What causes an infinite loop?

Forgetting to change the value that the condition tests. If the condition never becomes False, the loop runs forever. Always make sure the body modifies the variable in the condition.

When should I use while instead of for?

Use while when you don't know in advance how many times you need to loop — for example, keep asking for input until it's valid. Use for when you know exactly how many iterations you need.

break exits the loop immediately, skipping any remaining iterations. Useful for 'stop as soon as we find what we're looking for'.

Python while loops

A while loop repeats a block of code for as long as a condition is true. It is one of the two iteration constructs required by every UK GCSE Computer Science specification (the other is the for loop). This unit covers the basic structure, how to make sure the loop actually stops (the classic infinite-loop mistake), using while loops for input validation, and the break statement for early exit. Every example runs in the browser so learners can watch the loop counter change on each iteration. Perfect for classroom teaching, GCSE and A Level revision, and independent or home-schooled learners.

What you'll learn

Write a while loop with a clear stopping condition
Update the loop variable inside the body to avoid infinite loops
Use while for input validation (keep asking until the input is valid)
Exit early with break when needed
Distinguish condition-controlled (while) from count-controlled (for)

UK GCSE topic coverage

Condition-controlled iteration
Running totals
Sentinel-controlled iteration
Looping over characters
Linear search preview
String building
Input validation
Selection inside iteration
Applied iteration

Worked example

Keep asking until the user enters a positive number

n = int(input("Enter a positive number: "))
while n <= 0:
    print("That isn't positive.")
    n = int(input("Try again: "))
print("Thanks, got", n)

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Lessons in this unit

Full interactive lessons with checked tasks are available to signed-in RunPy users. Unit 1 and a selection of exam-style questions are free with a RunPy account.

Introducing while loops
The accumulator pattern
Sentinel-controlled loops
Iterating over a string with while
Searching inside a string
Building a new string in a loop
Input validation loops
Nested logic inside loops
Composite algorithm challenge

Frequently asked questions

What causes an infinite loop?: Forgetting to change the value that the condition tests. If the condition never becomes False, the loop runs forever. Always make sure the body modifies the variable in the condition.
When should I use while instead of for?: Use while when you don't know in advance how many times you need to loop — for example, keep asking for input until it's valid. Use for when you know exactly how many iterations you need.
What does break do?: break exits the loop immediately, skipping any remaining iterations. Useful for 'stop as soon as we find what we're looking for'.