# Creating Functions

Last updated on 2023-04-26 | Edit this page

## Overview

### Questions

- How can I teach MATLAB how to do new things?

### Objectives

- Compare and contrast MATLAB function files with MATLAB scripts.
- Define a function that takes arguments.
- Test a function.
- Recognize 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 future. In this lesson, we’ll learn how to write a function so that we can repeat several operations with a single command.

Let’s start by defining a function `fahr_to_kelvin`

that
converts temperatures from Fahrenheit to Kelvin:

### MATLAB

```
function ktemp = fahr_to_kelvin(ftemp)
%FAHR_TO_KELVIN Convert Fahrenheit to Kelvin
ktemp = ((ftemp - 32) * (5/9)) + 273.15;
end
```

A MATLAB function *must* be saved in a text file with a
`.m`

extension. The name of that file must be the same as the
function defined inside it. The name must start with a letter and cannot
contain spaces. So, you will need to save the above code in a file
called `fahr_to_kelvin.m`

. Remember to save your m-files in
the current directory.

The first line of our function is called the *function
definition*, and it declares that we’re writing a function named
`fahr_to_kelvin`

, that has a single input
parameter,`ftemp`

, and a single output parameter,
`ktemp`

. Anything following the function definition line is
called the *body* of the function. The keyword `end`

marks the end of the function body, and the function won’t know about
any code after `end`

.

A function can have multiple input and output parameters if required, but isn’t required to have any of either. The general form of a function is shown in the pseudo-code below:

### MATLAB

```
function [out1, out2] = function_name(in1, in2)
%FUNCTION_NAME Function description
% This section below is called the body of the function
out1 = something calculated;
out2 = something else;
end
```

Just as we saw with scripts, functions must be *visible* to
MATLAB, i.e., a file containing a function has to be placed in a
directory that MATLAB knows about. The most convenient of those
directories is the current working directory.

We can call our function from the command line like any other MATLAB function:

### OUTPUT

`ans = 273.15`

When we pass a value, like `32`

, to the function, the
value is assigned to the variable `ftemp`

so that it can be
used inside the function. If we want to return a value from the
function, we must assign that value to a variable named
`ktemp`

-–in the first line of our function, we promised that
the output of our function would be named `ktemp`

.

Outside of the function, the variables `ftemp`

and
`ktemp`

aren’t visible; they are only used by the function
body to refer to the input and output values.

This is one of the major differences between scripts and functions: a script can be thought of as automating the command line, with full access to all variables in the base workspace, whereas a function can only read and write variables from the calling workspace if they are passed as arguments — i.e. a function has its own separate workspace.

Now that we’ve seen how to convert Fahrenheit to Kelvin, it’s easy to convert Kelvin to Celsius.

### MATLAB

```
function ctemp = kelvin_to_celsius(ktemp)
%KELVIN_TO_CELSIUS Convert from Kelvin to Celcius
ctemp = ktemp - 273.15;
end
```

Again, we can call this function like any other:

### OUTPUT

`ans = -273.15`

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

### MATLAB

```
function ctemp = fahr_to_celsius(ftemp)
%FAHR_TO_CELSIUS Convert Fahrenheit to Celcius
ktemp = fahr_to_kelvin(ftemp);
ctemp = kelvin_to_celsius(ktemp);
end
```

Calling this function,

we get, as expected:

### OUTPUT

`ans = 0`

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.

### OUTPUT

```
259.8167
278.1500
273.1500
0
```

`ktemp`

is 0 because the function
`fahr_to_kelvin`

has no knowledge of the variable
`ktemp`

which exists outside of the function.

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 value:

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 matrix of 0’s, and then center that around 3:

### OUTPUT

```
ans =
3 3
3 3
```

That looks right, so let’s try out `center`

function on
our real data:

It’s hard to tell from the default output whether the result is correct–this is often the case when working with fairly large arrays–but, there are a few simple tests that will reassure us.

