Designing, creating and refining algorithms: Identify common errors

The idea

Identifying common errors means reading an algorithm and finding where it will not do what is intended. You check each line against the purpose: syntax errors break the rules of the language, such as a missing bracket or a misspelled keyword; logic errors run but give wrong output, such as using < instead of > or adding instead of subtracting; runtime errors stop the program, such as dividing by zero or reading past the end of an array. A useful method is to trace a small input by hand, compare the result with the expected result, then fix the first error and retest. In an exam you may be shown pseudocode or a program and asked to state the error and the correction, so name the line and explain the effect.

What earns the marks

• Distinguishes syntax, logic and runtime errors and gives a correct example of each in pseudocode or a high-level language.
• Locates an error by tracing a short input through the algorithm and comparing the actual output with the expected output.
• Explains the effect of an error, such as an infinite loop, an incorrect total, or a program crash, rather than only naming the line.
• States a valid correction, for example changing a comparison operator, swapping two statements, or adding a missing initialisation.
• Checks boundary cases such as an empty list, a zero value, or the first and last items, where errors often appear.
• Uses test data that includes normal, boundary and erroneous values to expose different faults.

Common mistakes and how to put them right

• Confusing a logic error with a syntax error.

  Put it right: A syntax error prevents the program running at all, while a logic error runs but produces wrong output.

• Assuming an algorithm is correct because it works for one input.

  Put it right: Test several inputs, including boundary values, before deciding there is no error.

• Changing many lines at once when correcting an algorithm.

  Put it right: Fix one error, retest, then continue so you can see which change solved the problem.

Exam tips

• Quote the line or statement that contains the error, then state the correction separately so both parts are clear.
• Use the words syntax, logic and runtime accurately; do not use them as synonyms.
• When asked to identify an error, describe what the code does now and what it should do instead.