Data Types and Type Conversion

Overview

Teaching: 10 min
Exercises: 10 min
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.

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

print(type(52))
<class 'int'>
title = 'Biochemistry'
print(type(title))
<class 'str'>

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

print(5 - 3)
2
print('hello' - 'h')
---------------------------------------------------------------------------
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.

full_name = 'Ahmed' + ' ' + 'Walsh'
print(full_name)
Ahmed Walsh
separator = '=' * 10
print(separator)
==========

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

print(len(full_name))
11
print(len(52))
---------------------------------------------------------------------------
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.

print(1 + 'A')
---------------------------------------------------------------------------
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'
print(1 + int('2'))
print(str(1) + '2')
3
12

Can mix integers and floats freely in operations.

print('half is', 1 / 2.0)
print('three squared is', 3.0 ** 2)
half is 0.5
three squared is 9.0

Variables only change value when something is assigned to them.

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

Fractions

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

Solution

It is a floating-point number (often abbreviated “float”).

print(type(3.4))
<class 'float'>

Automatic Type Conversion

What type of value is 3.25 + 4?

Solution

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

result = 3.25 + 4
print(result, 'is', type(result))
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 since the start of the year.
  3. Call number of a book.
  4. Standard book loan period.
  5. Number of reference queries in a year.
  6. Average library classes taught per semester.

Solution

  1. Integer
  2. Float
  3. String
  4. Integer
  5. Integer
  6. Float

Division Types

There are three different types of division:

  1. ‘Normal’ division (aka floating-point division) is what most people may be familiar with: 5 / 2 = 2.5
  2. Floor division, which cuts out the part after the period: 5 / 2 = 2
  3. Modulo division, which only keeps the remained after division: 5 / 2 = 1

In Python 3, the / operator performs floating-point division, the // operator performs floor division, and the ‘%’ (or modulo) operator calculates the modulo division:

print('5 // 3:', 5//3)
print('5 / 3:', 5/3)
print('5 % 3:', 5%3)
5 // 3: 1
5 / 3: 1.6666666666666667
5 % 3: 2

If num_students is the number of students enrolled in a course (let say 600), and num_per_class is the number that can attend a single class (let say 42), write an expression that calculates the number of classes needed to teach everyone.

Solution

Depending on requirements it might be important to detect when the number of students per class doesn’t divide the number of students evenly. Detect it with the % operator and test if the remainder that it returns is greater than 0.

num_students = 600
num_per_class = 42
num_classes = num_students // num_per_class
remainder = num_students % num_per_class

print(num_students, 'students,', num_per_class, 'per class')
print(num_classes, ' full classes, plus an extra class with only ', remainder, 'students')
600 students, 42 per class
14  full classes, plus an extra class with only  12 students

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:

print("string to float:", float("3.4"))
print("float to int:", int(3.4))
string to float: 3.4
float to int: 3

Note: conversion is some times also called typecast.

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

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

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

Solution

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

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

num_as_string = "3.4"
num_as_float = float(num_as_string)
num_as_int = int(num_as_float)
print(num_as_int)
3

We could also write it in a single line like this: int(float("3.4"))

Arithmetic with Different Types

Which of the following will print 2.0? Note: there may be more than one right answer.

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

Solution

Answer: 1 and 4.

  1. is correct
  2. gives 2.1
  3. gives an arror because we cannot convert text to int directly
  4. is correct
  5. gives 2 (as an integer not as a float)
  6. gives an error because second is a string.

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.