Error Detection, Debugging & Algorithm Correction

A central objective of Cambridge IGCSE Paper 2 is assessing a student's ability to inspect algorithms, detect deliberate flaws, explain the nature of the error, and provide the exact corrected code.

1. The Three Error Classes

2. Common Algorithm Bugs to Look For in Paper 2

1. Incorrect Initialisation:

• Total initialized inside the loop (resets every pass).
• Min initialized to 0 (locks minimum at 0 for positive numbers).
• Max initialized to 999 (locks maximum at 999).

1. Off-by-One Loop Bounds:

• Loops running 1 TO N - 1 when all $N$ items need processing.
• Loops running 1 TO N during bubble sort comparisons, causing Array[i + 1] to access index $N + 1$ (Array Index Out of Bounds runtime crash).

1. Loop Condition Inversion:

• Using < instead of > or <= instead of >=.
• In REPEAT ... UNTIL, using the continuation condition instead of the termination condition.

1. Sentinel Inclusion:

• Reading sentinel values and adding them to the total before testing the condition.

Written practice and self-review

Consider this flawed algorithm intended to calculate the sum of 10 input numbers:

1  FOR Counter <- 1 TO 10
2      Total <- 0
3      INPUT Number
4      Total <- Total + Number
5  NEXT Counter
6  OUTPUT "Final Total: ", Total

1. Identify the line number containing the logic error and explain what problem it causes during execution.
2. State how to correct this error.

Write an explanation before checking the theory. Check that each claim answers the question, uses the correct concept and explains why it applies. Discuss uncertain points with a teacher. The structured exercise below checks specific facts; it does not automatically mark this explanation.