
#'
#' This function converts input temperatures in Kelvin to Fahrenheit.
#' @param temp_K The temperature in Kelvin.
#' @return The temperature in Fahrenheit.
#' @export
#' @examples
#' kelvin_to_fahrenheit(273.15)

kelvin_to_fahrenheit <- function(temp_K) {
  temp_C <- kelvin_to_celsius(temp_K)
  temp_F <- celsius_to_fahrenheit(temp_C)
  temp_F
}

Key Points

  • A package is the basic unit of reusability in R.

  • Every package must have a DESCRIPTION file and an R directory containing code. These are created by us.

  • A NAMESPACE file is needed as well, and a man directory containing documentation, but both can be autogenerated.


Introduction to RStudio

Overview

Teaching: 15 min
Exercises: 0 min
Questions
  • How do I use the RStudio graphical user interface?

Objectives
  • Get familiar with RStudio interface.

  • Understand the difference between script file and console.

  • Demonstrate useful shortcuts.

Let’s start by learning about our tool.

Interacting with R

There are two main ways of interacting with R: using the console or by using script files (plain text files that contain your code).

The console window (in RStudio, the bottom left panel) is the place where R is waiting for you to tell it what to do, and where it will show the results of a command. You can type commands directly into the console, but they will be forgotten when you close the session. It is better to enter the commands in the script editor, and save the script. This way, you have a complete record of what you did, you can easily show others how you did it and you can do it again later on if needed. You can copy-paste into the R console, but the Rstudio script editor allows you to ‘send’ the current line or the currently selected text to the R console using the Ctrl+Return shortcut.

At some point in your analysis you may want to check the content of variable or the structure of an object, without necessarily keep a record of it in your script. You can type these commands directly in the console. RStudio provides the Ctrl+1 and Ctrl+2 shortcuts allow you to jump between the script and the console windows.

If R is ready to accept commands, the R console shows a > prompt. If it receives a command (by typing, copy-pasting or sent from the script editor using Ctrl+Return), R will try to execute it, and when ready, show the results and come back with a new >-prompt to wait for new commands.

If R is still waiting for you to enter more data because it isn’t complete yet, the console will show a + prompt. It means that you haven’t finished entering a complete command. This is because you have not ‘closed’ a parenthesis or quotation. If you’re in RStudio and this happens, click inside the console window and press Esc; this should help you out of trouble.

Commenting

Use # signs to comment. Comment liberally in your R scripts. Anything to the right of a # is ignored by R.

Assignment Operator

<- is the assignment operator. It assigns values on the right to objects on the left. So, after executing x <- 3, the value of x is 3. The arrow can be read as 3 goes into x. You can also use = for assignments but not in all contexts so it is good practice to use <- for assignments. = should only be used to specify the values of arguments in functions, see below.

In RStudio, typing Alt+- (push Alt, the key next to your space bar at the same time as the - key) will write <- in a single keystroke.

Key Points

  • Using RStudio can make programming in R much more productive.


Addressing Data

Overview

Teaching: 20 min
Exercises: 0 min
Questions
  • What are the different methods for accessing parts of a data frame?

Objectives
  • Understand the three different ways R can address data inside a data frame.

  • Combine different methods for addressing data with the assignment operator to update subsets of data.

R is a powerful language for data manipulation. There are three main ways for addressing data inside R objects.

Lets start by loading some sample data:

dat <- read.csv(file = 'data/sample.csv', header = TRUE, stringsAsFactors = FALSE)

Interpreting Rows as Headers

The first row of this csv file is a list of column names. We used the header = TRUE argument to read.csv so that R can interpret the file correctly. We are using the stringsAsFactors = FALSE argument to override the default behaviour for R. Using factors in R is covered in a separate lesson.

Lets take a look at this data.

class(dat)
[1] "data.frame"

R has loaded the contents of the .csv file into a variable called dat which is a data frame.

We can compactly display the internal structure of a data frame using the structure function str.

str(dat)
'data.frame':	100 obs. of  9 variables:
 $ ID           : chr  "Sub001" "Sub002" "Sub003" "Sub004" ...
 $ Gender       : chr  "m" "m" "m" "f" ...
 $ Group        : chr  "Control" "Treatment2" "Treatment2" "Treatment1" ...
 $ BloodPressure: int  132 139 130 105 125 112 173 108 131 129 ...
 $ Age          : num  16 17.2 19.5 15.7 19.9 14.3 17.7 19.8 19.4 18.8 ...
 $ Aneurisms_q1 : int  114 148 196 199 188 260 135 216 117 188 ...
 $ Aneurisms_q2 : int  140 209 251 140 120 266 98 238 215 144 ...
 $ Aneurisms_q3 : int  202 248 122 233 222 320 154 279 181 192 ...
 $ Aneurisms_q4 : int  237 248 177 220 228 294 245 251 272 185 ...

The str function tell us that the data has 100 rows and 9 columns. It is also tell us that the data frame is made up of character chr, integer int and numeric vectors.

head(dat)
      ID Gender      Group BloodPressure  Age Aneurisms_q1 Aneurisms_q2
1 Sub001      m    Control           132 16.0          114          140
2 Sub002      m Treatment2           139 17.2          148          209
3 Sub003      m Treatment2           130 19.5          196          251
4 Sub004      f Treatment1           105 15.7          199          140
5 Sub005      m Treatment1           125 19.9          188          120
6 Sub006      M Treatment2           112 14.3          260          266
  Aneurisms_q3 Aneurisms_q4
1          202          237
2          248          248
3          122          177
4          233          220
5          222          228
6          320          294

The data is the results of an (not real) experiment, looking at the number of aneurysms that formed in the eyes of patients who undertook 3 different treatments.

Addressing by Index

Data can be accessed by index. We have already seen how square brackets [ can be used to subset data (sometimes also called “slicing”). The generic format is dat[row_numbers,column_numbers].

Selecting Values

What will be returned by dat[1, 1]? Think about the number of rows and columns you would expect as the result.

Solution

dat[1, 1]
[1] "Sub001"

If we leave out a dimension R will interpret this as a request for all values in that dimension.

Selecting More Values

What will be returned by dat[, 2]?

Solution

dat[, 2]
  [1] "m" "m" "m" "f" "m" "M" "f" "m" "m" "f" "m" "f" "f" "m" "m" "m" "f" "m"
 [19] "m" "F" "f" "m" "f" "f" "m" "M" "M" "f" "m" "f" "f" "m" "m" "m" "m" "f"
 [37] "f" "m" "M" "m" "f" "m" "m" "m" "f" "f" "M" "M" "m" "m" "m" "f" "f" "f"
 [55] "m" "f" "m" "m" "m" "f" "f" "f" "f" "M" "f" "m" "f" "f" "M" "m" "m" "m"
 [73] "F" "m" "m" "f" "M" "M" "M" "f" "m" "M" "M" "m" "m" "f" "f" "f" "m" "m"
 [91] "f" "m" "F" "f" "m" "m" "F" "m" "M" "M"

The colon : can be used to create a sequence of integers.

6:9
[1] 6 7 8 9

Creates a vector of numbers from 6 to 9.

This can be very useful for addressing data.

Subsetting with Sequences

Use the colon operator to index just the aneurism count data (columns 6 to 9).

Solution

dat[, 6:9]
    Aneurisms_q1 Aneurisms_q2 Aneurisms_q3 Aneurisms_q4
1            114          140          202          237
2            148          209          248          248
3            196          251          122          177
4            199          140          233          220
5            188          120          222          228
6            260          266          320          294
7            135           98          154          245
8            216          238          279          251
9            117          215          181          272
10           188          144          192          185
11           134          155          247          223
12           152          177          323          245
13           112          220          225          195
14           109          150          177          189
15           146          140          239          223
16            97          172          203          207
17           165          157          200          193
18           158          265          243          187
19           178          109          206          182
20           107          188          167          218
21           174          160          203          183
22            97          110          194          133
23           187          239          281          214
24           188          191          256          265
25           114          199          242          195
26           115          160          158          228
27           128          249          294          315
28           112          230          281          126
29           136          109          105          155
30           103          148          219          228
31           132          151          234          162
32           118          154          260          160
33           166          176          253          233
34           152          105          197          299
35           191          148          166          185
36           152          178          158          170
37           161          270          232          284
38           239          184          317          269
39           132          137          193          206
40           168          255          273          274
41           140          184          239          202
42           166           85          179          196
43           141          160          179          239
44           161          168          212          181
45           103          111          254          126
46           231          240          260          310
47           192          141          180          225
48           178          180          169          183
49           167          123          236          224
50           135          150          208          279
51           150          166          153          204
52           192           80          138          222
53           153          153          236          216
54           205          264          269          207
55           117          194          216          211
56           199          119          183          251
57           182          129          226          218
58           180          196          250          294
59           111          111          244          201
60           101           98          178          116
61           166          167          232          241
62           158          171          237          212
63           189          178          177          238
64           189          101          193          172
65           239          189          297          300
66           185          224          151          182
67           224          112          304          288
68           104          139          211          204
69           222          199          280          196
70           107           98          204          138
71           153          255          218          234
72           118          165          220          227
73           102          184          246          222
74           188          125          191          157
75           180          283          204          298
76           178          214          291          240
77           168          184          184          229
78           118          170          249          249
79           169          114          248          233
80           156          138          218          258
81           232          211          219          246
82           188          108          180          136
83           169          168          180          211
84           241          233          292          182
85            65          207          234          235
86           225          185          195          235
87           104          116          173          221
88           179          158          216          244
89           103          140          209          186
90           112          130          175          191
91           226          170          307          244
92           228          221          316          259
93           209          142          199          184
94           153          104          194          214
95           111          118          173          191
96           148          132          200          194
97           141          196          322          273
98           193          112          123          181
99           130          226          286          281
100          126          157          129          160

