Identifying Errors

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Notes

Types of Errors

Three main categories: **syntax errors**, **logic errors**, and **runtime errors**.
Syntax errors break grammatical rules of the programming language and **prevent the program from running**.
Logic errors cause the program to run but **produce incorrect output**.
Runtime errors **cause the program to crash** during execution.
Trace tables help identify all types of errors by simulating program execution.

Syntax Errors

Examples: missing brackets, misplaced semicolons, typos in variable names, incorrect operator use.
Example: `def generate_username(first_name, last_name)` missing colon (`:`) at end.
Example: `first_name = imput(...)` misspelling `input` as `imput`.
Example: `usdrname = ...` typo in variable name.
Example: `main` missing parentheses `()` to call the function.

Logic Errors

Program runs but gives wrong results due to flawed logic.
Examples: incorrect comparison operators (`<` instead of `≤`), using `AND` instead of `OR`, off-by-one loops, array index errors.
Example: `if length < 0 or width < 0:` should be `≤ 0` to reject zero as non-positive.
Logic errors are often found using **trace tables** and test data.

Runtime Errors

Errors that occur during program execution, causing a crash.
Examples: division by zero, array index out of bounds, file not found, type conversion failure.
Example: entering `"abc"` for a float input crashes without a loop to retry.
Fix: wrap input in a `while True` loop with try-except to handle invalid input gracefully.

Using Trace Tables

A trace table records the state of variables at each step of algorithm execution.
Helps detect logic errors by comparing expected vs actual outcomes.
Each column represents a variable; each row shows values after a line of code.
Useful for dry-running algorithms in exams.

Writing & Amending Algorithms

Algorithmic thinking uses **abstraction** and **decomposition** to break problems into steps.
Plan algorithm in parts: declare variables, input, process, loop, output.
Use variable names and data exactly as given in the question to avoid losing marks.
Comment code to explain logic and help examiners award marks.
Pseudocode has no strict syntax, but follow IGCSE specification for consistency.

Exam Tips

Describe syntax errors as 'prevents the program from running'.
Describe logic errors as 'produces incorrect output'.
Describe runtime errors as 'causes the program to crash'.
Name the error type, locate it, and explain its impact on output/execution.
Use scrap paper to draft algorithms before writing final answer.

Example series circuit for testing logic: if switch is open, bulb off; if closed, ammeter reads current.

Practice questions

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1.Which type of error prevents a program from running?
Easy
- ASyntax error
- BLogic error
- CRuntime error
- DOff-by-one error
2.Which type of error causes a program to crash while it is running?
Easy
- ASyntax error
- BLogic error
- CRuntime error
- DCompilation error
3.A program runs but produces an incorrect result. What type of error is most likely present?
Medium
- ASyntax error
- BLogic error
- CRuntime error
- DInput error
4.Which of the following is an example of a syntax error?
Medium
- AUsing a variable before it is assigned
- BDividing by zero
- CMissing a colon at the end of a function definition
- DAn array index going out of bounds
5.In the following pseudocode, what type of error is present? FOR i ← 1 TO 10 OUTPUT i NEXT i (Assume the loop should output numbers 1 to 10, but it outputs 1 to 11.)
Hard
- ASyntax error
- BLogic error
- CRuntime error
- DNo error
6.Which of the following is NOT a common type of error in programming?
Easy
- ASyntax error
- BLogic error
- CRuntime error
- DDesign error
7.A programmer writes: IF age > 18 THEN OUTPUT 'Adult' ELSE OUTPUT 'Child'. The program always outputs 'Child' even when age is 20. What type of error is this?
Medium
- ASyntax error
- BLogic error
- CRuntime error
- DTypo error
8.A program crashes when the user enters a string instead of a number. This is an example of a:
Hard
- ASyntax error
- BLogic error
- CRuntime error
- DCompilation error

