Use pseudocode to write: a 'pre-condition' loop

Topic 11.2 – Constructs

Objective

Use pseudocode to write a pre‑condition loop (also known as a while loop) and understand its role in algorithm design.

What is a Pre‑condition Loop?

A pre‑condition loop repeats a block of statements while a given condition is true. The condition is evaluated before each iteration, so if the condition is false initially, the loop body may never execute.

Pseudocode Syntax

The standard pseudocode format used in the Cambridge syllabus is:

WHILE condition DO
statements
END WHILE

Key Characteristics

• The loop condition is a pre‑condition – it must be true for the loop to start.
• Potentially zero executions if the condition is false at the start.
• Often used when the number of repetitions is not known in advance.

Example: Sum of Positive Numbers

Read a sequence of integers from the user until a negative number is entered, then output the sum of the positive numbers entered.

DECLARE sum, number AS INTEGER
SET sum ← 0
READ number
WHILE number ≥ 0 DO
SET sum ← sum + number
READ number
END WHILE
OUTPUT "Sum =", sum

Step‑by‑Step Execution

Common Pitfalls

1. Forgetting to update a variable that influences the loop condition, leading to an infinite loop.
2. Using the wrong relational operator (e.g., > instead of ≥).
3. Assuming the loop will execute at least once – that is a post‑condition loop (REPEAT … UNTIL).

Comparison with Other Loop Types

Practice Exercise

Write pseudocode for a program that asks the user for a password and keeps prompting until the correct password "Cambridge" is entered. Use a pre‑condition loop.

Solution Sketch

DECLARE input AS STRING
READ input
WHILE input ≠ "Cambridge" DO
OUTPUT "Incorrect, try again."
READ input
END WHILE
OUTPUT "Access granted."

Summary

• A pre‑condition loop tests its condition before each iteration.
• Syntax: WHILE … DO … END WHILE.
• Useful when the loop may need to execute zero times.
• Always ensure the loop condition will eventually become false.