Finally we can use the c() (combine) function to address non-sequential rows and columns.

dat[c(1, 5, 7, 9), 1:5]
      ID Gender      Group BloodPressure  Age
1 Sub001      m    Control           132 16.0
5 Sub005      m Treatment1           125 19.9
7 Sub007      f    Control           173 17.7
9 Sub009      m Treatment2           131 19.4

Returns the first 5 columns for patients in rows 1,5,7 and 9

Subsetting Non-Sequential Data

Write code to return the age and gender values for the first 5 patients.

Solution

dat[1:5, c(5, 2)]
   Age Gender
1 16.0      m
2 17.2      m
3 19.5      m
4 15.7      f
5 19.9      m

Addressing by Name

Columns in an R data frame are named.

colnames(dat)
[1] "ID"            "Gender"        "Group"         "BloodPressure"
[5] "Age"           "Aneurisms_q1"  "Aneurisms_q2"  "Aneurisms_q3" 
[9] "Aneurisms_q4" 

Default Names

If column names are not specified e.g. using headers = FALSE in a read.csv() function, R assigns default names V1, V2, ..., Vn

We usually use the $ operator to address a column by name

dat$Gender
  [1] "m" "m" "m" "f" "m" "M" "f" "m" "m" "f" "m" "f" "f" "m" "m" "m" "f" "m"
 [19] "m" "F" "f" "m" "f" "f" "m" "M" "M" "f" "m" "f" "f" "m" "m" "m" "m" "f"
 [37] "f" "m" "M" "m" "f" "m" "m" "m" "f" "f" "M" "M" "m" "m" "m" "f" "f" "f"
 [55] "m" "f" "m" "m" "m" "f" "f" "f" "f" "M" "f" "m" "f" "f" "M" "m" "m" "m"
 [73] "F" "m" "m" "f" "M" "M" "M" "f" "m" "M" "M" "m" "m" "f" "f" "f" "m" "m"
 [91] "f" "m" "F" "f" "m" "m" "F" "m" "M" "M"

When we extract a single column from a data frame using the $ operator, R will return a vector of that column class and not a data frame.

class(dat$Gender)
[1] "character"
class(dat$BloodPressure)
[1] "integer"

Named addressing can also be used in square brackets.

head(dat[, c('Age', 'Gender')])
   Age Gender
1 16.0      m
2 17.2      m
3 19.5      m
4 15.7      f
5 19.9      m
6 14.3      M

Best Practice

Best practice is to address columns by name. Often, you will create or delete columns and the column position will change.


Rows in an R data frame can also be named, and rows can also be addressed by their names.
By default, row names are indices (i.e. position of each row in the data frame):

rownames(dat)
  [1] "1"   "2"   "3"   "4"   "5"   "6"   "7"   "8"   "9"   "10"  "11"  "12" 
 [13] "13"  "14"  "15"  "16"  "17"  "18"  "19"  "20"  "21"  "22"  "23"  "24" 
 [25] "25"  "26"  "27"  "28"  "29"  "30"  "31"  "32"  "33"  "34"  "35"  "36" 
 [37] "37"  "38"  "39"  "40"  "41"  "42"  "43"  "44"  "45"  "46"  "47"  "48" 
 [49] "49"  "50"  "51"  "52"  "53"  "54"  "55"  "56"  "57"  "58"  "59"  "60" 
 [61] "61"  "62"  "63"  "64"  "65"  "66"  "67"  "68"  "69"  "70"  "71"  "72" 
 [73] "73"  "74"  "75"  "76"  "77"  "78"  "79"  "80"  "81"  "82"  "83"  "84" 
 [85] "85"  "86"  "87"  "88"  "89"  "90"  "91"  "92"  "93"  "94"  "95"  "96" 
 [97] "97"  "98"  "99"  "100"

We can add row names as we read in the file with the row.names parameter in read.csv.
In the following example, we choose the first column ID to become the vector of row names of the data frame, with row.names = 1.

dat2 <- read.csv(file = 'data/sample.csv', header = TRUE, stringsAsFactors = FALSE, row.names=1)
rownames(dat2)
  [1] "Sub001" "Sub002" "Sub003" "Sub004" "Sub005" "Sub006" "Sub007" "Sub008"
  [9] "Sub009" "Sub010" "Sub011" "Sub012" "Sub013" "Sub014" "Sub015" "Sub016"
 [17] "Sub017" "Sub018" "Sub019" "Sub020" "Sub021" "Sub022" "Sub023" "Sub024"
 [25] "Sub025" "Sub026" "Sub027" "Sub028" "Sub029" "Sub030" "Sub031" "Sub032"
 [33] "Sub033" "Sub034" "Sub035" "Sub036" "Sub037" "Sub038" "Sub039" "Sub040"
 [41] "Sub041" "Sub042" "Sub043" "Sub044" "Sub045" "Sub046" "Sub047" "Sub048"
 [49] "Sub049" "Sub050" "Sub051" "Sub052" "Sub053" "Sub054" "Sub055" "Sub056"
 [57] "Sub057" "Sub058" "Sub059" "Sub060" "Sub061" "Sub062" "Sub063" "Sub064"
 [65] "Sub065" "Sub066" "Sub067" "Sub068" "Sub069" "Sub070" "Sub071" "Sub072"
 [73] "Sub073" "Sub074" "Sub075" "Sub076" "Sub077" "Sub078" "Sub079" "Sub080"
 [81] "Sub081" "Sub082" "Sub083" "Sub084" "Sub085" "Sub086" "Sub087" "Sub088"
 [89] "Sub089" "Sub090" "Sub091" "Sub092" "Sub093" "Sub094" "Sub095" "Sub096"
 [97] "Sub097" "Sub098" "Sub099" "Sub100"

We can now extract one or more rows using those row names:

dat2["Sub072", ]
       Gender   Group BloodPressure  Age Aneurisms_q1 Aneurisms_q2 Aneurisms_q3
Sub072      m Control           116 17.4          118          165          220
       Aneurisms_q4
Sub072          227
dat2[c("Sub009", "Sub072"), ]
       Gender      Group BloodPressure  Age Aneurisms_q1 Aneurisms_q2
Sub009      m Treatment2           131 19.4          117          215
Sub072      m    Control           116 17.4          118          165
       Aneurisms_q3 Aneurisms_q4
Sub009          181          272
Sub072          220          227

Note that row names must be unique!
For example, if we try and read in the data setting the Group column as row names, R will throw an error because values in that column are duplicated:

dat2 <- read.csv(file = 'data/sample.csv', header = TRUE, stringsAsFactors = FALSE, row.names=3)
Error in read.table(file = file, header = header, sep = sep, quote = quote, : duplicate 'row.names' are not allowed

Addressing by Logical Vector

A logical vector contains only the special values TRUE and FALSE.

c(TRUE, TRUE, FALSE, FALSE, TRUE)
[1]  TRUE  TRUE FALSE FALSE  TRUE

Truth and Its Opposite

Note the values TRUE and FALSE are all capital letters and are not quoted.

Logical vectors can be created using relational operators e.g. <, >, ==, !=, %in%.

x <- c(1, 2, 3, 11, 12, 13)
x < 10
[1]  TRUE  TRUE  TRUE FALSE FALSE FALSE
x %in% 1:10
[1]  TRUE  TRUE  TRUE FALSE FALSE FALSE

We can use logical vectors to select data from a data frame. This is often referred to as logical indexing.

index <- dat$Group == 'Control'
dat[index,]$BloodPressure
 [1] 132 173 129  77 158  81 137 111 135 108 133 139 126 125  99 122 155 133  94
[20]  98  74 116  97 104 117  90 150 116 108 102

Often this operation is written as one line of code:

plot(dat[dat$Group == 'Control', ]$BloodPressure)

plot of chunk logical_vectors_indexing2

Using Logical Indexes

  1. Create a scatterplot showing BloodPressure for subjects not in the control group.
  2. How many ways are there to index this set of subjects?