More questions

9.Which of the following is an example of a runtime error?
Easy
- AMisspelling a variable name
- BUsing = instead of == in a condition
- CDividing by zero
- DForgetting to declare a variable
10.A trace table is used to help identify which type of error?
Medium
- ASyntax errors
- BLogic errors
- CRuntime errors
- DAll of the above
11.Consider the code: x = 5; y = 0; result = x / y. This code will cause a:
Hard
- ASyntax error
- BLogic error
- CRuntime error
- DCompilation error
12.Which of the following best describes a logic error?
Easy
- AThe program crashes when running
- BThe program does not run at all
- CThe program runs but produces wrong output
- DThe program runs too slowly
13.A program is designed to calculate the average of three test scores. The code is: sum = score1 + score2 + score3 average = sum / 3 If the user enters 10, 20, and 30, the program outputs 20.0 as expected. However, if the user enters 0, 0, and 0, the program outputs 0.0. What type of error, if any, is present in this code?
Medium
- ANo error; the program works correctly for all inputs.
- BSyntax error because division by zero is not allowed.
- CLogic error because the average of zeros should be undefined.
- DRuntime error because the program crashes when all scores are zero.
14.A program contains the line: IF score ≥ 50 THEN OUTPUT "Pass" ELSE OUTPUT "Fail". A student enters a score of 50 and the program outputs "Fail". What type of error is this?
Medium
- ASyntax error
- BLogic error
- CRuntime error
- DNo error
15.A program uses an array of size 10 (index 0 to 9). The following loop is intended to output all elements: FOR i = 0 TO 10 OUTPUT array[i] NEXT i What type of error does this cause?
Hard
- ASyntax error
- BLogic error – outputs an extra element beyond the array bounds
- CRuntime error – index out of bounds when i = 10
- DNo error
16.Which of the following best describes a syntax error?
Easy
- AAn error that causes the program to produce incorrect output
- BAn error that breaks the grammatical rules of the programming language and prevents the program from running
- CAn error that occurs during program execution, such as division by zero
- DAn error that occurs when the program runs out of memory
17.A programmer writes the following pseudocode to find the largest of three numbers: IF num1 > num2 THEN IF num1 > num3 THEN OUTPUT num1 ELSE OUTPUT num3 ELSE IF num2 > num3 THEN OUTPUT num2 ELSE OUTPUT num3 If num1 = 5, num2 = 5, num3 = 3, what is the output?
Medium
- A5
- B3
- C5 (but it should output 5, so no error)
- DThe program crashes
18.A program calculates the average of a list of numbers. The code is: sum = 0 FOR i = 1 TO LENGTH(list) sum = sum + list[i] NEXT i average = sum / LENGTH(list) Assuming the list contains 5 numbers, what type of error is present if the list is indexed starting from 0?
Hard
- ASyntax error
- BLogic error – the average will be incorrect because the first element is skipped
- CRuntime error – index out of bounds when i = LENGTH(list)
- DNo error
19.A program reads a number from the user and then computes 10 / number. If the user enters 0, what type of error occurs?
Medium
- ASyntax error
- BLogic error
- CRuntime error
- DNo error
20.Which of the following is an example of a logic error?
Easy
- AUsing a variable before it has been assigned a value
- BMissing a closing bracket
- CDividing by zero
- DTyping 'pritn' instead of 'print'
21.A program uses a while loop that never ends because the condition never becomes false. What type of error is this?
Hard
- ASyntax error
- BLogic error
- CRuntime error
- DNo error
22.A programmer writes the following pseudocode to count down from 5 to 1: FOR i = 5 TO 1 OUTPUT i NEXT i What type of error does this contain?
Medium
- ASyntax error – the FOR loop syntax is incorrect
- BLogic error – the loop will not execute because the start value is greater than the end value
- CRuntime error – the program crashes when i becomes less than 1
- DNo error
23.A program contains the line: x = 5 y = 10 (without any operator between them). What type of error is this?
Easy
- ALogic error
- BRuntime error
- CSyntax error
- DNo error
24.A program is supposed to output the sum of numbers from 1 to 10. The code is: sum = 0 FOR i = 1 TO 10 sum = sum + i NEXT i OUTPUT sum The output is 55, which is correct. However, the programmer notices that if the loop is changed to 'FOR i = 0 TO 10', the output becomes 55 as well. Why is there no error in the original code?
Hard
- AThe original code has a logic error, but it accidentally gives the correct answer.
- BThe original code is correct; starting from 1 to 10 sums the numbers 1 through 10 inclusive, giving 55.
