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

flowchart TD
    P[Programming error] --> S[Syntax error]
    P --> L[Logic error]
    P --> R[Runtime error]
    S --> S1["Detected before run<br/>by compiler/interpreter"]
    S --> S2["Examples: missing bracket,<br/>missing colon, typo in keyword"]
    L --> L1["Detected by comparing<br/>actual vs expected output"]
    L --> L2["Examples: wrong operator,<br/>off-by-one, wrong variable"]
    R --> R1["Detected during execution<br/>— program crashes"]
    R --> R2["Examples: divide by zero,<br/>index out of range, type mismatch"]

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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:

scores = [80, 70, 90, 60, 50]
total = 0
for i = 0 to scores.length - 1
    total = total + scores[i]
next i
average = total / 4    // LOGIC ERROR: should divide by 5 (scores.length)
print("Average: " + str(average))

The program runs fine but gives 70 instead of the correct answer 70... actually both give 70 in this case. Let us use a clearer example:

scores = [80, 70, 90, 60]
total = 0
for i = 0 to scores.length - 1
    total = total + scores[i]
next i
average = total / 3    // LOGIC ERROR: should divide by 4
print("Average: " + str(average))
// Outputs 100 instead of correct answer 75

Key Point About Logic Errors

Logic errors are the hardest to find because:

• The program does not crash — it runs "normally"
• No error messages are displayed
• You only notice the error by checking the output against expected results
• Trace tables and testing are the main tools for finding them

OCR Exam Tip: Logic errors are the most commonly asked about in the exam. A typical question shows code with a logic error and asks you to identify it, explain what it does wrong, and suggest a fix. Always trace through the code step by step to find the error.

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Runtime Errors

A runtime error occurs while the program is running, causing it to crash or terminate unexpectedly. The code is syntactically correct but encounters a situation it cannot handle.

Common Runtime Errors

Error	Cause	Example
Division by zero	Dividing a number by zero	result = 10 / 0
File not found	Trying to open a file that does not exist	openRead("missing.txt")
Index out of range	Accessing an array index that does not exist	list[10] when list has 5 items
Type mismatch	Performing an operation on incompatible types	int("hello")
Stack overflow	Infinite recursion	A function that calls itself forever
Out of memory	Program uses more memory than available	Creating an infinitely growing list

Python example:

numbers = [10, 20, 30]
print(numbers[5])  # Runtime error: IndexError — list index out of range

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Comparing the Three Error Types

Feature	Syntax Error	Logic Error	Runtime Error
When detected	Before running	During or after running	During running
Program runs?	No	Yes	Yes, but then crashes
Error message?	Yes — with line number	No	Yes — when it crashes
Difficulty to find	Easy	Hard	Medium
Difficulty to fix	Easy	Can be hard	Medium
Detection method	Compiler/interpreter	Testing and trace tables	Testing