Solution

  1. The code for such a plot:

     plot(dat[dat$Group != 'Control', ]$BloodPressure)
    

    plot of chunk plot-logical

  2. In addition to dat$Group != 'Control', one could use dat$Group %in% c("Treatment1", "Treatment2").

Combining Addressing and Assignment

The assignment operator <- can be combined with addressing.

x <- c(1, 2, 3, 11, 12, 13)
x[x < 10] <- 0
x
[1]  0  0  0 11 12 13

Updating a Subset of Values

In this dataset, values for Gender have been recorded as both uppercase M, F and lowercase m, f. Combine the addressing and assignment operations to convert all values to lowercase.

Solution

dat[dat$Gender == 'M', ]$Gender <- 'm'
dat[dat$Gender == 'F', ]$Gender <- 'f'

Key Points

  • Data in data frames can be addressed by index (subsetting), by logical vector, or by name (columns only).

  • Use the $ operator to address a column by name.


Reading and Writing CSV Files

Overview

Teaching: 30 min
Exercises: 0 min
Questions
  • How do I read data from a CSV file into R?

  • How do I write data to a CSV file?

Objectives
  • Read in a .csv, and explore the arguments of the csv reader.

  • Write the altered data set to a new .csv, and explore the arguments.

The most common way that scientists store data is in Excel spreadsheets. While there are R packages designed to access data from Excel spreadsheets (e.g., gdata, RODBC, XLConnect, xlsx, RExcel), users often find it easier to save their spreadsheets in comma-separated values files (CSV) and then use R’s built in functionality to read and manipulate the data. In this short lesson, we’ll learn how to read data from a .csv and write to a new .csv, and explore the arguments that allow you read and write the data correctly for your needs.

Read a .csv and Explore the Arguments

Let’s start by opening a .csv file containing information on the speeds at which cars of different colors were clocked in 45 mph zones in the four-corners states (CarSpeeds.csv). We will use the built in read.csv(...) function call, which reads the data in as a data frame, and assign the data frame to a variable (using <-) so that it is stored in R’s memory. Then we will explore some of the basic arguments that can be supplied to the function. First, open the RStudio project containing the scripts and data you were working on in episode ‘Analyzing Patient Data’.

# Import the data and look at the first six rows
carSpeeds <- read.csv(file = 'data/car-speeds.csv')
head(carSpeeds)
  Color Speed     State
1  Blue    32 NewMexico
2   Red    45   Arizona
3  Blue    35  Colorado
4 White    34   Arizona
5   Red    25   Arizona
6  Blue    41   Arizona

Changing Delimiters

The default delimiter of the read.csv() function is a comma, but you can use other delimiters by supplying the ‘sep’ argument to the function (e.g., typing sep = ';' allows a semi-colon separated file to be correctly imported - see ?read.csv() for more information on this and other options for working with different file types).

The call above will import the data, but we have not taken advantage of several handy arguments that can be helpful in loading the data in the format we want. Let’s explore some of these arguments.

The header Argument

The default for read.csv(...) is to set the header argument to TRUE. This means that the first row of values in the .csv is set as header information (column names). If your data set does not have a header, set the header argument to FALSE:

# The first row of the data without setting the header argument:
carSpeeds[1, ]
  Color Speed     State
1  Blue    32 NewMexico
# The first row of the data if the header argument is set to FALSE:
carSpeeds <- read.csv(file = 'data/car-speeds.csv', header = FALSE)

carSpeeds[1, ]
     V1    V2    V3
1 Color Speed State

Clearly this is not the desired behavior for this data set, but it may be useful if you have a dataset without headers.

The stringsAsFactors Argument

In older versions of R (prior to 4.0) this was perhaps the most important argument in read.csv(), particularly if you were working with categorical data. This is because the default behavior of R was to convert character strings into factors, which may make it difficult to do such things as replace values. It is important to be aware of this behaviour, which we will demonstrate. For example, let’s say we find out that the data collector was color blind, and accidentally recorded green cars as being blue. In order to correct the data set, let’s replace ‘Blue’ with ‘Green’ in the $Color column:

# Here we will use R's `ifelse` function, in which we provide the test phrase,
# the outcome if the result of the test is 'TRUE', and the outcome if the
# result is 'FALSE'. We will also assign the results to the Color column,
# using '<-'

# First - reload the data with a header
carSpeeds <- read.csv(file = 'data/car-speeds.csv', stringsAsFactors = TRUE)

carSpeeds$Color <- ifelse(carSpeeds$Color == 'Blue', 'Green', carSpeeds$Color)
carSpeeds$Color
  [1] "Green" "1"     "Green" "5"     "4"     "Green" "Green" "2"     "5"    
 [10] "4"     "4"     "5"     "Green" "Green" "2"     "4"     "Green" "Green"
 [19] "5"     "Green" "Green" "Green" "4"     "Green" "4"     "4"     "4"    
 [28] "4"     "5"     "Green" "4"     "5"     "2"     "4"     "2"     "2"    
 [37] "Green" "4"     "2"     "4"     "2"     "2"     "4"     "4"     "5"    
 [46] "2"     "Green" "4"     "4"     "2"     "2"     "4"     "5"     "4"    
 [55] "Green" "Green" "2"     "Green" "5"     "2"     "4"     "Green" "Green"
 [64] "5"     "2"     "4"     "4"     "2"     "Green" "5"     "Green" "4"    
 [73] "5"     "5"     "Green" "Green" "Green" "Green" "Green" "5"     "2"    
 [82] "Green" "5"     "2"     "2"     "4"     "4"     "5"     "5"     "5"    
 [91] "5"     "4"     "4"     "4"     "5"     "2"     "5"     "2"     "2"    
[100] "5"    

What happened?!? It looks like ‘Blue’ was replaced with ‘Green’, but every other color was turned into a number (as a character string, given the quote marks before and after). This is because the colors of the cars were loaded as factors, and the factor level was reported following replacement.

To see the internal structure, we can use another function, str(). In this case, the dataframe’s internal structure includes the format of each column, which is what we are interested in. str() will be reviewed a little more in the lesson Data Types and Structures.

str(carSpeeds)
'data.frame':	100 obs. of  3 variables:
 $ Color: chr  "Green" "1" "Green" "5" ...
 $ Speed: int  32 45 35 34 25 41 34 29 31 26 ...
 $ State: Factor w/ 4 levels "Arizona","Colorado",..: 3 1 2 1 1 1 3 2 1 2 ...

We can see that the $Color and $State columns are factors and $Speed is a numeric column.

Now, let’s load the dataset using stringsAsFactors=FALSE, and see what happens when we try to replace ‘Blue’ with ‘Green’ in the $Color column:

carSpeeds <- read.csv(file = 'data/car-speeds.csv', stringsAsFactors = FALSE)
str(carSpeeds)
'data.frame':	100 obs. of  3 variables:
 $ Color: chr  "Blue" " Red" "Blue" "White" ...
 $ Speed: int  32 45 35 34 25 41 34 29 31 26 ...
 $ State: chr  "NewMexico" "Arizona" "Colorado" "Arizona" ...
carSpeeds$Color <- ifelse(carSpeeds$Color == 'Blue', 'Green', carSpeeds$Color)
carSpeeds$Color
  [1] "Green" " Red"  "Green" "White" "Red"   "Green" "Green" "Black" "White"
 [10] "Red"   "Red"   "White" "Green" "Green" "Black" "Red"   "Green" "Green"
 [19] "White" "Green" "Green" "Green" "Red"   "Green" "Red"   "Red"   "Red"  
 [28] "Red"   "White" "Green" "Red"   "White" "Black" "Red"   "Black" "Black"
 [37] "Green" "Red"   "Black" "Red"   "Black" "Black" "Red"   "Red"   "White"
 [46] "Black" "Green" "Red"   "Red"   "Black" "Black" "Red"   "White" "Red"  
 [55] "Green" "Green" "Black" "Green" "White" "Black" "Red"   "Green" "Green"
 [64] "White" "Black" "Red"   "Red"   "Black" "Green" "White" "Green" "Red"  
 [73] "White" "White" "Green" "Green" "Green" "Green" "Green" "White" "Black"
 [82] "Green" "White" "Black" "Black" "Red"   "Red"   "White" "White" "White"
 [91] "White" "Red"   "Red"   "Red"   "White" "Black" "White" "Black" "Black"
[100] "White"

That’s better! And we can see how the data now is read as character instead of factor. From R version 4.0 onwards we do not have to specify stringsAsFactors=FALSE, this is the default behavior.

