Iteration (Loops) (Years 7–11) — Computer Science

Repeating code with count-controlled FOR loops and condition-controlled WHILE and REPEAT UNTIL loops.

Computers are brilliant at one thing above all: doing the same job over and over without getting bored. Iteration — looping — is how you tell a program to repeat a block of code. Instead of writing "print star" fifty times, you write it once and say "do that fifty times". Loops turn long, repetitive tasks into a few tidy lines.

Why this matters: Nearly every real program loops — scrolling a feed, checking each item in a shopping basket, dealing a hand of cards. Loops are the third building block of programming, and knowing which type to reach for is a core GCSE skill.

A loop runs its body again and again, then exits.

Count-controlled loops (FOR)

A FOR loop steps a counter through a fixed range and stops by itself.

When you know exactly how many times to repeat, use a FOR loop. A counter variable steps through a range of numbers automatically.

FOR i ← 1 TO 5
    OUTPUT i
NEXT i

This outputs 1, 2, 3, 4, 5. Picture a runner doing five laps of a track: the loop counts each lap for you and stops on its own. This is the loop to reach for when the number of repeats is fixed.

Condition-controlled loops (WHILE)

WHILE tests before the body, so it can run zero times.

Sometimes you do not know how many repeats you need — you keep going until something happens. A WHILE loop checks a condition before each pass and stops as soon as it becomes False.

total = 0
WHILE total < 10
    total = total + 3
ENDWHILE

Like eating chips until you are full: you cannot say in advance how many, you just keep checking.

REPEAT UNTIL: check at the end

REPEAT UNTIL tests after the body, so it always runs at least once.

A REPEAT ... UNTIL loop tests its condition after the body, so the body always runs at least once. Handy for asking a user to try again.

Loop	Use when	Test happens
FOR	Repeats are counted / fixed	—
WHILE	Repeats unknown, may be zero	Before the body
REPEAT UNTIL	Repeats unknown, at least one	After the body

Worked trace of the WHILE loop above:
Start: total = 0. Is 0 < 10? Yes → total = 3.
Is 3 < 10? Yes → total = 6.
Is 6 < 10? Yes → total = 9.
Is 9 < 10? Yes → total = 12.
Is 12 < 10? No → stop. Final total = 12 (the loop ran 4 times).

Common mistake: an infinite loop. If a WHILE condition never becomes False — for example you forget to change total inside the loop — the program repeats forever. Always make sure something inside the loop moves you towards the exit.

Ask yourself one question to pick the loop: "Do I know the number of repeats?" Yes → FOR. No → WHILE (or REPEAT UNTIL if it must run at least once).

One or two questions pick the right loop every time.