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

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

Critical Operational Characteristics:

  1. Zero Iterations Possible: If <Condition> is initially FALSE, the loop body is bypassed entirely and runs zero times.
  2. Continuation Condition: The loop continues executing as long as the condition evaluates to TRUE. It terminates the instant the condition becomes FALSE.
  3. 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.

2. Choosing Between the Three Loop Types

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:
Task 1 Task 2 Task 3

Independent Practice: Bounded powers

+30 XP

Read a non-negative integer limit. Output powers of two starting at 1, each on a separate line, while the value is no greater than the limit. Output nothing when the limit is zero.

Write a complete program. Read inputs in the stated order. Output only the requested results, one per line, without input prompts. Test with your own values before submitting.

solution.pseudo
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○ Zero iterations
○ Exact power
○ Between powers