The as.is Argument

This is an extension of the stringsAsFactors argument, but gives you control over individual columns. For example, if we want the colors of cars imported as strings, but we want the names of the states imported as factors, we would load the data set as:

carSpeeds <- read.csv(file = 'data/car-speeds.csv', as.is = 1)

# Note, the 1 applies as.is to the first column only

Now we can see that if we try to replace ‘Blue’ with ‘Green’ in the $Color column everything looks fine, while trying to replace ‘Arizona’ with ‘Ohio’ in the $State column returns the factor numbers for the names of states that we haven’t replaced:

str(carSpeeds)
'data.frame':	100 obs. of  3 variables:
 $ Color: chr  "Blue" " Red" "Blue" "White" ...
 $ Speed: int  32 45 35 34 25 41 34 29 31 26 ...
 $ State: Factor w/ 4 levels "Arizona","Colorado",..: 3 1 2 1 1 1 3 2 1 2 ...
carSpeeds$Color <- ifelse(carSpeeds$Color == 'Blue', 'Green', carSpeeds$Color)
carSpeeds$Color
  [1] "Green" " Red"  "Green" "White" "Red"   "Green" "Green" "Black" "White"
 [10] "Red"   "Red"   "White" "Green" "Green" "Black" "Red"   "Green" "Green"
 [19] "White" "Green" "Green" "Green" "Red"   "Green" "Red"   "Red"   "Red"  
 [28] "Red"   "White" "Green" "Red"   "White" "Black" "Red"   "Black" "Black"
 [37] "Green" "Red"   "Black" "Red"   "Black" "Black" "Red"   "Red"   "White"
 [46] "Black" "Green" "Red"   "Red"   "Black" "Black" "Red"   "White" "Red"  
 [55] "Green" "Green" "Black" "Green" "White" "Black" "Red"   "Green" "Green"
 [64] "White" "Black" "Red"   "Red"   "Black" "Green" "White" "Green" "Red"  
 [73] "White" "White" "Green" "Green" "Green" "Green" "Green" "White" "Black"
 [82] "Green" "White" "Black" "Black" "Red"   "Red"   "White" "White" "White"
 [91] "White" "Red"   "Red"   "Red"   "White" "Black" "White" "Black" "Black"
[100] "White"
carSpeeds$State <- ifelse(carSpeeds$State == 'Arizona', 'Ohio', carSpeeds$State)
carSpeeds$State
  [1] "3"    "Ohio" "2"    "Ohio" "Ohio" "Ohio" "3"    "2"    "Ohio" "2"   
 [11] "4"    "4"    "4"    "4"    "4"    "3"    "Ohio" "3"    "Ohio" "4"   
 [21] "4"    "4"    "3"    "2"    "2"    "3"    "2"    "4"    "2"    "4"   
 [31] "3"    "2"    "2"    "4"    "2"    "2"    "3"    "Ohio" "4"    "2"   
 [41] "2"    "3"    "Ohio" "4"    "Ohio" "2"    "3"    "3"    "3"    "2"   
 [51] "Ohio" "4"    "4"    "Ohio" "3"    "2"    "4"    "2"    "4"    "4"   
 [61] "4"    "2"    "3"    "2"    "3"    "2"    "3"    "Ohio" "3"    "4"   
 [71] "4"    "2"    "Ohio" "4"    "2"    "2"    "2"    "Ohio" "3"    "Ohio"
 [81] "4"    "2"    "2"    "Ohio" "Ohio" "Ohio" "4"    "Ohio" "4"    "4"   
 [91] "4"    "Ohio" "Ohio" "3"    "2"    "2"    "4"    "3"    "Ohio" "4"   

We can see that $Color column is a character while $State is a factor.

Updating Values in a Factor

Suppose we want to keep the colors of cars as factors for some other operations we want to perform. Write code for replacing ‘Blue’ with ‘Green’ in the $Color column of the cars dataset without importing the data with stringsAsFactors=FALSE.

Solution

carSpeeds <- read.csv(file = 'data/car-speeds.csv')
# Replace 'Blue' with 'Green' in cars$Color without using the stringsAsFactors
# or as.is arguments
carSpeeds$Color <- ifelse(as.character(carSpeeds$Color) == 'Blue',
                         'Green',
                         as.character(carSpeeds$Color))
# Convert colors back to factors
carSpeeds$Color <- as.factor(carSpeeds$Color)

The strip.white Argument

It is not uncommon for mistakes to have been made when the data were recorded, for example a space (whitespace) may have been inserted before a data value. By default this whitespace will be kept in the R environment, such that ‘\ Red’ will be recognized as a different value than ‘Red’. In order to avoid this type of error, use the strip.white argument. Let’s see how this works by checking for the unique values in the $Color column of our dataset:

Here, the data recorder added a space before the color of the car in one of the cells:

# We use the built-in unique() function to extract the unique colors in our dataset
unique(carSpeeds$Color)
[1] Green  Red  White Red   Black
Levels:  Red Black Green Red White

Oops, we see two values for red cars.

Let’s try again, this time importing the data using the strip.white argument. NOTE - this argument must be accompanied by the sep argument, by which we indicate the type of delimiter in the file (the comma for most .csv files)

carSpeeds <- read.csv(
  file = 'data/car-speeds.csv',
  stringsAsFactors = FALSE, 
  strip.white = TRUE,
  sep = ','
  )

unique(carSpeeds$Color)
[1] "Blue"  "Red"   "White" "Black"

That’s better!

Specify Missing Data When Loading

It is common for data sets to have missing values, or mistakes. The convention for recording missing values often depends on the individual who collected the data and can be recorded as n.a., --, or empty cells “ “. R recognises the reserved character string NA as a missing value, but not some of the examples above. Let’s say the inflamation scale in the data set we used earlier inflammation-01.csv actually starts at 1 for no inflamation and the zero values (0) were a missed observation. Looking at the ?read.csv help page is there an argument we could use to ensure all zeros (0) are read in as NA? Perhaps, in the car-speeds.csv data contains mistakes and the person measuring the car speeds could not accurately distinguish between “Black or “Blue” cars. Is there a way to specify more than one ‘string’, such as “Black” and “Blue”, to be replaced by NA

Solution

read.csv(file = "data/inflammation-01.csv", na.strings = "0")

or , in car-speeds.csv use a character vector for multiple values.

read.csv(
  file = 'data/car-speeds.csv',
  na.strings = c("Black", "Blue")
)

Write a New .csv and Explore the Arguments

After altering our cars dataset by replacing ‘Blue’ with ‘Green’ in the $Color column, we now want to save the output. There are several arguments for the write.csv(...) function call, a few of which are particularly important for how the data are exported. Let’s explore these now.

# Export the data. The write.csv() function requires a minimum of two
# arguments, the data to be saved and the name of the output file.

write.csv(carSpeeds, file = 'data/car-speeds-cleaned.csv')

If you open the file, you’ll see that it has header names, because the data had headers within R, but that there are numbers in the first column.

csv written without row.names argument

The row.names Argument

This argument allows us to set the names of the rows in the output data file. R’s default for this argument is TRUE, and since it does not know what else to name the rows for the cars data set, it resorts to using row numbers. To correct this, we can set row.names to FALSE:

write.csv(carSpeeds, file = 'data/car-speeds-cleaned.csv', row.names = FALSE)

Now we see:

csv written with row.names argument

Setting Column Names

There is also a col.names argument, which can be used to set the column names for a data set without headers. If the data set already has headers (e.g., we used the headers = TRUE argument when importing the data) then a col.names argument will be ignored.

The na Argument

There are times when we want to specify certain values for NAs in the data set (e.g., we are going to pass the data to a program that only accepts -9999 as a nodata value). In this case, we want to set the NA value of our output file to the desired value, using the na argument. Let’s see how this works:

# First, replace the speed in the 3rd row with NA, by using an index (square
# brackets to indicate the position of the value we want to replace)
carSpeeds$Speed[3] <- NA
head(carSpeeds)
  Color Speed     State
1  Blue    32 NewMexico
2   Red    45   Arizona
3  Blue    NA  Colorado
4 White    34   Arizona
5   Red    25   Arizona
6  Blue    41   Arizona
write.csv(carSpeeds, file = 'data/car-speeds-cleaned.csv', row.names = FALSE)

Now we’ll set NA to -9999 when we write the new .csv file:

# Note - the na argument requires a string input
write.csv(carSpeeds,
          file = 'data/car-speeds-cleaned.csv',
          row.names = FALSE,
          na = '-9999')

And we see:

csv written with -9999 as NA

Key Points

  • Import data from a .csv file using the read.csv(...) function.

  • Understand some of the key arguments available for importing the data properly, including header, stringsAsFactors, as.is, and strip.white.

