Lesson 10: While Loops¶

In this lesson we will learn

the difference between definite and indefinite iteration
how to make Python pick random numbers
how to use comparison operators
how to use a while loop to repeat until something happens
how to keep asking the user until they give a valid answer

Terminology

while loop – a loop that keeps repeating its code while its condition is True.
definite iteration – a loop where we know how many times it will run, usually written as a for loop.
indefinite iteration – a loop where we don't know how many times it will run, usually written as a while loop.
count-controlled loop – a loop that runs a set number of times, like a for loop.
condition-controlled loop – a loop that runs until a condition changes, like a while loop.
comparison operator – a symbol such as ==, !=, > or < that compares two values and gives back True or False.
infinite loop – a loop whose condition is always True, so it never stops on its own.
break – a command that exits a loop straight away.

Two types of loops¶

Python has two types of loops. We've already used the for loop. Now we'll learn about the while loop. They match two different ways of repeating code:

definite iteration is when we know how many times the loop will run. We use a for loop, because it runs a set number of times.
indefinite iteration is when we don't know how many times the loop will run. We use a while loop, because it keeps going while something is True.

Think about dealing cards:

In Uno, each player gets 7 cards, so we deal 7 times. This is definite iteration.
In Snap, we keep dealing until the deck is empty, and we don't know how many cards each player will get. This is indefinite iteration.

So:

a for loop runs a set number of times (count controlled)
a while loop runs until a condition changes (condition controlled)

To see how while loops work, let's build a number guessing game.

Number guessing game¶

Type the code below into a new file, or open one_guess.py from the lesson_10 folder of the tutorial files. Save it as guessing_game.py.

import random


def get_number(prompt):
    num = input(prompt)
    if num.isdigit():
        return int(num)
    else:
        print("Invalid input")
        quit()


number = random.randint(1, 100)

guess = get_number("Guess a number between 1 and 100> ")

if guess == number:
    print("Correct!")
else:
    print("Incorrect. The number was", number)

PRIMM

Predict what will happen when we run the code.
Run the code. Did it match your prediction?
Time to investigate the code. What does each line do?

Code explanation

line 1 → imports the random module, so we can use its random number tools.
lines 4–10 → define get_number, which asks a question and returns a whole number, or stops the program.
line 13 → picks a random whole number from 1 to 100 (including both 1 and 100) and stores it in number.
line 15 → asks the user to guess the number and stores their guess in guess.
line 17 → checks whether guess is equal to number.
line 18 → if it is, prints Correct!…
line 19 → …otherwise…
line 20 → …prints Incorrect. and shows the number.

The random module

The random module has lots of functions for getting random values. To see them all, visit the W3Schools Python random module page.

== is a comparison operator. It compares two values and gives back True or False.

Comparison operators

Operator	Meaning
`==`	checks if two values are the same
`!=`	checks if two values are different
`>`	checks if the left value is greater than the right value
`<`	checks if the left value is less than the right value
`>=`	checks if the left value is greater than or equal to the right value
`<=`	checks if the left value is less than or equal to the right value

A better game¶

With only one guess, the game isn't much fun. Let's give the player 10 guesses. That means repeating code, which is iteration. We know it will repeat 10 times, so this is definite iteration, and we use a for loop.

Change our code so it matches the code below.

import random


def get_number(prompt):
    num = input(prompt)
    if num.isdigit():
        return int(num)
    else:
        print("Invalid input")
        quit()


number = random.randint(1, 100)

print("You have 10 turns to guess a number between 1 and 100")

for turn in range(10):
    guess = get_number("Guess a number between 1 and 100> ")

    if guess == number:
        print("Correct!")
    else:
        print("Incorrect. Try again")

print("The number was", number)

PRIMM

Predict what will happen when we run the code.
Run the code. Did it match your prediction?
Time to investigate the code. What does each line do?

Code explanation

line 1 → imports the random module.
lines 4–10 → define get_number, which asks a question and returns a whole number, or stops the program.
line 13 → picks a random whole number from 1 to 100 and stores it in number.
line 15 → tells the user they have 10 guesses.
line 17 → starts a for loop that repeats 10 times.
line 18 → asks the user for a guess and stores it in guess.
line 20 → checks whether guess is equal to number.
line 21 → if it is, prints Correct!…
line 22 → …otherwise…
line 23 → …prints Incorrect. Try again.
line 25 → shows the number after all 10 guesses are finished.

An even better game¶

This version is better, but players don't learn anything from their earlier guesses. Let's give them hints by telling them if their guess is too high or too low. Change the if … else into an if … elif … else so our code matches the code below.

import random


def get_number(prompt):
    num = input(prompt)
    if num.isdigit():
        return int(num)
    else:
        print("Invalid input")
        quit()


number = random.randint(1, 100)

print("You have 10 turns to guess a number between 1 and 100")

for turn in range(10):
    guess = get_number("Guess a number between 1 and 100> ")

    if guess > number:
        print("Guess is too high")
    elif guess < number:
        print("Guess is too low")
    else:
        print("Correct!")

print("The number was", number)

PRIMM

Predict what will happen when:
- the guess is too high
- the guess is too low
- the guess is correct
- all 10 guesses are used without finding the number
Run the code several times until you've seen all four. Did they match your predictions?
Time to investigate the code. What does each line do?

