Totalling, Counting & Sentinel Input Loops

Two of the most frequently examined standard algorithmic operations in Cambridge IGCSE Computer Science are Totalling (accumulating sums) and Counting (tracking occurrences).

1. Totalling vs Counting

2. Sentinel Values and Rogue Values

When the exact number of items to be entered is unknown in advance, a Sentinel Value (also called a rogue value) is used to terminate input:

• Common examples: -1 (for positive numbers), 999, "XXX", or "STOP".
• Exam Rule: The sentinel value must never be included in the calculation or added to the counter!

The Priming Read Pattern:

To prevent the sentinel value from polluting the accumulator, developers use the "priming read" pattern:

Total <- 0
Count <- 0

INPUT Value             // 1. Priming read before loop starts

WHILE Value <> -1 DO    // 2. Test condition BEFORE processing
    Total <- Total + Value
    Count <- Count + 1
    INPUT Value         // 3. Read subsequent value at end of loop body
ENDWHILE

If the user enters -1 immediately on the very first prompt, the WHILE condition evaluates to FALSE, the loop never executes, and Total correctly remains 0.