- CThe original code has a syntax error because the loop variable should start at 0.
- DThe original code has a runtime error because i is not declared.
25.A program is designed to calculate the average of a list of numbers. The code is: sum = 0 FOR i = 1 TO LENGTH(list) sum = sum + list[i] NEXT i average = sum / LENGTH(list) The program runs but the average is always too high. What type of error is most likely present?
Medium
- ALogic error
- BSyntax error
- CRuntime error
- DOff-by-one error
26.A program uses an array of size 10 (index 0 to 9). The following loop is intended to output all elements: FOR i = 0 TO 10 OUTPUT array[i] NEXT i What type of error occurs?
Hard
- ARuntime error
- BSyntax error
- CLogic error
- DOff-by-one error
27.A program calculates the average of three test scores. The code is: sum = score1 + score2 + score3 average = sum / 3 If the user enters 10, 20, and 30, the output is 20.0, which is correct. However, if the user enters 10, 20, and 'abc', the program crashes. What type of error is this?
Medium
- ARuntime error
- BSyntax error
- CLogic error
- DInput error
28.A programmer writes the following pseudocode to find the largest of three numbers: IF num1 > num2 THEN IF num1 > num3 THEN OUTPUT num1 ELSE OUTPUT num3 ENDIF ELSE IF num2 > num3 THEN OUTPUT num2 ELSE OUTPUT num3 ENDIF ENDIF If the numbers are 5, 5, and 3, what is the output?
Hard
- A5
- B3
- CNo output
- DError
29.What is a logic error?
Easy
- AAn error that causes the program to crash.
- BAn error that prevents the program from running.
- CAn error that causes the program to run but produce incorrect output.
- DAn error that occurs when dividing by zero.
30.A programmer writes the following pseudocode to calculate the average of three numbers: sum = num1 + num2 + num3 average = sum / 3 OUTPUT average If the user enters 10, 20, and 30, the output is 20.0, which is correct. However, if the user enters 0, 0, and 0, the output is 0.0. What type of error, if any, is present?
Medium
- ASyntax error
- BLogic error
- CRuntime error
- DNo error
31.Which of the following is a common example of a logic error?
Medium
- AMissing a closing bracket.
- BUsing a variable before it is assigned a value.
- CTrying to open a file that does not exist.
- DSpelling a keyword incorrectly.
32.A program uses an array of size 10 (index 0 to 9). The following loop is intended to output all elements: FOR i = 0 TO 10 OUTPUT array[i] NEXT i What type of error does this contain?
Hard
- ASyntax error
- BLogic error
- CRuntime error
- DBoth logic and runtime error
33.Which of the following best describes the use of a trace table?
Hard
- ATo translate high-level code into machine code.
- BTo help identify logic errors by manually tracking variable values.
- CTo automatically fix syntax errors in the code.
- DTo measure the execution time of a program.
34.What type of error causes a program to crash while it is running?
Easy
- ASyntax error
- BLogic error
- CRuntime error
- DDesign error
35.A program is designed to calculate the area of a rectangle. The code includes: if length < 0 or width < 0: raise ValueError("Length and width must be positive values.") area = length×width If the user enters length = 10 and width = 0, the program outputs 0.0 instead of an error message. What type of error is present?
Hard
- ASyntax error
- BLogic error
- CRuntime error
- DNo error
36.A programmer writes the following pseudocode to calculate the average of a list of numbers: sum = 0 FOR i = 1 TO LENGTH(list) sum = sum + list[i] NEXT i average = sum / LENGTH(list) If the list contains the numbers [10, 20, 30], what will be the output?
Medium
- A20
- B30
- C10
- D60
37.A program contains the following code: x = 5 WHILE x > 0 OUTPUT x x = x - 1 ENDWHILE What type of error is present?
Hard
- ASyntax error
- BLogic error
- CRuntime error
- DNo error
38.A program is designed to find the largest number in a list. The code is: largest = 0 FOR i = 1 TO LENGTH(list) IF list[i] > largest THEN largest = list[i] ENDIF NEXT i OUTPUT largest If the list contains [-5, -2, -10], what will be the output?
Medium
- A0
- B-2
- C-5
- D-10
39.A programmer writes the following pseudocode to output the numbers 1 to 5: FOR i = 1 TO 5 OUTPUT i NEXT i The program outputs 1, 2, 3, 4, 5. What type of error is present?
Medium
- ASyntax error
- BLogic error
- CRuntime error
- DNo error
40.A program uses the following code to count the number of even numbers in a list: count = 0 FOR i = 1 TO LENGTH(list) IF list[i] MOD 2 = 0 THEN count = count + 1 ENDIF NEXT i OUTPUT count If the list is [1, 3, 5, 7], what is the output?
Hard
- A0
- B1
- C4
- DError

