6.1.5 Identifying and correcting errors

Three Types of Error in Programs

The three types of programming error are syntax, runtime and logic. Each requires a different approach to locate and correct. This benchmark focuses on identifying and fixing them in actual code.

Error type	When detected	Program runs?	How to find it
Syntax	Before execution (compile/parse time)	No	Error message shows line number and type
Runtime	During execution	Crashes partway	Exception message; trace to the line that executes
Logic	During testing	Yes, but wrong output	Trace tables; test data; print statements; debugger

Worked Examples

# Python — Syntax errors prevent execution
score = int(input("Enter score: ")
if score >= 50              # Missing colon
    print("Pass")
else:
    print("Fail")

Errors: (1) missing closing parenthesis ) on line 1; (2) missing colon : after the if condition on line 2. Python raises a SyntaxError and shows the approximate line. The program will not start until both are fixed.

# Fixed
score = int(input("Enter score: "))
if score >= 50:
    print("Pass")
else:
    print("Fail")

# Python — Runtime errors occur during execution
numbers = [10, 20, 30]
index = int(input("Enter index: "))
print(numbers[index])         # IndexError if index >= 3 or < -3

total = int(input("Total: "))
people = int(input("People: "))
share = total / people        # ZeroDivisionError if people == 0

Issues: no bounds check before accessing numbers[index]; no guard against dividing by zero.

# Fixed — add guards before the risky operations
numbers = [10, 20, 30]
index = int(input("Enter index (0-2): "))
if index >= 0 and index < len(numbers):
    print(numbers[index])
else:
    print("Index out of range")

total = int(input("Total: "))
people = int(input("People: "))
if people == 0:
    print("Cannot divide by zero")
else:
    share = total / people
    print("Share:", share)

# Python — Logic error: runs but gives wrong output
# Intended: calculate the average of a list
scores = [70, 85, 60, 90]
total = 0
for score in scores:
    total = total + score
average = total / 5          # BUG: divides by 5 but list has 4 items
print("Average:", average)   # Prints 61.25 instead of 76.25

Issue: hardcoded divisor 5 does not match the list length 4. No error message — the wrong answer is silently produced.

# Fixed — use len() to get the actual list length
scores = [70, 85, 60, 90]
total = 0
for score in scores:
    total = total + score
average = total / len(scores)   # Always correct regardless of list size
print("Average:", average)      # Output: 76.25

Key Takeaways

Syntax: breaks language rules — program will not run; error message gives line number.
Runtime: crashes during execution — add guards before risky operations (index access, division).
Logic: runs but gives wrong output — use trace tables, test with known inputs, use len() not hardcoded sizes.
Never hardcode the length of a list — use len() so the code works correctly if the list changes size.