Types of Errors: Syntax, Logic, and Runtime Errors

This lesson covers the three types of programming errors — syntax errors, logic errors, and runtime errors — as required by OCR J277 Section 2.4. Being able to identify, explain, and fix these errors is essential for the OCR GCSE Computer Science exam.

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Overview of Error Types

Error Type	When Detected	Stops the Program?	Example
Syntax error	Before the program runs (at compile/parse time)	Yes — program will not start	Missing bracket, misspelled keyword
Logic error	During execution (but program does not crash)	No — runs but gives wrong results	Using + instead of -
Runtime error	During execution	Yes — program crashes	Division by zero, file not found

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Syntax Errors

A syntax error occurs when the code breaks the rules (grammar) of the programming language. The interpreter or compiler cannot understand the instruction and refuses to run the program.

Common Syntax Errors

Error	Example (Python)	Fix
Missing colon	if x == 5	if x == 5:
Missing bracket	print("hello"	print("hello")
Misspelled keyword	whle x > 0:	while x > 0:
Missing quotation mark	print("hello)	print("hello")
Wrong indentation	if x == 5:\nprint("yes")	Indent the print line
Using = instead of ==	if x = 5:	if x == 5:

OCR Pseudocode example with syntax errors:

// INCORRECT — missing 'then' and 'endif'
if mark >= 50
    print("Pass")

// CORRECT
if mark >= 50 then
    print("Pass")
endif

Key Point About Syntax Errors

Syntax errors are the easiest to fix because:

• The compiler/interpreter tells you where the error is (line number)
• The program cannot run until they are fixed
• They are caused by typos or forgetting syntax rules

OCR Exam Tip: When asked to identify a syntax error in code, look for: missing brackets, missing colons (Python), misspelled keywords, missing then/endif (pseudocode), and incorrect indentation. Always state what the error is AND how to fix it for full marks.

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Logic Errors

A logic error occurs when the program runs without crashing but produces incorrect results. The code is syntactically valid but the algorithm or logic is wrong.

Common Logic Errors

Error	Incorrect Code	Correct Code
Wrong operator	average = total * count	average = total / count
Wrong comparison	if age > 18 (excludes 18)	if age >= 18 (includes 18)
Off-by-one	for i = 0 to 10 (11 items)	for i = 0 to 9 (10 items)
Wrong variable	total = total + price (should be quantity * price)	total = total + quantity * price
Infinite loop	while x > 0\n print(x) (x never decreases)	Add x = x - 1 inside the loop
Condition always true	while true (no exit condition)	Add a condition that can become false

Example — a logic error that calculates the wrong average: