CAMBRIDGE IGCSE 0478

Pre-Condition Loops (WHILE ... DO ... ENDWHILE)

Instructional Theory

A Pre-Condition Loop (the WHILE loop) evaluates its entry condition before executing the body of the loop.

1. Syntax of `WHILE ... DO ... ENDWHILE`

WHILE <Condition> DO
    // Statements executed while Condition remains TRUE
ENDWHILE

Zero Iterations Possible: If <Condition> is initially FALSE, the loop body is bypassed entirely and runs zero times.
Continuation Condition: The loop continues executing as long as the condition evaluates to TRUE. It terminates the instant the condition becomes FALSE.
Updating the Condition: Statements inside the loop body must modify at least one variable present in <Condition>, otherwise the loop will never terminate, resulting in an infinite loop.

Question to Ask	Appropriate Loop Construct	Cambridge Syntax
Is the exact number of iterations known in advance?	Count-Controlled Loop	`FOR i <- 1 TO N ... NEXT i`
Must the loop execute at least once (e.g. user input)?	Post-Condition Loop	`REPEAT ... UNTIL <Condition>`
May the loop need to execute zero times (e.g. searching)?	Pre-Condition Loop	`WHILE <Condition> DO ... ENDWHILE`

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Exercise:

• Multiple Choice

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Exam-Style Question

Question

If the condition of a WHILE ... DO ... ENDWHILE loop evaluates to FALSE before the loop begins, how many times will the loop body execute?

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