Loops – repeating things

What you'll learn

Run code several times with for and range()
Repeat with while as long as something is true
Recognise and avoid endless loops

Why loops?

Imagine you have to print “Hello” a hundred times. A hundred lines of print("Hello")? Far too much work. A loop repeats code for you.

The for loop

for goes through a sequence of values one after the other. range(5) produces the numbers 0, 1, 2, 3, 4 – five numbers, starting at 0.

for lap in range(5):
    print(f"Lap {lap}")

print("Done!")

Press Steps and watch the variable lap. You’ll see Python jumping back to the for line again and again.

range can do more:

Call	Numbers
`range(5)`	0, 1, 2, 3, 4
`range(1, 6)`	1, 2, 3, 4, 5
`range(0, 20, 5)`	0, 5, 10, 15
`range(3, 0, -1)`	3, 2, 1

Calculating in a loop

Loops are great for adding things up. Here we add all numbers from 1 to 100:

total = 0

for number in range(1, 101):
    total = total + number

print(f"The total is {total}.")

The while loop

while repeats as long as a condition is true. That’s handy when you don’t know in advance how often.

money = 20

while money >= 7:
    money = money - 7
    print(f"Bought a cinema ticket, {money} euros left.")

print("Not enough money any more.")

Watch out, endless loop: If the condition never changes, while runs forever. Try while True: print("Help") – and then press Stop.

Remember: Use for when you know how often. Use while when you repeat until something happens.

Quick check

Before you start the task: what does this program print? Type your guess – then Python works it out.

total = 0
for number in range(1, 4):
    total = total + number
    print(total)

Your task

Your turn

Program the countdown for a rocket launch: print the numbers from 10 down to 1, each on its own line, followed by Liftoff!. Use a loop – ten print() calls don't count.

# Countdown from 10 to 1, then "Liftoff!"

Something to think about
The first ten lines must be 10, 9, 8 … 1. Tip – range(10, 0, -1) counts backwards.
Which command helps?
After the 1 there should be exactly one line “Liftoff!” – and nothing else.
Use for or while instead of printing every number separately.

Gap code – fill in the ___

                        for number in ___:
    print(___)

print(___)

Solve it without hints to earn a ⭐ on your progress page.

Show solution

Try it yourself first – mistakes teach you the most. Here is one possible solution:

for number in range(10, 0, -1):
    print(number)

print("Liftoff!")

For fast learners

Pro task

Build a number pyramid with height rows. Row 1 shows 1, row 2 1 2, row 3 1 2 3 – and so on up to row height. The numbers are separated by exactly one space.

height = 4

# One row per pass of the loop

Show pro solution

height = 4

for row in range(1, height + 1):
    numbers = []
    for number in range(1, row + 1):
        numbers.append(str(number))
    print(" ".join(numbers))

Practise more in the training roomEndless new tasks on this topic – with XP and streaks.