Comparing Error Types

Not every mistake in a program is the same type. You can compare the three types of error by when the computer finds them and by what happens to the program as a result.

Error type	When it is found	Effect on the program	Example
Syntax error	Before execution, when the code is checked	The program will not run at all until it is fixed	A missing colon after `if score > 10`
Runtime error	During execution, when the faulty line is reached	The program starts, then stops at that line	Using `players[5]` when the list holds only three names
Logic error	Not found automatically; you notice it by testing	The program runs to the end but gives the wrong result	Printing `loser` when you meant to print `winner`

A syntax error breaks the rules of the language, so the code cannot be translated into machine code. Nothing runs: not even the lines before the mistake. A runtime error is different because the code is valid enough to start. The program may work for some inputs and then crash when an impossible operation is attempted. A logic error does not crash the program. The computer follows your instructions exactly, but the instructions themselves are wrong, so the output is incorrect and no error message appears.

Syntax, runtime and logic errors on a timeline: syntax errors are found when the code is checked, runtime errors when the faulty line runs, and logic errors only show as wrong output.

One program can contain more than one error, and they may be of different types. Fixing a syntax error might let the program run for the first time, revealing a runtime error that was hidden. After that is fixed, testing could show a logic error. Each fault has to be identified and corrected separately.

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