GCSEComputer ScienceEdexcelRevision Notes6. Problem Solving with ProgrammingDevelop CodeIdentify, Locate & Correct Program Errors

Identify, Locate & Correct Program Errors (Edexcel GCSE Computer Science)

Revision Note

Robert Hampton

How do you identify, locate & correct program errors?

  • In the exam students are expected to be able to:

    • Fix syntax errors in code

    • Use test data to identify run-time and logic errors which undetected would prevent code from functioning correctly

  • A level-based mark scheme for functionality awards marks for error-free, robust solutions

Examiner Tip

Before working through the examples below, it is important you fully understand the different types of errors in programming. To revise them click here

Syntax Errors

Task

  • Locate the syntax errors and correct them so that the program works

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 & Runtime Errors

Task

  • Use suitable test data to identify and locate logic & runtime errors in the following program

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

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

    Expertise: Computer Science Content Creator

    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.

    James Woodhouse

    Author: James Woodhouse

    Expertise: Computer Science

    James graduated from the University of Sunderland with a degree in ICT and Computing education. He has over 14 years of experience both teaching and leading in Computer Science, specialising in teaching GCSE and A-level. James has held various leadership roles, including Head of Computer Science and coordinator positions for Key Stage 3 and Key Stage 4. James has a keen interest in networking security and technologies aimed at preventing security breaches.