  • Write data to a new .csv file using the write.csv(...) function

  • Understand some of the key arguments available for exporting the data properly, such as row.names, col.names, and na.


Understanding Factors

Overview

Teaching: 20 min
Exercises: 0 min
Questions
  • How is categorical data represented in R?

  • How do I work with factors?

Objectives
  • Understand how to represent categorical data in R.

  • Know the difference between ordered and unordered factors.

  • Be aware of some of the problems encountered when using factors.

Factors are used to represent categorical data. Factors can be ordered or unordered and are an important class for statistical analysis and for plotting.

Factors are stored as integers, and have labels associated with these unique integers. While factors look (and often behave) like character vectors, they are actually integers under the hood, and you need to be careful when treating them like strings.

Once created, factors can only contain a pre-defined set values, known as levels. By default, R always sorts levels in alphabetical order. For instance, if you have a factor with 2 levels:

The factor() Command

The factor() command is used to create and modify factors in R:

sex <- factor(c("male", "female", "female", "male"))

R will assign 1 to the level "female" and 2 to the level "male" (because f comes before m, even though the first element in this vector is "male"). You can check this by using the function levels(), and check the number of levels using nlevels():

levels(sex)
[1] "female" "male"  
nlevels(sex)
[1] 2

Sometimes, the order of the factors does not matter, other times you might want to specify the order because it is meaningful (e.g., “low”, “medium”, “high”) or it is required by particular type of analysis. Additionally, specifying the order of the levels allows us to compare levels:

food <- factor(c("low", "high", "medium", "high", "low", "medium", "high"))
levels(food)
[1] "high"   "low"    "medium"
food <- factor(food, levels = c("low", "medium", "high"))
levels(food)
[1] "low"    "medium" "high"  
min(food) # doesn't work
Error in Summary.factor(structure(c(1L, 3L, 2L, 3L, 1L, 2L, 3L), .Label = c("low", : 'min' not meaningful for factors
food <- factor(food, levels = c("low", "medium", "high"), ordered = TRUE)
levels(food)
[1] "low"    "medium" "high"  
min(food) # works!
[1] low
Levels: low < medium < high

In R’s memory, these factors are represented by numbers (1, 2, 3). They are better than using simple integer labels because factors are self describing: "low", "medium", and "high"” is more descriptive than 1, 2, 3. Which is low? You wouldn’t be able to tell with just integer data. Factors have this information built in. It is particularly helpful when there are many levels (like the subjects in our example data set).

Representing Data in R

You have a vector representing levels of exercise undertaken by 5 subjects

“l”, “n”, “n”, “i”, “l” ; n=none, l=light, i=intense

What is the best way to represent this in R?

a) exercise <- c(“l”, “n”, “n”, “i”, “l”)

b) exercise <- factor(c(“l”, “n”, “n”, “i”, “l”), ordered = TRUE)

c) exercise < -factor(c(“l”, “n”, “n”, “i”, “l”), levels = c(“n”, “l”, “i”), ordered = FALSE)

d) exercise <- factor(c(“l”, “n”, “n”, “i”, “l”), levels = c(“n”, “l”, “i”), ordered = TRUE)

Solution

Correct solution is d)

exercise <- factor(c("l", "n", "n", "i", "l"), levels = c("n", "l", "i"), ordered = TRUE)

We only expect three cathegories (“n”, “l”, “i”). We can order these from least intense to most intense, so let’s use ordered.

Converting Factors

Converting from a factor to a number can cause problems:

f <- factor(c(3.4, 1.2, 5))
as.numeric(f)
[1] 2 1 3

This does not behave as expected (and there is no warning).

The recommended way is to use the integer vector to index the factor levels:

levels(f)[f]
[1] "3.4" "1.2" "5"  

This returns a character vector, the as.numeric() function is still required to convert the values to the proper type (numeric).

f <- levels(f)[f]
f <- as.numeric(f)

Using Factors

Lets load our example data to see the use of factors:

dat <- read.csv(file = 'data/sample.csv', stringsAsFactors = TRUE)

Default Behavior

stringsAsFactors = TRUE was the default behavior for R prior to version 4.0. We are using it here to override the default behaviour for R version 4.0 which is stringsAsFactors = FALSE. It is included here for clarity.

str(dat)
'data.frame':	100 obs. of  9 variables:
 $ ID           : Factor w/ 100 levels "Sub001","Sub002",..: 1 2 3 4 5 6 7 8 9 10 ...
 $ Gender       : Factor w/ 4 levels "f","F","m","M": 3 3 3 1 3 4 1 3 3 1 ...
 $ Group        : Factor w/ 3 levels "Control","Treatment1",..: 1 3 3 2 2 3 1 3 3 1 ...
 $ BloodPressure: int  132 139 130 105 125 112 173 108 131 129 ...
 $ Age          : num  16 17.2 19.5 15.7 19.9 14.3 17.7 19.8 19.4 18.8 ...
 $ Aneurisms_q1 : int  114 148 196 199 188 260 135 216 117 188 ...
 $ Aneurisms_q2 : int  140 209 251 140 120 266 98 238 215 144 ...
 $ Aneurisms_q3 : int  202 248 122 233 222 320 154 279 181 192 ...
 $ Aneurisms_q4 : int  237 248 177 220 228 294 245 251 272 185 ...

Notice the first 3 columns have been converted to factors. These values were text in the data file so R automatically interpreted them as categorical variables.

summary(dat)
       ID     Gender        Group    BloodPressure        Age       
 Sub001 : 1   f:35   Control   :30   Min.   : 62.0   Min.   :12.10  
 Sub002 : 1   F: 4   Treatment1:35   1st Qu.:107.5   1st Qu.:14.78  
 Sub003 : 1   m:46   Treatment2:35   Median :117.5   Median :16.65  
 Sub004 : 1   M:15                   Mean   :118.6   Mean   :16.42  
 Sub005 : 1                          3rd Qu.:133.0   3rd Qu.:18.30  
 Sub006 : 1                          Max.   :173.0   Max.   :20.00  
 (Other):94                                                         
  Aneurisms_q1    Aneurisms_q2    Aneurisms_q3    Aneurisms_q4  
 Min.   : 65.0   Min.   : 80.0   Min.   :105.0   Min.   :116.0  
 1st Qu.:118.0   1st Qu.:131.5   1st Qu.:182.5   1st Qu.:186.8  
 Median :158.0   Median :162.5   Median :217.0   Median :219.0  
 Mean   :158.8   Mean   :168.0   Mean   :219.8   Mean   :217.9  
 3rd Qu.:188.0   3rd Qu.:196.8   3rd Qu.:248.2   3rd Qu.:244.2  
 Max.   :260.0   Max.   :283.0   Max.   :323.0   Max.   :315.0  
                                                                

Notice the summary() function handles factors differently to numbers (and strings), the occurrence counts for each value is often more useful information.

The summary() Function

The summary() function is a great way of spotting errors in your data (look at the dat$Gender column). It’s also a great way for spotting missing data.

Reordering Factors

The function table() tabulates observations and can be used to create bar plots quickly. For instance:

table(dat$Group)

   Control Treatment1 Treatment2 
        30         35         35 
barplot(table(dat$Group))

plot of chunk reordering-factors Use the factor() command to modify the column dat$Group so that the control group is plotted last.

Solution

dat$Group <- factor(dat$Group, levels = c("Treatment1", "Treatment2", "Control"))
barplot(table(dat$Group))

plot of chunk reordering-factors-2

Removing Levels from a Factor

Some of the Gender values in our dataset have been coded incorrectly. Let’s remove levels from this factor.

barplot(table(dat$Gender))

plot of chunk gender-counts

Values should have been recorded as lowercase ‘m’ and ‘f’. We should correct this.

dat$Gender[dat$Gender == 'M'] <- 'm'

Updating Factors

plot(x = dat$Gender, y = dat$BloodPressure)

plot of chunk updating-factors

Why does this plot show 4 levels?

Hint how many levels does dat$Gender have?

Solution

dat$Gender has 4 levels, so the plot shows 4 levels.

We need to tell R that “M” is no longer a valid value for this column. We use the droplevels() function to remove extra levels.

dat$Gender <- droplevels(dat$Gender)
plot(x = dat$Gender, y = dat$BloodPressure)

plot of chunk dropping-levels

Adjusting Factor Levels

Adjusting the levels() of a factor provides a useful shortcut for reassigning values in this case.

levels(dat$Gender)[2] <- 'f'
plot(x = dat$Gender, y = dat$BloodPressure)

plot of chunk adjusting-levels

Key Points

  • Factors are used to represent categorical data.

  • Factors can be ordered or unordered.

  • Some R functions have special methods for handling factors.


