← IGCSE Computer Science Paper 2

Choosing between FOR, WHILE and REPEAT

Use FOR when the number of repeats is known, WHILE when the loop may run zero times, and REPEAT when it must run at least once. WHILE tests first; REPEAT tests last.

Part of: IGCSE Computer Science 0478 — 7a Program Development Life Cycle — Module 48

Understanding Choosing between FOR, WHILE and REPEAT in IGCSE Computer Science Paper 2

What it does: REPEAT ... UNTIL is a post-condition loop. The body runs once before the condition is tested, so it always runs at least once. The count of 12 repeats is known before the loop starts, so a count-controlled FOR loop is the best choice. FOR repeats a set number of times. WHILE is condition-controlled because it repeats until its condition changes.

Common mistakes: A FOR loop is used when the number of repeats is known, and its range is written with TO, not UNTIL. It should be FOR Count ← 1 TO 5. REPEAT...UNTIL tests at the end and stops when the condition is TRUE. The loop should stop when the PIN is correct, so it should be UNTIL Pin = 1234.

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Learning

Duo

A program must display a message exactly 10 times. Which loop is the best choice?

REPEAT ... UNTILFOR ... TO ... NEXT

Matches

FOR Count ← 1 TO TotalPressed INPUT Key NEXT Count (used to keep asking until the user types STOP)

↓

A FOR loop is the wrong choice because the number of repeats is not known. A condition-controlled loop such as REPEAT ... UNTIL or WHILE is needed.

Applying

Meanings

Term

FOR, WHILE or REPEAT

Meaning

Use FOR when the number of repeats is known, WHILE when the loop may run zero times, and REPEAT when it must run at least once. WHILE tests first; REPEAT tests last.

Practice

// REPEAT runs at least once

Testing

Gaps

Count ← 1 TO 10

Check Answer

Spots

1Total ← 0

2FOR Count ← 1 UNTIL 5

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Quick Questions

1. A program must display a message exactly 10 times. Which loop is the best choice?

A. REPEAT ... UNTIL

B. FOR ... TO ... NEXT ✓

The number of repeats is known (10), so it is count-controlled. FOR is the best fit.

2. A loop might need to run zero times, for example if the user has no items to enter. Which loop is the best choice?

A. WHILE ... DO ... ENDWHILE ✓

B. REPEAT ... UNTIL

WHILE tests its condition before the first pass, so the body can be skipped completely. REPEAT always runs at least once.

3. A password must be asked for at least once, then asked again until it is correct. Which loop is the best choice?

A. REPEAT ... UNTIL ✓

B. FOR ... TO ... NEXT

The user must be asked at least once, so a post-condition loop (REPEAT) is best. The number of attempts is not known, so FOR does not fit.

4. Which statement is true?

A. REPEAT tests its condition first; WHILE tests its condition last.

B. WHILE tests its condition first; REPEAT tests its condition last. ✓

WHILE is a pre-condition loop. REPEAT is a post-condition loop, so its body always runs at least once.

5. Which loop is count-controlled?

A. FOR Count ← 1 TO 20 ✓

B. WHILE Total < 100 DO

FOR repeats a set number of times. WHILE is condition-controlled because it repeats until its condition changes.

Match the Pairs

FOR Count ← 1 TO 10 → A count-controlled loop: use it when the number of repeats is known before the loop starts

WHILE Password <> "open" DO → A pre-condition loop: the test happens first, so the body may run zero times

A loop must run at least once, such as asking for a password before it can be checked, with the test at the end → REPEAT INPUT Password UNTIL Password = "open"

A scenario where the number of repeats is known: output the 12 times table → FOR Number ← 1 TO 12 OUTPUT Number * 7 NEXT Number

FOR Count ← 1 TO TotalPressed INPUT Key NEXT Count (used to keep asking until the user types STOP) → A FOR loop is the wrong choice because the number of repeats is not known. A condition-controlled loop such as REPEAT ... UNTIL or WHILE is needed.

REPEAT OUTPUT Item Count ← Count + 1 UNTIL Count > Total (Total could be 0 and nothing should print) → REPEAT always runs at least once, so it would output an item even when Total is 0. A WHILE loop should be used because it can run zero times.

Key Term

FOR, WHILE or REPEAT

Use FOR when the number of repeats is known, WHILE when the loop may run zero times, and REPEAT when it must run at least once. WHILE tests first; REPEAT tests last.

Fill in the Blanks

1. FOR Count ← 1 TO 10 OUTPUT Count NEXT Count

Hint: Type the keyword for a count-controlled loop used when the number of repeats is known in advance.

2. WHILE Stock > 0 DO Stock ← Stock - 1 ENDWHILE

Hint: Type the keyword for the loop that tests its condition first, so it may run zero times.

3. REPEAT INPUT Guess UNTIL Guess = 7

Hint: Type the keyword that starts the loop that must always run at least once.

4. REPEAT INPUT Guess UNTIL Guess = Secret

Hint: Type the keyword that ends a loop by testing the condition last (a post-condition).

5. FOR Day ← 1 TO 7 OUTPUT Day NEXT Day

Hint: Type the keyword that closes the count-controlled loop and moves on to the next value of the counter.

Spot the Error

Question 1

A Total ← 0

B FOR Count ← 1 UNTIL 5

C INPUT Mark

D Total ← Total + Mark

E NEXT Count

Line B — A FOR loop is used when the number of repeats is known, and its range is written with TO, not UNTIL. It should be FOR Count ← 1 TO 5.

Question 2

A REPEAT

B INPUT Pin

C UNTIL Pin <> 1234

Line C — REPEAT...UNTIL tests at the end and stops when the condition is TRUE. The loop should stop when the PIN is correct, so it should be UNTIL Pin = 1234.

Question 3

A INPUT Mark

B WHILE Mark >= 0 THEN

C Total ← Total + Mark

D INPUT Mark

E ENDWHILE

Line B — A WHILE loop header ends with DO, not THEN (THEN belongs to IF). It should be WHILE Mark >= 0 DO.

Question 4

A FOR Day ← 1 TO 7

B INPUT Steps

C Total ← Total + Steps

D ENDFOR

Line D — A FOR loop is closed with NEXT followed by the loop variable, not ENDFOR. It should be NEXT Day.

Question 5

A WHILE Choice <> 0 DO

B OUTPUT "1 Play 0 Quit"

C INPUT Choice

D UNTIL Choice = 0

Line A — The menu must be shown at least once, so REPEAT is the right loop. WHILE tests first, before Choice has any value, and would not match the UNTIL at the end. Line 1 should be REPEAT.

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