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Lesson 11: Boolean Logic

In this lesson we will learn

  • what Boolean values are
  • how comparison operators give back True or False
  • how to use the Boolean operators not, and and or
  • how to join comparisons to build more powerful conditions

Terminology

  • Boolean operator – an operator that works with True and False values and always gives back True or False.
  • not operator – a Boolean operator that flips a value, so not True is False and not False is True.
  • and operator – a Boolean operator that gives back True only if every value is True.
  • or operator – a Boolean operator that gives back True if at least one value is True.

Video link

Booleans

In programming, Boolean means working with two possible values: True and False.

  • a Boolean variable can only store True or False
  • comparison operators (==, !=, >, <, >=, <=) check something and give back True or False
  • Boolean operators (we'll learn these soon) also give back True or False

True and False are special words in Python. When we type them in Thonny, they're coloured differently to show they're special.

Comparison operators

The conditions in if and while statements check whether something is True or False, using comparison operators. Let's quickly review them. Create a new file, type in the code below, and save it as boolean_logic.py.

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print("jeff" == "jeff")
print(1 != 1)
print(500 > 300)
print(100 >= 250)
print("a" < "q")
print(-30 <= 3)

PRIMM

  1. Predict the six results. Each one will be either True or False.
  2. Run the code. Were your predictions correct?
  3. Time to investigate the code. What does each line do?
Code explanation
  • line 1 → prints whether "jeff" is equal to "jeff".
  • line 2 → prints whether 1 is not equal to 1.
  • line 3 → prints whether 500 is greater than 300.
  • line 4 → prints whether 100 is greater than or equal to 250.
  • line 5 → prints whether "a" is less than "q". For letters, "less than" means "comes earlier in the alphabet".
  • line 6 → prints whether -30 is less than or equal to 3.

PRIMM

Time to modify the code. Can you change the values so each result switches: every True becomes False, and every False becomes True?

Comparisons work the same way whether the values are typed straight into the code or stored in variables. Change our code so it matches the code below.

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score = 10
print(score > 5)

PRIMM

  1. Predict whether the code will print True or False.
  2. Run the code. Was your prediction correct?
  3. Time to investigate the code. What does each line do?
Code explanation
  • line 1 → stores 10 in score.
  • line 2 → prints whether the value in score is greater than 5.

Boolean operators

Boolean operators work a bit like maths, but instead of numbers they use True and False, and the result is always True or False. They're useful when we want to check more than one thing at the same time. There are three: not, and and or.

The not operator

The easiest operator is not. It flips the value: not True is False, and not False is True. Change our code so it matches the code below.

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print("not True is:", not True)
print("not False is:", not False)

PRIMM

  1. Predict what the Shell will show.
  2. Run the code. Was your prediction correct?
  3. Time to investigate the code. What does each line do?
Code explanation
  • line 1 → prints not True is: followed by the result of not True, which is False.
  • line 2 → prints not False is: followed by the result of not False, which is True.

The and operator

The and operator only gives back True if every value is True. Change our code so it matches the code below.

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print("True and True is:", True and True)
print("True and False is:", True and False)
print("False and True is:", False and True)
print("False and False is:", False and False)
print("True and True and True is:", True and True and True)
print("True and True and False is:", True and True and False)

PRIMM

  1. Predict what the Shell will show.
  2. Run the code. Were your predictions correct?
  3. Time to investigate the code. What does each line do?
Code explanation
  • line 1 → prints True, because both values are True.
  • line 2 → prints False, because one value is False.
  • line 3 → prints False, because one value is False.
  • line 4 → prints False, because both values are False.
  • line 5 → prints True, because all three values are True.
  • line 6 → prints False, because one of the three values is False.

The or operator

The or operator works the opposite way to and. It gives back True if at least one value is True. Change our code so it matches the code below.

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print("True or True is:", True or True)
print("True or False is:", True or False)
print("False or True is:", False or True)
print("False or False is:", False or False)
print("True or True or True is:", True or True or True)
print("True or False or False is:", True or False or False)

PRIMM

  1. Predict what the Shell will show.
  2. Run the code. Were your predictions correct?
  3. Time to investigate the code. What does each line do?
Code explanation
  • line 1 → prints True, because at least one value (in fact both) is True.
  • line 2 → prints True, because one value is True.
  • line 3 → prints True, because one value is True.
  • line 4 → prints False, because no values are True.
  • line 5 → prints True, because all three values are True.
  • line 6 → prints True, because one of the three values is True.

Joining comparisons

Using True and False on their own isn't very useful. But remember, comparison operators give back True or False. So we can use Boolean operators to join comparisons together, and build more powerful conditions for our if and while statements. Change our code so it matches the code below.

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print(7 < 8 and "a" < "o")

PRIMM

  1. Predict what the Shell will show.
  2. Run the code. Was your prediction correct?
  3. Time to investigate the code. What does each line do?
Code explanation
  • line 1 → works out both comparisons first (7 < 8 is True, and "a" < "o" is True), then works out True and True, and prints the result, True.

Comparisons on both sides

When we join comparisons, we need a full comparison on both sides of the Boolean operator.

  • score > 5 and score < 13 is correct: both sides are comparisons.
  • score > 5 and < 13 is a SyntaxError, because the second part isn't a full comparison.
  • score == 5 or score == 13 checks whether score is 5 or 13. Writing score == 5 or 13 runs, but doesn't do what we expect: an if statement using it always runs its code, because Python treats 13 on its own as True.

Exercises

In this course, the exercises are the make part of PRIMM. Work through them to make your own code.

Starter files are in the lesson_11 folder of the tutorial files. Solutions are on the Exercise Solutions page.

Exercise 1

Starter: lesson_11/ex1_all_true

Each line in this program prints False. Can you change the values so every line prints True? Only change the numbers and the Boolean values.

Exercise 2

Starter: lesson_11/ex2_pass_mark

Can you write a program that asks the user for their test score out of 100, then prints True if the score is a pass (from 50 to 100), and False if it isn't? Use and to join two comparisons.

Exercise 3

Starter: lesson_11/ex3_weekend

Can you write a program that asks the user what day it is, then prints True if it's Saturday or Sunday, and False if it isn't? Use or to join two comparisons.