3.2.2c Iteration — AQA 8525

What is Iteration?

Iteration (repetition) allows a block of code to run multiple times without having to rewrite it. AQA specifies three loop types: FOR, WHILE, and REPEAT…UNTIL.

FOR Loop (Count-controlled)

Use when you know exactly how many times the loop should run.

FOR i ← 1 TO 5
    OUTPUT i
ENDFOR
// Outputs: 1 2 3 4 5

The loop variable i automatically increments from the start value to the end value (inclusive). The default step is 1.

// Summing 1 to 10
total ← 0
FOR i ← 1 TO 10
    total ← total + i
ENDFOR
OUTPUT total   // 55

WHILE Loop (Pre-condition)

Use when the number of repetitions is not known in advance. The condition is checked before each iteration — so the body may never run at all if the condition starts False.

count ← 1
WHILE count <= 5
    OUTPUT count
    count ← count + 1
ENDWHILE
// Outputs: 1 2 3 4 5

Input validation is a classic WHILE use case — keep asking until valid input is received:

age ← int(INPUT("Enter age: "))
WHILE age < 0 OR age > 120
    OUTPUT "Invalid — try again"
    age ← int(INPUT("Enter age: "))
ENDWHILE

REPEAT…UNTIL Loop (Post-condition)

The condition is checked after each iteration — so the body always runs at least once.

REPEAT
    password ← INPUT("Enter password: ")
UNTIL password == "secret123"

Keeps asking until the password is correct. Perfect when you want the user to enter data at least once before checking validity.

Comparison of the Three Loops

Loop	When to use	Condition checked	Min runs
FOR	Know exact count	Start/end of count	0 (if start > end)
WHILE	Don't know count; check first	Before each iteration	0
REPEAT…UNTIL	Don't know count; run first	After each iteration	1

Infinite Loops

A loop with a condition that is always True (or never becomes False) will run forever — this is a bug. Common cause: forgetting to update the loop variable inside a WHILE loop.

// INFINITE LOOP — bug!
count ← 1
WHILE count <= 5
    OUTPUT count
    // Missing: count ← count + 1
ENDWHILE

Exam tip: REPEAT…UNTIL loops once guaranteed — the condition is at the bottom. WHILE may run zero times — the condition is at the top. Exam questions often ask you to pick the correct loop type for a scenario or trace through one.

⚠️ Common Mistakes

Using ENDFOR after a WHILE loop (and vice versa) — every loop has its own closing keyword
Forgetting to update the counter in a WHILE loop → infinite loop
FOR i ← 1 TO 5 runs 5 times (1,2,3,4,5) — remember the end value is inclusive
UNTIL condition is when to STOP (opposite of WHILE condition which is when to CONTINUE)

▶

Video coming soon

Key points

FOR loop — count-controlled, inclusive range
WHILE loop — pre-condition, may run 0 times
REPEAT…UNTIL — post-condition, always runs at least once
Input validation using WHILE

Click slide or press arrow keys to navigate

✍️

Worksheet — 3.2.2c Iteration

8 questions · 24 marks

Q1State the three types of iteration in AQA pseudo-code and state when each is used.[6]

✅ Mark scheme

Mark scheme

FOR — when exact number of iterations known [2]; WHILE — condition checked before, used when count unknown/may run 0 times [2]; REPEAT…UNTIL — condition checked after, always runs at least once [2].

Q2Trace this loop and state all output: FOR i ← 1 TO 5, OUTPUT i * i, ENDFOR[2]

✅ Mark scheme

Mark scheme

1 4 9 16 25 [2] — one mark if 4 correct.

Q3Write a WHILE loop that keeps asking the user for a number until they enter a number greater than 0.[4]

✅ Mark scheme

Mark scheme

Initial INPUT before loop [1]; WHILE num <= 0 [1]; OUTPUT error message [1]; INPUT inside loop [1]; ENDWHILE [implicit in structure].

Q4What is the key difference between a WHILE loop and a REPEAT…UNTIL loop?[2]

✅ Mark scheme

Mark scheme

WHILE checks condition before each iteration — may run 0 times [1]; REPEAT…UNTIL checks condition after each iteration — always runs at least once [1].

Q5Write pseudo-code using a FOR loop to output all even numbers between 2 and 20 inclusive.[3]

✅ Mark scheme

Mark scheme

FOR i ← 2 TO 20 (or 1 TO 10) [1]; IF i MOD 2 == 0 THEN OUTPUT i (or OUTPUT i*2 if using 1-10) [1]; ENDFOR [1]. Accept step-2 approach if known.

Q6Identify the bug in this code and explain the consequence: count ← 1, WHILE count <= 10, OUTPUT count, ENDWHILE[2]

✅ Mark scheme

Mark scheme

count is never incremented [1]; this creates an infinite loop that outputs 1 forever [1].

Q7How many times does this loop run: FOR i ← 5 TO 1, OUTPUT i, ENDFOR? Explain your answer.[2]

✅ Mark scheme

Mark scheme

0 times [1]; the start (5) is already greater than the end (1), so the loop does not execute [1].

Q8Write a REPEAT…UNTIL loop that asks the user to guess a number until they guess 7.[3]

✅ Mark scheme

Mark scheme

REPEAT [1]; guess ← int(INPUT("Guess: ")) [1]; UNTIL guess == 7 [1].

✓

Check your answers above.

Topic Quiz

Q 1 of 10

You scored

out of 10

Card 1 of 6

Click to flip

🎉

All done!

Term	Definition

🔒

Pro Content

Subscribe to unlock all AQA 8525 lessons, worksheets, quizzes and mini tests.

£7.99/month

or £59/year

Upgrade to Pro

What is Iteration?

FOR Loop (Count-controlled)

WHILE Loop (Pre-condition)

REPEAT…UNTIL Loop (Post-condition)

Comparison of the Three Loops

Infinite Loops

Key points

Worksheet — 3.2.2c Iteration

Mini Test — 3.2.2c Iteration