AQA 8525 · Section 3.2 · 3.2.2c

Iteration
Loops

FOR Loop · WHILE Loop · REPEAT UNTIL · Choosing the Right Loop

CSZoneAQA GCSE Computer Science 8525

Three AQA Loop Types

AQA 8525 — Three Types of Loop

🔢

FOR … ENDFOR

Known number of repetitions

⏳

WHILE … ENDWHILE

Check condition before each loop

🔄

REPEAT … UNTIL

Always runs at least once

FOR Loop

Count-Controlled Loop

FOR i ← 1 TO 5
OUTPUT i
ENDFOR

← Outputs: 1, 2, 3, 4, 5

total ← 0
FOR i ← 1 TO 10
total ← total + i
ENDFOR
OUTPUT total

← Outputs: 55 (sum of 1 to 10)

Use FOR when:You know exactly how many times the loop should run.

WHILE & REPEAT UNTIL

Condition-Controlled Loops

WHILE — pre-condition

count ← 1
WHILE count <= 5
OUTPUT count
count ← count + 1
ENDWHILE

May run 0 times if condition is False immediately

REPEAT UNTIL — post-condition

REPEAT
num ← INT(USERINPUT)
UNTIL num > 0
OUTPUT num

Always runs at least once — good for input validation

Choosing a Loop

Which Loop Should I Use?

Situation	Best Loop
Repeat exactly 10 times	FOR 1 TO 10
Loop while password is wrong (check first)	WHILE
Ask for valid input (must run at least once)	REPEAT UNTIL

⚡ Exam Tip:REPEAT UNTIL is ideal for input validation — it guarantees the prompt is shown at least once.

Exam Practice

Have a go at this question

AQA-style question

Write a program using a REPEAT UNTIL loop that repeatedly asks the user to enter a password until they enter the correct password 'secret'. Output 'Access granted' when correct.

4 marks

REPEAT
password ← USERINPUT
UNTIL password = 'secret'
OUTPUT 'Access granted'

Key Takeaways

What to Remember

FOR = count-controlled (known repetitions) → ends with ENDFOR

WHILE = pre-condition (may run 0 times) → ends with ENDWHILE

REPEAT UNTIL = post-condition (runs at least once) — ideal for input validation

WHILE checks condition before · REPEAT UNTIL checks after each iteration