Iteration — Loops

Spec & Goals 3 min

AQA Spec 3.2.2 · Programming constructs — iteration

By the end of this lesson you can:

1. Explain what iteration is and write a count-controlled FOR loop.
2. Write condition-controlled loops using WHILE … ENDWHILE and REPEAT … UNTIL.
3. Choose the right loop for a problem and trace a loop to find its output.

Warm-Up 5 min

Last lesson selection chose which code runs. Iteration decides how many times code runs.

Quick starter

A teacher marks 30 exam papers, one after another. Which describes this best?

• Repeating a task a known number of times.
• Repeating until something becomes true.
• Doing the task only once.

Key Concept — repeating code with loops 14 min

Iteration means repeating a block of code. The repeated block is called a loop. There are two kinds.

Count-controlled iteration — FOR

Use a count-controlled loop when you know how many times to repeat. A counter steps from a start to an end value.

AQA pseudo-code

FOR i ← 1 TO 5
  OUTPUT i
ENDFOR

Condition-controlled iteration — WHILE and REPEAT

Use a condition-controlled loop when you do not know the number of repeats in advance — it depends on a condition.

WHILE tests the condition before the loop body, so the body may run zero times:

WHILE password ≠ 'open' DO
  password ← USERINPUT
ENDWHILE

REPEAT tests the condition after the loop body, so the body always runs at least once:

REPEAT
  password ← USERINPUT
UNTIL password = 'open'

Worked Example — a sum and a validation loop 12 min

Problem 1: use a FOR loop to add the numbers 1 to 5 and output the total.

AQA pseudo-code

total ← 0
FOR i ← 1 TO 5
  total ← total + i
ENDFOR
OUTPUT total

The same loop in Python

total = 0
for i in range(1, 6):
    total = total + i
print(total)

Trace the loop. After the loop, total is 15:

Problem 2: keep re-asking for an age until the user gives a valid one (1–120). The body must run at least once, so use REPEAT … UNTIL.

REPEAT
  age ← USERINPUT
UNTIL age ≥ 1 AND age ≤ 120
OUTPUT 'Thank you'

while True:
    age = int(input("Enter your age: "))
    if age >= 1 and age <= 120:
        break
print("Thank you")

Try It Yourself 12 min

📝 Exam Practice 10 min

Answer the way the examiner expects — the command word and the marks tell you how much to write.

FOR count ← __________ TO __________
  OUTPUT __________
ENDFOR

Recap & Key Terms 3 min

Iteration repeats code. Use a count-controlled FOR loop for a known number of repeats. Use a condition-controlled loop — WHILE (tests before) or REPEAT … UNTIL (tests after) — when the number of repeats is unknown.

Iteration

Repeating a block of code one or more times.

Count-controlled

A loop that repeats a known, fixed number of times.

Condition-controlled

A loop that repeats while/until a condition is met, an unknown number of times.

FOR

A count-controlled loop: FOR i ← 1 TO n … ENDFOR.

WHILE

A condition-controlled loop that tests its condition before the body, so it may run zero times.

REPEAT-UNTIL

A condition-controlled loop that tests its condition after the body, so it runs at least once.

Homework 1 min

Task (≤ 15 min): Write AQA pseudo-code that uses a WHILE loop to output the 5 times table (5, 10, …, 50).

count ← 1
WHILE count ≤ 10 DO
  OUTPUT count * 5
  count ← count + 1
ENDWHILE