Slides

Identifying Errors

When designing algorithms, mistakes and issues will occur.
There are three main categories of errors: syntax, logic, and runtime.
Trace tables can help find any kind of error in a program or algorithm.

Syntax Errors

A syntax error breaks the grammatical rules of a programming language and stops it from running.
Examples: typos, missing brackets or quotes, misplaced semicolons, invalid variable names.
Incorrect use of operators or incorrectly nested loops/blocks also cause syntax errors.

Which of the following is a syntax error?

Using a variable before it is assigned
Missing a colon at the end of a function definition
Dividing by zero
Looping one extra time

Logic Errors

A logic error causes the program to run but produce incorrect output.
Examples: incorrect use of operators (< vs >), using AND instead of OR, off-by-one loops.
Logic errors can be found using trace tables or test data.

A program calculates the area of a rectangle. For length=10 and width=0, it outputs 0.00 instead of an error. What type of error is this?

Syntax error
Logic error
Runtime error
No error

Runtime Errors

A runtime error causes the program to crash during execution.
Examples: dividing by zero, index out of range, unable to read/write a drive.
These errors occur when the program tries to perform an impossible operation.

Which of the following is a runtime error?

Missing a closing bracket
Using '=' instead of '==' in a condition
Accessing an array element with an index of -1
Writing a variable name incorrectly

Describing Errors in Exams

Use 'prevents the program from running' for syntax errors.
Use 'produces incorrect output' for logic errors.
Use 'causes the program to crash' for runtime errors.
These exact phrases earn marks in the exam.

A ____ error prevents the program from running, while a ____ error causes the program to crash.

Identifying Errors with Trace Tables

Trace tables help track variable values and output step by step.
They are useful for finding logic errors that produce incorrect results.
Test data with expected and actual outcomes can reveal errors.

Which of these are true about identifying errors?

Syntax errors can be found by running the program.
Logic errors can be found using trace tables.
Runtime errors always produce incorrect output.
Test data helps identify logic errors.

Example: Syntax Errors in Python

Missing colon: def generate_username(first_name, last_name)
Misspelled function: imput instead of input
Typo in variable: usdrname instead of username
Missing parentheses: main instead of main()

In the code 'first____name = imput(...)', the error is a ____ error because 'imput' is not a valid function name.

Example: Logic Error in Area Calculation

Condition 'if length < 0 or width < 0' accepts width=0 as valid.
Should be '≤ 0' to reject zero values.
This causes incorrect output (area = 0) instead of an error.

What change fixes the logic error in the area calculation?

Change '<' to '>'
Change '<' to '≤'
Add a try-except block
Change 'or' to 'and'

Example: Runtime Error and Fix

Entering non-numeric input causes the program to crash.
The fix is to add a while loop around the input to retry on error.
The loop continues until valid input is entered, then breaks.

To handle invalid input, add a ____ loop around the input code.

Writing & Amending Algorithms

Algorithmic thinking is creating step-by-step instructions to solve a problem.
Use abstraction and decomposition to identify each step.
Plan your algorithm in parts before writing final code.
Comment your code to help understanding and earn marks.

Which of these are good practices when writing algorithms in exams?

Use variable names from the question.
Write the algorithm directly without planning.
Comment your code.
Use scrap paper to sketch ideas.

What is the first step in algorithmic thinking?

Write the final code
Test the algorithm
Identify each step using abstraction and decomposition
Output the results

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