Data Types and Type Conversion

Last updated on 2023-05-02 | Edit this page

Overview

Questions

  • What kinds of data do programs store?
  • How can I convert one type to another?

Objectives

  • Explain key differences between integers and floating point numbers.
  • Explain key differences between numbers and character strings.
  • Use built-in functions to convert between integers, floating point numbers, and strings.

Every value has a type.

  • Every value in a program has a specific type.
  • Integer (int): represents positive or negative whole numbers like 3 or -512.
  • Floating point number (float): represents real numbers like 3.14159 or -2.5.
  • Character string (usually called “string”, str): text.
    • Written in either single quotes or double quotes (as long as they match).
    • The quote marks aren’t printed when the string is displayed.

Use the built-in function type to find the type of a value.

  • Use the built-in function type to find out what type a value has.
  • Works on variables as well.
    • But remember: the value has the type — the variable is just a label.

PYTHON 

print(type(52))

OUTPUT 

<class 'int'>

PYTHON 

fitness = 'average'
print(type(fitness))

OUTPUT 

<class 'str'>

Types control what operations (or methods) can be performed on a given value.

  • A value’s type determines what the program can do to it.

PYTHON 

print(5 - 3)

OUTPUT 

2

PYTHON 

print('hello' - 'h')

ERROR 

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-2-67f5626a1e07> in <module>()
----> 1 print('hello' - 'h')

TypeError: unsupported operand type(s) for -: 'str' and 'str'

You can use the “+” and “*” operators on strings.

  • “Adding” character strings concatenates them.

PYTHON 

full_name = 'Ahmed' + ' ' + 'Walsh'
print(full_name)

OUTPUT 

Ahmed Walsh
  • Multiplying a character string by an integer N creates a new string that consists of that character string repeated N times.
    • Since multiplication is repeated addition.

PYTHON 

separator = '=' * 10
print(separator)

OUTPUT 

==========

Strings have a length (but numbers don’t).

  • The built-in function len counts the number of characters in a string.

PYTHON 

print(len(full_name))

OUTPUT 

11
  • But numbers don’t have a length (not even zero).

PYTHON 

print(len(52))

ERROR 

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-3-f769e8e8097d> in <module>()
----> 1 print(len(52))

TypeError: object of type 'int' has no len()

Must convert numbers to strings or vice versa when operating on them.

  • Cannot add numbers and strings.

PYTHON 

print(1 + '2')

ERROR 

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-4-fe4f54a023c6> in <module>()
----> 1 print(1 + '2')

TypeError: unsupported operand type(s) for +: 'int' and 'str'
  • Not allowed because it’s ambiguous: should 1 + '2' be 3 or '12'?
  • Some types can be converted to other types by using the type name as a function.

PYTHON 

print(1 + int('2'))
print(str(1) + '2')

OUTPUT 

3
12

Can mix integers and floats freely in operations.

  • Integers and floating-point numbers can be mixed in arithmetic.
    • Python 3 automatically converts integers to floats as needed.

PYTHON 

print('half is', 1 / 2.0)
print('three squared is', 3.0 ** 2)

OUTPUT 

half is 0.5
three squared is 9.0

Variables only change value when something is assigned to them.

  • If we make one cell in a spreadsheet depend on another, and update the latter, the former updates automatically.
  • This does not happen in programming languages.

PYTHON 

variable_one = 1
variable_two = 5 * variable_one
variable_one = 2
print('first is', variable_one, 'and second is', variable_two)

OUTPUT 

first is 2 and second is 5
  • The computer reads the value of variable_one when doing the multiplication, creates a new value, and assigns it to variable_two.
  • Afterwards, the value of variable_two is set to the new value and not dependent on variable_one so its value does not automatically change when variable_one changes.

Fractions

What type of value is 3.4? How can you find out?

It is a floating-point number (often abbreviated “float”). It is possible to find out by using the built-in function type().

PYTHON 

print(type(3.4))

OUTPUT 

<class 'float'>

Automatic Type Conversion

What type of value is 3.25 + 4?

It is a float: integers are automatically converted to floats as necessary.

PYTHON 

result = 3.25 + 4
print(result, 'is', type(result))

OUTPUT 

7.25 is <class 'float'>

Choose a Type

What type of value (integer, floating point number, or character string) would you use to represent each of the following? Try to come up with more than one good answer for each problem. For example, in # 1, when would counting days with a floating point variable make more sense than using an integer?

  1. Number of days since the start of the year.
  2. Time elapsed from the start of the year until now in days.
  3. Serial number of a piece of lab equipment.
  4. A lab specimen’s age
  5. Current population of a city.
  6. Average population of a city over time.

