Identifying and Correcting Errors

Types of errors

Error	What happens	Example
syntax error	the rules of the language are broken; the program does not run	a missing bracket
runtime error	the program fails while running	dividing by zero, an index outside a list
logic error	the program runs but gives the wrong result	using > instead of ≥
overflow error	a number is too large for the space reserved for it	adding past the largest value

Logic errors are the hardest to find because nothing crashes.

Finding errors

• Test cases: choose inputs and the expected outputs, including boundary and unusual values (0, negative numbers, empty lists, the largest allowed value).
• Hand tracing: follow the code line by line, recording variable values in a table.
• Visualizations and debuggers: step through code.
• Adding output statements to display intermediate values.

Example of a logic error

The procedure should count the scores of at least 50:

count ← 0
FOR EACH s IN scores
{
  IF (s > 50)
  {
    count ← count + 1
  }
}

A score of exactly 50 is not counted. The condition should be s ≥ 50. A test case containing the boundary value 50 reveals the error.

Quick summary

• Syntax errors stop the program; logic errors give wrong answers silently.
• Test boundaries and unusual inputs.
• Hand-trace with a table of values.