Data Types and Structures

Overview

Teaching: 45 min
Exercises: 0 min
Questions
  • What are the different data types in R?

  • What are the different data structures in R?

  • How do I access data within the various data structures?

Objectives
  • Expose learners to the different data types in R and show how these data types are used in data structures.

  • Learn how to create vectors of different types.

  • Be able to check the type of vector.

  • Learn about missing data and other special values.

  • Get familiar with the different data structures (lists, matrices, data frames).

Understanding Basic Data Types and Data Structures in R

To make the best of the R language, you’ll need a strong understanding of the basic data types and data structures and how to operate on them.

Data structures are very important to understand because these are the objects you will manipulate on a day-to-day basis in R. Dealing with object conversions is one of the most common sources of frustration for beginners.

Everything in R is an object.

R has 6 basic data types. (In addition to the five listed below, there is also raw which will not be discussed in this workshop.)

Elements of these data types may be combined to form data structures, such as atomic vectors. When we call a vector atomic, we mean that the vector only holds data of a single data type. Below are examples of atomic character vectors, numeric vectors, integer vectors, etc.

R provides many functions to examine features of vectors and other objects, for example

# Example
x <- "dataset"
typeof(x)
[1] "character"
attributes(x)
NULL
y <- 1:10
y
 [1]  1  2  3  4  5  6  7  8  9 10
typeof(y)
[1] "integer"
length(y)
[1] 10
z <- as.numeric(y)
z
 [1]  1  2  3  4  5  6  7  8  9 10
typeof(z)
[1] "double"

R has many data structures. These include

Vectors

A vector is the most common and basic data structure in R and is pretty much the workhorse of R. Technically, vectors can be one of two types:

although the term “vector” most commonly refers to the atomic types not to lists.

The Different Vector Modes

A vector is a collection of elements that are most commonly of mode character, logical, integer or numeric.

You can create an empty vector with vector(). (By default the mode is logical. You can be more explicit as shown in the examples below.) It is more common to use direct constructors such as character(), numeric(), etc.

vector() # an empty 'logical' (the default) vector
logical(0)
vector("character", length = 5) # a vector of mode 'character' with 5 elements
[1] "" "" "" "" ""
character(5) # the same thing, but using the constructor directly
[1] "" "" "" "" ""
numeric(5)   # a numeric vector with 5 elements
[1] 0 0 0 0 0
logical(5)   # a logical vector with 5 elements
[1] FALSE FALSE FALSE FALSE FALSE

You can also create vectors by directly specifying their content. R will then guess the appropriate mode of storage for the vector. For instance:

x <- c(1, 2, 3)

will create a vector x of mode numeric. These are the most common kind, and are treated as double precision real numbers. If you wanted to explicitly create integers, you need to add an L to each element (or coerce to the integer type using as.integer()).

x1 <- c(1L, 2L, 3L)

Using TRUE and FALSE will create a vector of mode logical:

y <- c(TRUE, TRUE, FALSE, FALSE)

While using quoted text will create a vector of mode character:

z <- c("Sarah", "Tracy", "Jon")

Examining Vectors

The functions typeof(), length(), class() and str() provide useful information about your vectors and R objects in general.

typeof(z)
[1] "character"
length(z)
[1] 3
class(z)
[1] "character"
str(z)
 chr [1:3] "Sarah" "Tracy" "Jon"

Adding Elements

The function c() (for combine) can also be used to add elements to a vector.

z <- c(z, "Annette")
z
[1] "Sarah"   "Tracy"   "Jon"     "Annette"
z <- c("Greg", z)
z
[1] "Greg"    "Sarah"   "Tracy"   "Jon"     "Annette"

Vectors from a Sequence of Numbers

You can create vectors as a sequence of numbers.

series <- 1:10
seq(10)
 [1]  1  2  3  4  5  6  7  8  9 10
seq(from = 1, to = 10, by = 0.1)
 [1]  1.0  1.1  1.2  1.3  1.4  1.5  1.6  1.7  1.8  1.9  2.0  2.1  2.2  2.3  2.4
[16]  2.5  2.6  2.7  2.8  2.9  3.0  3.1  3.2  3.3  3.4  3.5  3.6  3.7  3.8  3.9
[31]  4.0  4.1  4.2  4.3  4.4  4.5  4.6  4.7  4.8  4.9  5.0  5.1  5.2  5.3  5.4
[46]  5.5  5.6  5.7  5.8  5.9  6.0  6.1  6.2  6.3  6.4  6.5  6.6  6.7  6.8  6.9
[61]  7.0  7.1  7.2  7.3  7.4  7.5  7.6  7.7  7.8  7.9  8.0  8.1  8.2  8.3  8.4
[76]  8.5  8.6  8.7  8.8  8.9  9.0  9.1  9.2  9.3  9.4  9.5  9.6  9.7  9.8  9.9
[91] 10.0

Missing Data

R supports missing data in vectors. They are represented as NA (Not Available) and can be used for all the vector types covered in this lesson:

x <- c(0.5, NA, 0.7)
x <- c(TRUE, FALSE, NA)
x <- c("a", NA, "c", "d", "e")
x <- c(1+5i, 2-3i, NA)

The function is.na() indicates the elements of the vectors that represent missing data, and the function anyNA() returns TRUE if the vector contains any missing values:

x <- c("a", NA, "c", "d", NA)
y <- c("a", "b", "c", "d", "e")
is.na(x)
[1] FALSE  TRUE FALSE FALSE  TRUE
is.na(y)
[1] FALSE FALSE FALSE FALSE FALSE
anyNA(x)
[1] TRUE
anyNA(y)
[1] FALSE

Other Special Values

Inf is infinity. You can have either positive or negative infinity.

1/0
[1] Inf

NaN means Not a Number. It’s an undefined value.

0/0
[1] NaN

What Happens When You Mix Types Inside a Vector?

R will create a resulting vector with a mode that can most easily accommodate all the elements it contains. This conversion between modes of storage is called “coercion”. When R converts the mode of storage based on its content, it is referred to as “implicit coercion”. For instance, can you guess what the following do (without running them first)?

xx <- c(1.7, "a")
xx <- c(TRUE, 2)
xx <- c("a", TRUE)

You can also control how vectors are coerced explicitly using the as.<class_name>() functions:

as.numeric("1")
[1] 1
as.character(1:2)
[1] "1" "2"

Finding Commonalities

Do you see a property that’s common to all these vectors above?

Solution

All vectors are one-dimensional and each element is of the same type.

Objects Attributes

Objects can have attributes. Attributes are part of the object. These include:

You can also glean other attribute-like information such as length (works on vectors and lists) or number of characters (for character strings).

length(1:10)
[1] 10
nchar("Software Carpentry")
[1] 18

Matrix

In R matrices are an extension of the numeric or character vectors. They are not a separate type of object but simply an atomic vector with dimensions; the number of rows and columns. As with atomic vectors, the elements of a matrix must be of the same data type.

m <- matrix(nrow = 2, ncol = 2)
m
     [,1] [,2]
[1,]   NA   NA
[2,]   NA   NA
dim(m)
[1] 2 2

You can check that matrices are vectors with a class attribute of matrix by using class() and typeof().

m <- matrix(c(1:3))
class(m)
[1] "matrix" "array" 
typeof(m)
[1] "integer"

While class() shows that m is a matrix, typeof() shows that fundamentally the matrix is an integer vector.

Data types of matrix elements

Consider the following matrix:

FOURS <- matrix(
  c(4, 4, 4, 4),
  nrow = 2,
  ncol = 2)

Given that typeof(FOURS[1]) returns "double", what would you expect typeof(FOURS) to return? How do you know this is the case even without running this code?

Hint Can matrices be composed of elements of different data types?

Solution

We know that typeof(FOURS) will also return "double" since matrices are made of elements of the same data type. Note that you could do something like as.character(FOURS) if you needed the elements of FOURS as characters.

Matrices in R are filled column-wise.

m <- matrix(1:6, nrow = 2, ncol = 3)

Other ways to construct a matrix

m      <- 1:10
dim(m) <- c(2, 5)

This takes a vector and transforms it into a matrix with 2 rows and 5 columns.

Another way is to bind columns or rows using rbind() and cbind() (“row bind” and “column bind”, respectively).

x <- 1:3
y <- 10:12
cbind(x, y)
     x  y
[1,] 1 10
[2,] 2 11
[3,] 3 12
rbind(x, y)
  [,1] [,2] [,3]
x    1    2    3
y   10   11   12

You can also use the byrow argument to specify how the matrix is filled. From R’s own documentation:

mdat <- matrix(c(1, 2, 3, 11, 12, 13),
               nrow = 2,
               ncol = 3,
               byrow = TRUE)
mdat
     [,1] [,2] [,3]
[1,]    1    2    3
[2,]   11   12   13

Elements of a matrix can be referenced by specifying the index along each dimension (e.g. “row” and “column”) in single square brackets.

mdat[2, 3]
[1] 13

List

In R lists act as containers. Unlike atomic vectors, the contents of a list are not restricted to a single mode and can encompass any mixture of data types. Lists are sometimes called generic vectors, because the elements of a list can by of any type of R object, even lists containing further lists. This property makes them fundamentally different from atomic vectors.

