Logic Errors

A logic error is a mistake in the thinking behind a program rather than in the way it is written. The code is syntactically correct, so the computer accepts every line and runs the program right through to the end — but the result is not the one you wanted. The computer has done exactly what it was told to do; the fault is in what you told it to do.

Because the computer simply follows your instructions, no error message usually appears. The program finishes quietly with the wrong answer, the wrong message, or the wrong value stored in a variable.

Where logic errors come from

• Wrong operator — writing > where >= is needed, or + instead of -.
• Wrong condition — testing a score against the wrong pass mark, so every pupil is graded incorrectly.
• Wrong variable — adding the latest price to count when it should have gone into total.
• Wrong order of steps — working out an average before the last value has been added to the total.

Conditions are where logic errors hide most often, because they control both selection and loops. A loop condition that can never become true makes the loop repeat forever, which is a logic error too.

No compiler can find a logic error for you: every line is valid, so there is nothing for it to report. That is the difference from a syntax error, which breaks the grammar of the language and stops the program running at all.

In this program, anyone aged 12 or under should pay the child price of £6, and everyone else should pay £12.

age = int(input())
price = 12
if age >= 13:              # logic error: the condition is back to front
    price = 6
print("Ticket price: " + str(price))

Output when the user types 12:

12
Ticket price: 12

The rule is written back to front, so adults get the child price while children pay full price. Changing the condition to age < 13 puts it right.