Paper 2: Identifying and Correcting Errors in Given Code

A common question type in OCR J277 Paper 2 presents you with a piece of code that contains errors and asks you to identify and fix them. This lesson covers the types of errors you need to know and strategies for spotting them.

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Types of Errors

There are three main types of programming errors:

Error type	Description	Example
Syntax error	The code breaks the rules of the programming language	Missing endif, misspelled keyword, missing bracket
Logic error	The code runs but produces the wrong result	Using + instead of -, wrong comparison operator
Runtime error	The code crashes during execution	Division by zero, accessing an invalid array index

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Syntax Errors

Syntax errors are usually the easiest to spot. The code will not run at all if a syntax error is present.

Common Syntax Errors

Error	Incorrect	Correct
Missing closing keyword	if x > 5 then print("big")	if x > 5 then print("big") endif
Misspelled keyword	wihle x > 0	while x > 0
Missing bracket	print("Hello"	print("Hello")
Assignment vs comparison	if x = 5 then (in some contexts)	if x == 5 then
Wrong string delimiter	print('Hello")	print("Hello")

OCR Exam Tip: When looking for syntax errors, check every line for matching brackets, correct keywords, and proper structure. In OCR pseudocode, every if needs an endif, every for needs a next, and every while needs an endwhile.

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Logic Errors

Logic errors are harder to spot because the code runs without crashing — it just produces the wrong answer.

Common Logic Errors

Error	Description	Example
Wrong operator	Using < instead of >	if age < 18 when you mean if age > 18
Off-by-one	Loop runs one too many or one too few times	for i = 0 to 10 when it should be for i = 0 to 9 for an array of 10 items
Wrong variable	Using the wrong variable in a calculation	total = total + count instead of total = total + score
Wrong initial value	Starting a counter at 1 instead of 0	count = 1 when it should be count = 0
Condition always true/false	A while loop that never terminates or never runs	while x > 0 when x starts at -1 and is never changed

Example: Spot the Logic Error

// This program should find the smallest number in an array
numbers = [5, 3, 8, 1, 4]
smallest = 0
for i = 0 to 4
    if numbers[i] > smallest then
        smallest = numbers[i]
    endif
next i
print(smallest)

Errors:

1. smallest = 0 — should be smallest = numbers[0] (initialising to 0 may give wrong result if all numbers are positive... but actually here, no number is less than 0, so the logic error below is the main issue)
2. numbers[i] > smallest — should be numbers[i] < smallest (we want the smallest, not the largest)