A list is a special type of vector. Each element can be a different type.

Create lists using list() or coerce other objects using as.list(). An empty list of the required length can be created using vector()

x <- list(1, "a", TRUE, 1+4i)
x
[[1]]
[1] 1

[[2]]
[1] "a"

[[3]]
[1] TRUE

[[4]]
[1] 1+4i
x <- vector("list", length = 5) # empty list
length(x)
[1] 5

The content of elements of a list can be retrieved by using double square brackets.

x[[1]]
NULL

Vectors can be coerced to lists as follows:

x <- 1:10
x <- as.list(x)
length(x)
[1] 10

Examining Lists

  1. What is the class of x[1]?
  2. What is the class of x[[1]]?

Solution

  1. class(x[1])
    
    [1] "list"
    
  2. class(x[[1]])
    
    [1] "integer"
    

Elements of a list can be named (i.e. lists can have the names attribute)

xlist <- list(a = "Karthik Ram", b = 1:10, data = head(mtcars))
xlist
$a
[1] "Karthik Ram"

$b
 [1]  1  2  3  4  5  6  7  8  9 10

$data
                   mpg cyl disp  hp drat    wt  qsec vs am gear carb
Mazda RX4         21.0   6  160 110 3.90 2.620 16.46  0  1    4    4
Mazda RX4 Wag     21.0   6  160 110 3.90 2.875 17.02  0  1    4    4
Datsun 710        22.8   4  108  93 3.85 2.320 18.61  1  1    4    1
Hornet 4 Drive    21.4   6  258 110 3.08 3.215 19.44  1  0    3    1
Hornet Sportabout 18.7   8  360 175 3.15 3.440 17.02  0  0    3    2
Valiant           18.1   6  225 105 2.76 3.460 20.22  1  0    3    1
names(xlist)
[1] "a"    "b"    "data"

Examining Named Lists

  1. What is the length of this object?
  2. What is its structure?

Solution

  1. length(xlist)
    
    [1] 3
    
  2. str(xlist)
    
    List of 3
     $ a   : chr "Karthik Ram"
     $ b   : int [1:10] 1 2 3 4 5 6 7 8 9 10
     $ data:'data.frame':	6 obs. of  11 variables:
      ..$ mpg : num [1:6] 21 21 22.8 21.4 18.7 18.1
      ..$ cyl : num [1:6] 6 6 4 6 8 6
      ..$ disp: num [1:6] 160 160 108 258 360 225
      ..$ hp  : num [1:6] 110 110 93 110 175 105
      ..$ drat: num [1:6] 3.9 3.9 3.85 3.08 3.15 2.76
      ..$ wt  : num [1:6] 2.62 2.88 2.32 3.21 3.44 ...
      ..$ qsec: num [1:6] 16.5 17 18.6 19.4 17 ...
      ..$ vs  : num [1:6] 0 0 1 1 0 1
      ..$ am  : num [1:6] 1 1 1 0 0 0
      ..$ gear: num [1:6] 4 4 4 3 3 3
      ..$ carb: num [1:6] 4 4 1 1 2 1
    

Lists can be extremely useful inside functions. Because the functions in R are able to return only a single object, you can “staple” together lots of different kinds of results into a single object that a function can return.

A list does not print to the console like a vector. Instead, each element of the list starts on a new line.

Elements are indexed by double brackets. Single brackets will still return a(nother) list. If the elements of a list are named, they can be referenced by the $ notation (i.e. xlist$data).

Data Frame

A data frame is a very important data type in R. It’s pretty much the de facto data structure for most tabular data and what we use for statistics.

A data frame is a special type of list where every element of the list has same length (i.e. data frame is a “rectangular” list).

Data frames can have additional attributes such as rownames(), which can be useful for annotating data, like subject_id or sample_id. But most of the time they are not used.

Some additional information on data frames:

Creating Data Frames by Hand

To create data frames by hand:

dat <- data.frame(id = letters[1:10], x = 1:10, y = 11:20)
dat
   id  x  y
1   a  1 11
2   b  2 12
3   c  3 13
4   d  4 14
5   e  5 15
6   f  6 16
7   g  7 17
8   h  8 18
9   i  9 19
10  j 10 20

Useful Data Frame Functions

  • head() - shows first 6 rows
  • tail() - shows last 6 rows
  • dim() - returns the dimensions of data frame (i.e. number of rows and number of columns)
  • nrow() - number of rows
  • ncol() - number of columns
  • str() - structure of data frame - name, type and preview of data in each column
  • names() or colnames() - both show the names attribute for a data frame
  • sapply(dataframe, class) - shows the class of each column in the data frame

See that it is actually a special list:

is.list(dat)
[1] TRUE
class(dat)
[1] "data.frame"

Because data frames are rectangular, elements of data frame can be referenced by specifying the row and the column index in single square brackets (similar to matrix).

dat[1, 3]
[1] 11

As data frames are also lists, it is possible to refer to columns (which are elements of such list) using the list notation, i.e. either double square brackets or a $.

dat[["y"]]
 [1] 11 12 13 14 15 16 17 18 19 20
dat$y
 [1] 11 12 13 14 15 16 17 18 19 20

The following table summarizes the one-dimensional and two-dimensional data structures in R in relation to diversity of data types they can contain.

Dimensions Homogenous Heterogeneous
1-D atomic vector list
2-D matrix data frame

Lists can contain elements that are themselves muti-dimensional (e.g. a lists can contain data frames or another type of objects). Lists can also contain elements of any length, therefore list do not necessarily have to be “rectangular”. However in order for the list to qualify as a data frame, the length of each element has to be the same.

Column Types in Data Frames

Knowing that data frames are lists, can columns be of different type?

What type of structure do you expect to see when you explore the structure of the PlantGrowth data frame? Hint: Use str().

Solution

The weight column is numeric while group is a factor. Lists can have elements of different types. Since a Data Frame is just a special type of list, it can have columns of differing type (although, remember that type must be consistent within each column!).

str(PlantGrowth)
'data.frame':	30 obs. of  2 variables:
 $ weight: num  4.17 5.58 5.18 6.11 4.5 4.61 5.17 4.53 5.33 5.14 ...
 $ group : Factor w/ 3 levels "ctrl","trt1",..: 1 1 1 1 1 1 1 1 1 1 ...

Key Points

  • R’s basic data types are character, numeric, integer, complex, and logical.

  • R’s basic data structures include the vector, list, matrix, data frame, and factors. Some of these structures require that all members be of the same data type (e.g. vectors, matrices) while others permit multiple data types (e.g. lists, data frames).

  • Objects may have attributes, such as name, dimension, and class.


The Call Stack

Overview

Teaching: 15 min
Exercises: 0 min
Questions
  • What is the call stack, and how does R know what order to do things in?

  • How does scope work in R?

Objectives
  • Explain how stack frames are created and destroyed as functions are called.

  • Correctly identify the scope of a function’s local variables.

  • Explain variable scope in terms of the call stack.

The Call Stack

Let’s take a closer look at what happens when we call fahrenheit_to_kelvin(32). To make things clearer, we’ll start by putting the initial value 32 in a variable and store the final result in one as well:

original <- 32
final <- fahrenheit_to_kelvin(original)

The diagram below shows what memory looks like after the first line has been executed:

Call Stack (Initial State)

When we call fahrenheit_to_kelvin, R doesn’t create the variable temp_F right away. Instead, it creates something called a stack frame to keep track of the variables defined by fahrenheit_to_kelvin. Initially, this stack frame only holds the value of temp_F:

Call Stack Immediately After First Function Call

When we call fahrenheit_to_celsius inside fahrenheit_to_kelvin, R creates another stack frame to hold fahrenheit_to_celsius’s variables:

Call Stack During First Nested Function Call

It does this because there are now two variables in play called temp_F: the argument to fahrenheit_to_celsius, and the argument to fahrenheit_to_kelvin. Having two variables with the same name in the same part of the program would be ambiguous, so R (and every other modern programming language) creates a new stack frame for each function call to keep that function’s variables separate from those defined by other functions.

