Identifying Errors

Exam code: 2210

Written by: Ashika|Reviewed by: Caroline Carroll|Updated 2 July 2026

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) 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): 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
Missing semicolon `def generate_username(first_name, last_name)` Incorrectly spelt function name `first_name = imput("Enter your first name: ")` Typo in variable name `usdrname = generate_username(first_name, last_name)` 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()

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

Test number

Test data

Expected outcome

Actual outcome

Changes needed? (Y/N)

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."

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()

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

Test number

Test data

Expected outcome

Actual outcome

Changes needed? (Y/N)

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."

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.")