What is an Exception?

An exception is an error condition that arises during program execution — an event that disrupts normal program flow. Unlike syntax errors (detected at compile time), exceptions are runtime errors.

Examples of exceptions:

• Division by zero — attempting to divide a number by 0
• Invalid input type — user enters a letter when a number is expected
• File not found — attempting to open a file that doesn't exist
• Index out of range — accessing array index beyond its bounds
• Stack overflow — typically from infinite recursion

Exception Handling in AQA Pseudocode

AQA 7517 uses a TRY … EXCEPT … ENDTRY construct for exception handling:

TRY
    // Code that might raise an exception
    INPUT num
    result ← 100 / num
    OUTPUT result
EXCEPT
    // Code to handle the exception
    OUTPUT "Error: division by zero or invalid input"
ENDTRY

The TRY block contains code that might raise an exception. If an exception occurs, execution immediately jumps to the EXCEPT block, which handles it gracefully. Code after the exception in the TRY block is skipped.

Why Use Exception Handling?

• Prevents program crash — without handling, an exception terminates the program abruptly
• Provides meaningful feedback — a friendly error message rather than a cryptic runtime error
• Allows recovery — the program can continue or retry after handling an exception
• Separation of concerns — error-handling logic is separate from normal logic

Difference Between Error Types

Exception Handling in Python (for context)

AQA also expects awareness of how exceptions work in Python:

try:
    num = int(input("Enter a number: "))
    result = 100 / num
    print(result)
except ZeroDivisionError:
    print("Cannot divide by zero")
except ValueError:
    print("Please enter a valid number")

Python allows catching specific exception types — each except clause handles a different type. AQA pseudocode uses a single EXCEPT block.

FINALLY Block (Python)

Python also has a finally block that always executes regardless of whether an exception occurred — useful for cleanup (closing files, releasing resources):

try:
    file = open("data.txt")
finally:
    file.close()  # Always closes the file