When the call to fahrenheit_to_celsius returns a value, R throws away fahrenheit_to_celsius’s stack frame and creates a new variable in the stack frame for fahrenheit_to_kelvin to hold the temperature in Celsius:

Call Stack After Return From First Nested Function Call

It then calls celsius_to_kelvin, which means it creates a stack frame to hold that function’s variables:

Call Stack During Call to Second Nested Function

Once again, R throws away that stack frame when celsius_to_kelvin is done and creates the variable temp_K in the stack frame for fahrenheit_to_kelvin:

Call Stack After Second Nested Function Returns

Finally, when fahrenheit_to_kelvin is done, R throws away its stack frame and puts its result in a new variable called final that lives in the stack frame we started with:

Call Stack After All Functions Have Finished

This final stack frame is always there; it holds the variables we defined outside the functions in our code. What it doesn’t hold is the variables that were in the various stack frames. If we try to get the value of temp_F after our functions have finished running, R tells us that there’s no such thing:

temp_F
Error in eval(expr, envir, enclos): object 'temp_F' not found

Where to Learn More

The explanation of the stack frame above was very general and the basic concept will help you understand most languages you try to program with. However, R has some unique aspects that can be exploited when performing more complicated operations. We will not be writing anything that requires knowledge of these more advanced concepts. In the future when you are comfortable writing functions in R, you can learn more by reading the R Language Manual or this chapter from Advanced R Programming by Hadley Wickham. For context, R uses the terminology “environments” instead of frames.

Why go to all this trouble? Well, here’s a function called span that calculates the difference between the minimum and maximum values in an array:

span <- function(a) {
  diff <- max(a) - min(a)
  return(diff)
}

dat <- read.csv(file = "data/inflammation-01.csv", header = FALSE)
# span of inflammation data
span(dat)
[1] 20

Notice span assigns a value to variable called diff. We might very well use a variable with the same name (diff) to hold the inflammation data:

diff <- read.csv(file = "data/inflammation-01.csv", header = FALSE)
# span of inflammation data
span(diff)
[1] 20

We don’t expect the variable diff to have the value 20 after this function call, so the name diff cannot refer to the same variable defined inside span as it does in the main body of our program (which R refers to as the global environment). And yes, we could probably choose a different name than diff for our variable in this case, but we don’t want to have to read every line of code of the R functions we call to see what variable names they use, just in case they change the values of our variables.

The big idea here is encapsulation, and it’s the key to writing correct, comprehensible programs. A function’s job is to turn several operations into one so that we can think about a single function call instead of a dozen or a hundred statements each time we want to do something. That only works if functions don’t interfere with each other; if they do, we have to pay attention to the details once again, which quickly overloads our short-term memory.

Following the Call Stack

We previously wrote functions called highlight and edges. Draw a diagram showing how the call stack changes when we run the following:

inner_vec <- "carbon"
outer_vec <- "+"
result <- edges(highlight(inner_vec, outer_vec))

Key Points

  • R keeps track of active function calls using a call stack comprised of stack frames.

  • Only global variables and variables in the current stack frame can be accessed directly.


Loops in R

Overview

Teaching: 30 min
Exercises: 0 min
Questions
  • How can I do the same thing multiple times more efficiently in R?

  • What is vectorization?

  • Should I use a loop or an apply statement?

Objectives
  • Compare loops and vectorized operations.

  • Use the apply family of functions.

In R you have multiple options when repeating calculations: vectorized operations, for loops, and apply functions.

This lesson is an extension of Analyzing Multiple Data Sets. In that lesson, we introduced how to run a custom function, analyze, over multiple data files:

analyze <- function(filename) {
  # Plots the average, min, and max inflammation over time.
  # Input is character string of a csv file.
  dat <- read.csv(file = filename, header = FALSE)
  avg_day_inflammation <- apply(dat, 2, mean)
  plot(avg_day_inflammation)
  max_day_inflammation <- apply(dat, 2, max)
  plot(max_day_inflammation)
  min_day_inflammation <- apply(dat, 2, min)
  plot(min_day_inflammation)
}
filenames <- list.files(path = "data", pattern = "inflammation-[0-9]{2}.csv", full.names = TRUE)

Vectorized Operations

A key difference between R and many other languages is a topic known as vectorization. When you wrote the total function, we mentioned that R already has sum to do this; sum is much faster than the interpreted for loop because sum is coded in C to work with a vector of numbers. Many of R’s functions work this way; the loop is hidden from you in C. Learning to use vectorized operations is a key skill in R.

For example, to add pairs of numbers contained in two vectors

a <- 1:10
b <- 1:10

You could loop over the pairs adding each in turn, but that would be very inefficient in R.

Instead of using i in a to make our loop variable, we use the function seq_along to generate indices for each element a contains.

res <- numeric(length = length(a))
for (i in seq_along(a)) {
  res[i] <- a[i] + b[i]
}
res
 [1]  2  4  6  8 10 12 14 16 18 20

Instead, + is a vectorized function which can operate on entire vectors at once

res2 <- a + b
all.equal(res, res2)
[1] TRUE

Vector Recycling

When performing vector operations in R, it is important to know about recycling. If you perform an operation on two or more vectors of unequal length, R will recycle elements of the shorter vector(s) to match the longest vector. For example:

a <- 1:10
b <- 1:5
a + b
 [1]  2  4  6  8 10  7  9 11 13 15

The elements of a and b are added together starting from the first element of both vectors. When R reaches the end of the shorter vector b, it starts again at the first element of b and continues until it reaches the last element of the longest vector a. This behaviour may seem crazy at first glance, but it is very useful when you want to perform the same operation on every element of a vector. For example, say we want to multiply every element of our vector a by 5:

a <- 1:10
b <- 5
a * b
 [1]  5 10 15 20 25 30 35 40 45 50

Remember there are no scalars in R, so b is actually a vector of length 1; in order to add its value to every element of a, it is recycled to match the length of a.

When the length of the longer object is a multiple of the shorter object length (as in our example above), the recycling occurs silently. When the longer object length is not a multiple of the shorter object length, a warning is given:

a <- 1:10
b <- 1:7
a + b
Warning in a + b: longer object length is not a multiple of shorter object
length
 [1]  2  4  6  8 10 12 14  9 11 13

for or apply?

A for loop is used to apply the same function calls to a collection of objects. R has a family of functions, the apply family, which can be used in much the same way. You’ve already used one of the family, apply in the first lesson. The apply family members include

Each of these has an argument FUN which takes a function to apply to each element of the object. Instead of looping over filenames and calling analyze, as you did earlier, you could sapply over filenames with FUN = analyze:

sapply(filenames, FUN = analyze)

Deciding whether to use for or one of the apply family is really personal preference. Using an apply family function forces to you encapsulate your operations as a function rather than separate calls with for. for loops are often more natural in some circumstances; for several related operations, a for loop will avoid you having to pass in a lot of extra arguments to your function.

Loops in R Are Slow

No, they are not! If you follow some golden rules:

  1. Don’t use a loop when a vectorized alternative exists
  2. Don’t grow objects (via c, cbind, etc) during the loop - R has to create a new object and copy across the information just to add a new element or row/column
  3. Allocate an object to hold the results and fill it in during the loop

As an example, we’ll create a new version of analyze that will return the mean inflammation per day (column) of each file.

analyze2 <- function(filenames) {
  for (f in seq_along(filenames)) {
    fdata <- read.csv(filenames[f], header = FALSE)
    res <- apply(fdata, 2, mean)
    if (f == 1) {
      out <- res
    } else {
      # The loop is slowed by this call to cbind that grows the object
      out <- cbind(out, res)
    }
  }
  return(out)
}

system.time(avg2 <- analyze2(filenames))
   user  system elapsed 
  0.027   0.000   0.027 

Note how we add a new column to out at each iteration? This is a cardinal sin of writing a for loop in R.

Instead, we can create an empty matrix with the right dimensions (rows/columns) to hold the results. Then we loop over the files but this time we fill in the fth column of our results matrix out. This time there is no copying/growing for R to deal with.

analyze3 <- function(filenames) {
  out <- matrix(ncol = length(filenames), nrow = 40) # assuming 40 here from files
  for (f in seq_along(filenames)) {
    fdata <- read.csv(filenames[f], header = FALSE)
    out[, f] <- apply(fdata, 2, mean)
  }
  return(out)
}

system.time(avg3 <- analyze3(filenames))
   user  system elapsed 
  0.024   0.000   0.025 

In this simple example there is little difference in the compute time of analyze2 and analyze3. This is because we are only iterating over 12 files and hence we only incur 12 copy/grow operations. If we were doing this over more files or the data objects we were growing were larger, the penalty for copying/growing would be much larger.

Note that apply handles these memory allocation issues for you, but then you have to write the loop part as a function to pass to apply. At its heart, apply is just a for loop with extra convenience.

Key Points

  • Where possible, use vectorized operations instead of for loops to make code faster and more concise.

  • Use functions such as apply instead of for loops to operate on the values in a data structure.