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

The decision tree below helps you classify a bug quickly during an error-spotting question:

flowchart TD
    A[Run or trace the code] --> B{Does the code run at all?}
    B -->|No| C[Syntax error — missing keyword/bracket/spelling]
    B -->|Yes| D{Does it crash partway?}
    D -->|Yes| E[Runtime error — division by zero, array bounds, type mismatch]
    D -->|No| F{Output matches expected?}
    F -->|No| G[Logic error — wrong operator, off-by-one, bad initialisation]
    F -->|Yes| H[No error detected]

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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)

Corrected:

numbers = [5, 3, 8, 1, 4]
smallest = numbers[0]
for i = 1 to 4
    if numbers[i] < smallest then
        smallest = numbers[i]
    endif
next i
print(smallest)

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Runtime Errors

Runtime errors cause the program to crash during execution.

Common Runtime Errors

Error	Cause	Example
Division by zero	Dividing by a variable that is 0	average = total / count when count = 0
Index out of bounds	Accessing an array element that does not exist	names[10] when the array only has 5 elements
Type error	Performing an operation on incompatible types	"Hello" + 5 without type conversion
File not found	Trying to open a file that does not exist	openRead("missing.txt")

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Strategy for Error-Spotting Questions

1. Read the description of what the program should do
2. Trace through the code line by line with sample input
3. Compare the actual output with the expected output
4. Check for each error type systematically:
   • Syntax: missing keywords, brackets, spelling
   • Logic: wrong operators, wrong variables, wrong initial values
   • Runtime: division by zero, array bounds
5. Write the correction clearly — state the line number, the error, and the fix

OCR Exam Tip: When identifying errors, be specific. Do not just say "there is a logic error." Say "on line 5, the operator should be < instead of > because the program is supposed to find the smallest value."

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Worked Example: Multiple Errors

The following program should ask the user for 5 numbers, then display the total:

total = 1
for i = 1 to 6
    num = input("Enter a number: ")
    total = total + num
next
print(total)

Line	Error	Type	Correction
1	total = 1 should be total = 0	Logic	Starting total at 1 gives an incorrect result
2	for i = 1 to 6 should be for i = 1 to 5	Logic	The loop runs 6 times instead of 5
3	num = input(...) should be num = int(input(...))	Runtime/Logic	Input is read as a string, not a number
5	next should be next i	Syntax	The loop variable is missing (in OCR pseudocode)

Corrected program:

total = 0
for i = 1 to 5
    num = int(input("Enter a number: "))
    total = total + num
next i
print(total)

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Testing and Debugging Techniques