O LevelComputer ScienceCambridge (CIE)Revision Notes7. Algorithm Design & Problem-SolvingIdentifying ErrorsIdentifying Errors

Identifying Errors (Cambridge (CIE) O Level Computer Science)

Revision Note

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Robert Hampton

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Computer Science Content Creator

Identifying errors

  • Designing algorithms is a skill that must be developed and when designing algorithms, mistakes and issues will occur

  • Trace tables can also help to find any kind of error in a program or algorithm

  • There are three main categories of errors that when designing algorithms a programmer must be able to identify & fix, they are:

    • Syntax errors

    • Logic errors

    • Runtime errors

Syntax Errors

What is a syntax error?

  • A syntax error is an error that breaks the grammatical rules of a programming language and stops it from running

  • Examples of syntax errors are:

    • Typos and spelling errors 

    • Missing or extra brackets or quotes

    • Misplaced or missing semicolons

    • Invalid variable or function names

    • Incorrect use of operators

    • Incorrectly nested loops & blocks of code

Python code - with syntax errors

def generate_username(first_name, last_name)

"""

Generates a username based on the provided first and last name.

Inputs:

first_name (str): User's first name.

last_name (str): User's last name.

Returns:

str: A username combining initials and the first 3 characters of the last name.

"""

username = f"{first_name[0]}{last_name[0]}{last_name[:3]}"

return username

def main():

# -----------------------------------------------------------------------

# Prompts the user for personal information and displays a suggested username

# -----------------------------------------------------------------------

first_name = imput("Enter your first name: ")

last_name = input("Enter your last name: ")

usdrname = generate_username(first_name, last_name)

print(f"Here's a suggested username: {username}")

# ------------------------------------------------------------------------

# Main program

# ------------------------------------------------------------------------

main

Python code - without syntax errors

def generate_username(first_name, last_name):

"""

Generates a username based on the provided first and last name.

Inputs:

first_name (str): User's first name.

last_name (str): User's last name.

Returns:

str: A username combining initials and the first 3 characters of the last name.

"""

username = f"{first_name[0]}{last_name[0]}{last_name[:3]}"

return username

def main():

# -----------------------------------------------------------------------

# Prompts the user for personal information and displays a suggested username

# -----------------------------------------------------------------------

first_name = input("Enter your first name: ")

last_name = input("Enter your last name: ")

username = generate_username(first_name, last_name)

print(f"Here's a suggested username: {username}")

# ------------------------------------------------------------------------

# Main program

# ------------------------------------------------------------------------

main()

Syntax errors

  1. Missing semicolon def generate_username(first_name, last_name)

  2. Incorrectly spelt function name first_name = imput("Enter your first name: ")

  3. Typo in variable name usdrname = generate_username(first_name, last_name)

  4. Missing parenthesis main

Logic Errors

What is a logic error?

  • A logic error is where incorrect code is used that causes the program to run, but produces an incorrect output or result

  • Logic errors can be difficult to identify by the person who wrote the program, so one method of finding them is to use 'Trace Tables'

  • Examples of logic errors are:

    • Incorrect use of operators  (< and >)

    • Logical operator confusion (AND for OR)

    • Looping one extra time

    • Indexing arrays incorrectly (arrays indexing starts from 0)

    • Using variables before they are assigned

    • Infinite loops

Python code

def calculate_area(length, width):

"""

Calculates the area of a rectangle

Inputs:

length (float): The length of the rectangle (positive value)

width (float): The width of the rectangle (positive value)

Returns:

float: The calculated area of the rectangle

Raises:

ValueError: If either length or width is non-positive

"""

if length < 0 or width < 0:

raise ValueError("Length and width must be positive values.")

area = length * width

return area

def main():

# -----------------------------------------------------------------------

Prompts the user for rectangle dimensions and prints the calculated area

# -----------------------------------------------------------------------

try:

length = float(input("Enter the length of the rectangle: "))

width = float(input("Enter the width of the rectangle: "))

# Call the area calculation function

area = calculate_area(length, width)

print(f"The area of the rectangle is approximately {area:.2f} square units.")

except ValueError as error:

print(f"Error: {error}")

# -----------------------------------------------------------------------

# Main program

# -----------------------------------------------------------------------

main()

Logic errors

Test number

Test data

Expected outcome

Actual outcome

Changes needed? (Y/N)

1

Length = 5

Width = 5

"The area of the rectangle is approximately 25 square units."

"The area of the rectangle is approximately 25.00 square units."

N

2

Length = 10

Width = 0

"Length and width must be positive values."

"The area of the rectangle is approximately 0.00 square units."

Y
should not accept 0 input as not positive

  • Logic error located on line if length < 0 or width < 0:

  • Logic error identified in expression < 0, should be <= 0 so that 0 is not accepted as valid input for length or width

Runtime Errors

What is a runtime error?

  • A runtime error is where an error causes a program to crash

  • Examples of runtime errors are:

    • Dividing a number by 0

    • Index out of the range of an array

    • Unable to read or write a drive

Python code

def calculate_area(length, width):

"""

Calculates the area of a rectangle

Inputs:

length (float): The length of the rectangle (positive value)

width (float): The width of the rectangle (positive value)

Returns:

float: The calculated area of the rectangle

Raises:

ValueError: If either length or width is non-positive

"""

if length < 0 or width < 0:

raise ValueError("Length and width must be positive values.")

area = length * width

return area

def main():

# -----------------------------------------------------------------------

Prompts the user for rectangle dimensions and prints the calculated area

# -----------------------------------------------------------------------

try:

length = float(input("Enter the length of the rectangle: "))

width = float(input("Enter the width of the rectangle: "))

# Call the area calculation function

area = calculate_area(length, width)

print(f"The area of the rectangle is approximately {area:.2f} square units.")

except ValueError as error:

print(f"Error: {error}")

# -----------------------------------------------------------------------

# Main program

# -----------------------------------------------------------------------

main()

Runtime errors

Test number

Test data

Expected outcome

Actual outcome

Changes needed? (Y/N)

1

Length = 10

Width = 10

"The area of the rectangle is approximately 100 square units."

"The area of the rectangle is approximately 100.00 square units."

N

2

Length = "abc"

Width = 0

"Program could not convert string to float, try again"

Program crashed

Y
should give error message and ask user to enter again

  • Runtime error located in

try:

length = float(input("Enter the length of the rectangle: "))

width = float(input("Enter the width of the rectangle: "))

# Call the area calculation function

area = calculate_area(length, width)

print(f"The area of the rectangle is approximately {area:.2f} square units.")

except ValueError as error:

print(f"Error: {error}")

  • Runtime error identified as missing iteration (while loop) so program does not ask user to enter width and height again

  • Corrected code now includes a while loop

while True:

try:

length = float(input("Enter the length of the rectangle: "))

width = float(input("Enter the width of the rectangle: "))

# Call the area calculation function

area = calculate_area(length, width)


print(f"The area of the rectangle is approximately {area:.2f} square units.")

break # Exit the loop if successful

except ValueError as error:

print(f"Error: {error}")

print("Please enter positive values for length and width.")

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    Robert Hampton

    Author: Robert Hampton

    Rob has over 16 years' experience teaching Computer Science and ICT at KS3 & GCSE levels. Rob has demonstrated strong leadership as Head of Department since 2012 and previously supported teacher development as a Specialist Leader of Education, empowering departments to excel in Computer Science. Beyond his tech expertise, Robert embraces the virtual world as an avid gamer, conquering digital battlefields when he's not coding.