The Three Kinds of Loop — Pseudocode and Flowcharts | IGCSE Computer Science

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 11% of your exam marks.

stable

Medium

Stable11%

The most marks-dense topic. Both papers test pseudocode writing and trace tables every sitting.

CIE pseudocode supports three loop constructs. Each one fits a different situation; getting the right loop for the job makes algorithms cleaner and easier to read.

FOR loop

A FOR loop runs a block of code a fixed number of times, with a counter that takes each value in a stated range.

Syntax:

FOR i ← 1 TO 10
   OUTPUT i
NEXT i

Use a FOR loop when:

You know in advance how many times the loop should run.
A counter is naturally needed for the work (e.g. processing each element in an array).

Variations:

FOR i ← 1 TO 10 STEP 2 advances the counter by 2 each iteration (1, 3, 5, 7, 9).
FOR i ← 10 TO 1 STEP -1 counts down.

WHILE loop

A WHILE loop runs a block of code as long as a condition is true. The condition is checked at the top before each iteration.

Syntax:

WHILE Answer <> "stop" DO
   INPUT Answer
ENDWHILE

Use a WHILE loop when:

You do not know in advance how many iterations will be needed.
It may be appropriate to run the loop zero times if the condition is already false at the start.

REPEAT-UNTIL loop

A REPEAT-UNTIL loop runs a block of code repeatedly until a condition becomes true. The condition is checked at the bottom after each iteration.

Syntax:

REPEAT
   INPUT Number
UNTIL Number > 0

Use a REPEAT-UNTIL loop when:

You do not know in advance how many iterations will be needed.
The loop body must run at least once.

Side-by-side comparison

Feature	FOR	WHILE	REPEAT-UNTIL
Number of iterations	Known in advance	Not known	Not known
Condition checked	n/a (counter-controlled)	Before each iteration (top of loop)	After each iteration (bottom of loop)
Will it run at least once?	Only if the start ≤ end	No (zero iterations possible)	Yes (always at least one iteration)
Counter variable	Yes, the counter	Optional; the programmer manages it	Optional; the programmer manages it
Typical use	Traversing an array of known size; printing N lines; counting	Processing input until a sentinel value arrives; waiting for a condition	Validating user input; menu loops that should always run once

Choosing the right loop

Example — Which loop should you use in each scenario?

Scenario	Best loop	Why
Print the numbers from 1 to 100	FOR	Number of iterations is known: 100
Sum every score in a class array of 30 students	FOR	Number of iterations is known: 30
Keep asking the user for a positive number until they enter one	REPEAT-UNTIL	At least one input is needed; the loop continues until valid
Read lines from a file until the end-of-file marker	WHILE (`WHILE NOT EOF`)	May read zero lines if the file is empty
Display a menu and process choices until the user picks "Quit"	REPEAT-UNTIL	Menu must show at least once; loop ends on a specific choice
Search through unsorted data for a target value	WHILE (`WHILE NOT found AND index < size`)	May find the target on iteration 0, or never

Avoiding common loop pitfalls

Off-by-one errors: forgetting that FOR i ← 1 TO 10 runs 10 times, not 9 or 11.
Infinite loops: forgetting to update the variable in the loop's condition (e.g. forgetting to read fresh input inside a WHILE Answer <> "stop" loop).
Wrong loop choice: using a REPEAT-UNTIL when zero iterations should be possible, or a WHILE when at least one iteration is mandatory.
Counter inside the loop body: in CIE pseudocode, you generally should not modify the FOR loop's counter from inside the loop body.