Code explanation

line 1 → imports the random module.
lines 4–10 → define get_number, which asks a question and returns a whole number, or stops the program.
line 13 → picks a random whole number from 1 to 100 and stores it in number.
line 15 → tells the user they have 10 guesses.
line 17 → starts a for loop that repeats 10 times.
line 18 → asks the user for a guess and stores it in guess.
line 20 → checks whether guess is greater than number…
line 21 → …and if it is, prints Guess is too high…
line 22 → …otherwise, checks whether guess is less than number…
line 23 → …and if it is, prints Guess is too low…
line 24 → …otherwise, when the guess is neither too high nor too low…
line 25 → …prints Correct!.
line 27 → shows the number after the loop has finished.

Testing tip

To make testing easier, add a line that prints number just after it's chosen. Once you've finished testing, comment that line out so it doesn't spoil the game.

Did you notice the problem? When the user guesses the number, the game says Correct!… and then keeps asking for more guesses. That's because a for loop always runs a set number of times, no matter what.

We want the game to stop as soon as the correct number is guessed. We need a loop that keeps going until something happens: indefinite iteration, using a while loop.

Using a `while` loop¶

Make these changes so our code matches the code below:

replace line 15 with guess = 0
change the for loop on line 17 to while guess != number:

import random


def get_number(prompt):
    num = input(prompt)
    if num.isdigit():
        return int(num)
    else:
        print("Invalid input")
        quit()


number = random.randint(1, 100)

guess = 0

while guess != number:
    guess = get_number("Guess a number between 1 and 100> ")

    if guess > number:
        print("Guess is too high")
    elif guess < number:
        print("Guess is too low")
    else:
        print("Correct!")

print("The number was", number)

PRIMM

Predict what will happen when:
- the guess is too high
- the guess is too low
- the guess is correct
Run the code. Did it match your predictions?
Time to investigate the code. What does each line do?

Code explanation

line 1 → imports the random module.
lines 4–10 → define get_number, which asks a question and returns a whole number, or stops the program.
line 13 → picks a random whole number from 1 to 100 and stores it in number.
line 15 → gives guess a starting value of 0, so the while condition can be checked before the user has guessed.
line 17 → starts a while loop that keeps repeating while guess is not equal to number.
line 18 → asks the user for a guess and stores it in guess.
lines 20–25 → print Guess is too high, Guess is too low or Correct!.
line 27 → shows the number once the loop has finished.

Let's look closely at line 17, while guess != number::

guess != number is the loop's condition. It's True when the guess is not the number, and False when it is.
while tells Python to keep repeating the indented code while the condition is True.

And line 15, guess = 0:

the while condition uses guess before the user has entered anything, so guess needs a starting value, or the program will crash
the starting value must not be the same as the random number, or the loop won't run at all
the random number is between 1 and 100, so 0 guarantees the loop runs at least once

Flowchart of the guessing game with a while loop

Using `while` to catch errors¶

The game is better now, but there's still a problem. If the user types something that isn't a number, the game ends straight away. That's frustrating if they've already made a few guesses.

We can fix this with a while loop inside get_number, so it keeps asking until the user types a valid number. Change get_number so our code matches the code below.

import random


def get_number(prompt):
    while True:
        num = input(prompt)
        if num.isdigit():
            return int(num)
        else:
            print("Invalid input")


number = random.randint(1, 100)

guess = 0

while guess != number:
    guess = get_number("Guess a number between 1 and 100> ")

    if guess > number:
        print("Guess is too high")
    elif guess < number:
        print("Guess is too low")
    else:
        print("Correct!")

print("The number was", number)

PRIMM

Predict what will happen when we type dog as a guess.
Run the code. Test a guess that's too high, too low, correct, and not a number. Did it match your predictions?
Time to investigate the code. What does each line do?

Code explanation

line 1 → imports the random module.
line 4 → defines get_number with one parameter, prompt.
line 5 → starts a while loop whose condition is always True, so it repeats until the function returns.
line 6 → shows the question in prompt and stores the user's answer in num.
line 7 → checks whether num is made up only of digits.
line 8 → if it is, changes num into an integer and returns it, which ends the function and the loop…
line 9 → …otherwise…
line 10 → …tells the user their input is invalid, and the loop asks again.
line 13 → picks a random whole number from 1 to 100 and stores it in number.
line 15 → gives guess a starting value of 0.
line 17 → starts a while loop that keeps repeating while guess is not equal to number.
line 18 → asks the user for a guess and stores it in guess.
lines 20–25 → print Guess is too high, Guess is too low or Correct!.
line 27 → shows the number once the loop has finished.

while True: is called an infinite loop, because its condition is always True. Usually an infinite loop is a mistake, but here we're using it on purpose. We can leave the loop with:

break, which exits the loop
return, which exits the function, and so the loop too

The program now keeps asking until the user types a valid number, instead of crashing.

Flowchart of get_number with a while loop

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_10 folder of the tutorial files. Solutions are on the Exercise Solutions page.

Exercise 1¶

Starter: lesson_10/ex1_keep_asking

Can you change the get_number and get_color functions so they keep asking until the user types a valid answer, instead of stopping the program?

Lesson 10: While Loops¶

Two types of loops¶

Number guessing game¶

A better game¶

An even better game¶

Using a while loop¶

Using while to catch errors¶

Exercises¶

Exercise 1¶

Using a `while` loop¶

Using `while` to catch errors¶