The answers to the questions are:

  1. Integer, since the number of days would lie between 1 and 365.
  2. Floating point, since fractional days are required
  3. Character string if serial number contains letters and numbers, otherwise integer if the serial number consists only of numerals
  4. This will vary! How do you define a specimen’s age? whole days since collection (integer)? date and time (string)?
  5. Choose floating point to represent population as large aggregates (eg millions), or integer to represent population in units of individuals.
  6. Floating point number, since an average is likely to have a fractional part.

Division Types

In Python 3, the // operator performs integer (whole-number) floor division, the / operator performs floating-point division, and the % (or modulo) operator calculates and returns the remainder from integer division:

PYTHON 

print('5 // 3:', 5 // 3)
print('5 / 3:', 5 / 3)
print('5 % 3:', 5 % 3)

OUTPUT 

5 // 3: 1
5 / 3: 1.6666666666666667
5 % 3: 2

If num_subjects is the number of subjects taking part in a study, and num_per_survey is the number that can take part in a single survey, write an expression that calculates the number of surveys needed to reach everyone once.

We want the minimum number of surveys that reaches everyone once, which is the rounded up value of num_subjects/ num_per_survey. This is equivalent to performing a floor division with // and adding 1. Before the division we need to subtract 1 from the number of subjects to deal with the case where num_subjects is evenly divisible by num_per_survey.

PYTHON 

num_subjects = 600
num_per_survey = 42
num_surveys = (num_subjects - 1) // num_per_survey + 1

print(num_subjects, 'subjects,', num_per_survey, 'per survey:', num_surveys)

OUTPUT 

600 subjects, 42 per survey: 15

Strings to Numbers

Where reasonable, float() will convert a string to a floating point number, and int() will convert a floating point number to an integer:

PYTHON 

print("string to float:", float("3.4"))
print("float to int:", int(3.4))

OUTPUT 

string to float: 3.4
float to int: 3

If the conversion doesn’t make sense, however, an error message will occur.

PYTHON 

print("string to float:", float("Hello world!"))

ERROR 

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-5-df3b790bf0a2> in <module>
----> 1 print("string to float:", float("Hello world!"))

ValueError: could not convert string to float: 'Hello world!'

Given this information, what do you expect the following program to do?

What does it actually do?

Why do you think it does that?

PYTHON 

print("fractional string to int:", int("3.4"))

What do you expect this program to do? It would not be so unreasonable to expect the Python 3 int command to convert the string “3.4” to 3.4 and an additional type conversion to 3. After all, Python 3 performs a lot of other magic - isn’t that part of its charm?

PYTHON 

int("3.4")

OUTPUT 

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-2-ec6729dfccdc> in <module>
----> 1 int("3.4")
ValueError: invalid literal for int() with base 10: '3.4'

However, Python 3 throws an error. Why? To be consistent, possibly. If you ask Python to perform two consecutive typecasts, you must convert it explicitly in code.

PYTHON 

int(float("3.4"))

OUTPUT 

3

Arithmetic with Different Types

Which of the following will return the floating point number 2.0? Note: there may be more than one right answer.

PYTHON 

first = 1.0
second = "1"
third = "1.1"
  1. first + float(second)
  2. float(second) + float(third)
  3. first + int(third)
  4. first + int(float(third))
  5. int(first) + int(float(third))
  6. 2.0 * second

Answer: 1 and 4

Complex Numbers

Python provides complex numbers, which are written as 1.0+2.0j. If val is a complex number, its real and imaginary parts can be accessed using dot notation as val.real and val.imag.

PYTHON 

a_complex_number = 6 + 2j
print(a_complex_number.real)
print(a_complex_number.imag)

OUTPUT 

6.0
2.0
  1. Why do you think Python uses j instead of i for the imaginary part?
  2. What do you expect 1 + 2j + 3 to produce?
  3. What do you expect 4j to be? What about 4 j or 4 + j?
  1. Standard mathematics treatments typically use i to denote an imaginary number. However, from media reports it was an early convention established from electrical engineering that now presents a technically expensive area to change. Stack Overflow provides additional explanation and discussion.
  2. (4+2j)
  3. 4j and Syntax Error: invalid syntax. In the latter cases, j is considered a variable and the statement depends on if j is defined and if so, its assigned value.

Key Points

  • Every value has a type.
  • Use the built-in function type to find the type of a value.
  • Types control what operations can be done on values.
  • Strings can be added and multiplied.
  • Strings have a length (but numbers don’t).
  • Must convert numbers to strings or vice versa when operating on them.
  • Can mix integers and floats freely in operations.
  • Variables only change value when something is assigned to them.