Let’s calculate some simple statistics:

### OUTPUT

`0.00000 6.14875 20.00000`

And let’s do the same after applying our `center`

function
to the data:

### OUTPUT

` -6.1487 -0.0000 13.8513`

That seems almost right: the original mean was about 6.1, so the lower bound from zero is now about -6.1. The mean of the centered data isn’t quite zero–we’ll explore why not in the challenges–but it’s pretty close. We can even go further and check that the standard deviation hasn’t changed:

### OUTPUT

`5.3291e-15`

The difference is very small. It’s still possible that our function is wrong, but it seems unlikely enough that we should probably get back to doing our analysis. We have one more task first, though: we should write some documentation for our function to remind ourselves later what it’s for and how to use it.

### MATLAB

```
function out = center(data, desired)
%CENTER Center data around a desired value.
%
% center(DATA, DESIRED)
%
% Returns a new array containing the values in
% DATA centered around the value.
out = (data - mean(data(:))) + desired;
end
```

Comment lines immediately below the function definition line are
called “help text”. Typing `help function_name`

brings up the
help text for that function:

### OUTPUT

```
Center Center data around a desired value.
center(DATA, DESIRED)
Returns a new array containing the values in
DATA centered around the value.
```

### Testing a Function

Write a function called

`normalise`

that takes an array as input and returns an array of the same shape with its values scaled to lie in the range 0.0 to 1.0. (If L and H are the lowest and highest values in the input array, respectively, then the function should map a value v to (v - L)/(H - L).) Be sure to give the function a comment block explaining its use.Run

`help linspace`

to see how to use`linspace`

to generate regularly-spaced values. Use arrays like this to test your`normalise`

function.

```
```
function out = normalise(in)
%NORMALISE Return original array, normalised so that the
% new values lie in the range 0 to 1.
H = max(max(in));
L = min(min(in));
out = (in-L)/(H-L);
end
```
{: .language-matlab}
```

```
```
a = linspace(1, 10); % Create evenly-spaced vector
norm_a = normalise(a); % Normalise vector
plot(a, norm_a) % Visually check normalisation
```
{: .language-matlab}
```

### Convert a script into a function

Write a function called `plot_dataset`

which plots the
three summary graphs (max, min, std) for a given inflammation data
file.

The function should operate on a single data file, and should have
two parameters: `file_name`

and `plot_switch`

.
When called, the function should create the three graphs produced in the
previous lesson. Whether they are displayed or saved to the
`results`

directory should be controlled by the value of
`plot_switch`

i.e. `plot_dataset('data/inflammation-01.csv', 0)`

should
display the corresponding graphs for the first data set;
`plot_dataset('data/inflammation-02.csv', 1)`

should save the
figures for the second dataset to the `results`

directory.

You should mostly be reusing code from the `plot_all`

script.

Be sure to give your function help text.

### MATLAB

```
function plot_dataset(file_name, plot_switch)
%PLOT_DATASET Perform analysis for named data file.
% Create figures to show average, max and min inflammation.
% Display plots in GUI using plot_switch = 0,
% or save to disk using plot_switch = 1.
%
% Example:
% plot_dataset('data/inflammation-01.csv', 0)
% Generate string for image name:
img_name = replace(file_name, '.csv', '.png');
img_name = replace(img_name, 'data', 'results');
patient_data = readmatrix(file_name);
if plot_switch == 1
figure('visible', 'off')
else
figure('visible', 'on')
end
subplot(2, 2, 1)
plot(mean(patient_data, 1))
ylabel('average')
subplot(2, 2, 2)
plot(max(patient_data, [], 1))
ylabel('max')
subplot(2, 2, 3)
plot(min(patient_data, [], 1))
ylabel('min')
if plot_switch == 1
print(img_name, '-dpng')
close()
end
end
```

### MATLAB

```
%PLOT_ALL Analyse all inflammation datasets
% Create figures to show average, max and min inflammation.
% Save figures to 'results' directory.
files = dir('data/inflammation-*.csv');
for i = 1:length(files)
file_name = files(i).name;
file_name = fullfile('data', file_name);
% Process each data set, saving figures to disk.
plot_dataset(file_name, 1);
end
```

We have now solved our original problem: we can analyze any number of data files with a single command. More importantly, we have met two of the most important ideas in programming:

Use arrays to store related values, and loops to repeat operations on them.

Use functions to make code easier to re-use and easier to understand.