← IGCSE Computer Science Paper 2

REPEAT ... UNTIL loop (7a)

A post-condition loop: its statements run at least once, then the condition is tested after them. The loop stops when the condition is TRUE.

Part of: IGCSE Computer Science 0478 — 7a Program Development Life Cycle — Module 47

Understanding REPEAT ... UNTIL loop (7a) in IGCSE Computer Science Paper 2

What it does: The condition is TRUE straight away, but OUTPUT still runs once because the test comes after the body. REPEAT is a post-condition loop. The condition sits at UNTIL, which is at the end, so it is tested last. The loop should end when the correct password is typed, so use UNTIL Password = "secret".

Common mistakes: The loop needs to read a new value each time round. OUTPUT only displays Password, so the user is never asked and the condition keeps testing the same value. It should be INPUT Password. A REPEAT loop stops when the condition is TRUE. With <> the loop would stop on a wrong PIN and keep asking for the right one. It should be UNTIL PIN = 1234.

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Learning

Duo

When does a REPEAT ... UNTIL loop test its condition?

After the statements have runBefore the statements run

Matches

REPEAT INPUT Password ENDREPEAT

↓

A REPEAT loop must finish with UNTIL and a condition. ENDREPEAT does not exist in Cambridge pseudocode.

Applying

Meanings

Term

REPEAT ... UNTIL

Meaning

A post-condition loop: its statements run at least once, then the condition is tested after them. The loop stops when the condition is TRUE.

Practice

REPEAT

Testing

Gaps

Check Answer

Spots

1REPEAT

2 OUTPUT Password

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Quick Questions

1. When does a REPEAT ... UNTIL loop test its condition?

A. Before the statements run

B. After the statements have run ✓

REPEAT is a post-condition loop. The condition sits at UNTIL, which is at the end, so it is tested last.

2. What is the minimum number of times the statements in a REPEAT ... UNTIL loop run?

A. Zero times

B. At least once ✓

The condition is only checked after the first pass, so the body always runs at least once. A WHILE loop can run zero times.

3. When does a REPEAT ... UNTIL loop stop?

A. When the condition is TRUE ✓

B. When the condition is FALSE

UNTIL means 'keep going until this is true'. A WHILE loop is the opposite: it keeps going while the condition is TRUE.

4. Which loop is guaranteed to run its body at least once, even if the condition is already met at the start?

A. REPEAT ... UNTIL ✓

B. WHILE ... DO ... ENDWHILE

WHILE tests first, so its body can be skipped completely. REPEAT tests last, so its body always runs once.

5. Which is the correct way to ask for a password until it matches 'Secret1'?

A. REPEAT INPUT Pass UNTIL Pass = "Secret1" ✓

B. REPEAT INPUT Pass UNTIL Pass <> "Secret1"

The loop must stop when the password is correct. So the UNTIL condition is Pass = "Secret1". Using <> would stop on a wrong password.

Match the Pairs

REPEAT INPUT Password UNTIL Password = "secret" → Asks for a password at least once and keeps asking until the password typed in equals "secret".

UNTIL Count = 10 → The condition tested after the loop body; the loop stops when Count equals 10 and repeats while it does not.

A REPEAT loop always runs its statements at least once, because the condition is tested after them → REPEAT OUTPUT "Hello" UNTIL TRUE

A WHILE loop tests its condition first and can run zero times, but a REPEAT loop always runs once → WHILE Age < 0 DO INPUT Age ENDWHILE

REPEAT INPUT Password ENDREPEAT → A REPEAT loop must finish with UNTIL and a condition. ENDREPEAT does not exist in Cambridge pseudocode.

REPEAT INPUT Password UNTIL Password <> "secret" → The condition is the wrong way round. The loop stops when the condition is TRUE, so this stops on a wrong password.

Key Term

REPEAT ... UNTIL

A post-condition loop: its statements run at least once, then the condition is tested after them. The loop stops when the condition is TRUE.

Fill in the Blanks

1. REPEAT INPUT Password UNTIL Password = "Secret1"

Hint: Type the keyword that starts a loop whose condition is tested after the statements.

2. REPEAT INPUT Guess UNTIL Guess = 7

Hint: Type the keyword that ends the loop and introduces the stopping condition.

3. REPEAT INPUT Number UNTIL Number > 0

Hint: Type the relational operator so the loop stops once the number is positive.

4. REPEAT Count ← Count + 1 UNTIL Count = 5

Hint: Type the variable whose value is tested to decide when the loop stops.

5. REPEAT INPUT Password UNTIL Password = "Secret1"

Hint: Type the command that reads a new value from the user on every pass of the loop.

Spot the Error

Question 1

A REPEAT

B OUTPUT Password

C UNTIL Password = "Secret1"

Line B — The loop needs to read a new value each time round. OUTPUT only displays Password, so the user is never asked and the condition keeps testing the same value. It should be INPUT Password.

Question 2

A REPEAT

B OUTPUT "Enter PIN"

C INPUT PIN

D UNTIL PIN <> 1234

Line D — A REPEAT loop stops when the condition is TRUE. With <> the loop would stop on a wrong PIN and keep asking for the right one. It should be UNTIL PIN = 1234.

Question 3

A WHILE

B INPUT Guess

C UNTIL Guess = Answer

Line A — A loop that ends with UNTIL must begin with REPEAT. WHILE belongs with DO and ENDWHILE and tests its condition first. Line 1 should be REPEAT.

Question 4

A Count ← 0

B REPEAT

C Count ← Count

D UNTIL Count = 5

Line C — Count never changes, so Count = 5 never becomes TRUE and the loop runs forever. The counter must go up by 1 each time: Count ← Count + 1.

Question 5

A Total ← 0

B REPEAT

C INPUT Num

D Total = Total + Num

E UNTIL Num = 0

Line D — In Cambridge pseudocode, assignment uses the arrow ←. The = sign is a comparison. It should be Total ← Total + Num.

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