Testing algorithms with trace tables

A trace table tests an algorithm on paper before it becomes a program. You play the computer: carry out each step in order and write down every value a variable takes. This dry run (or desk check) shows whether the algorithm gives the right output, and Paper 1 asks for that output almost every year.

By the end of this page you can

• Set up a trace table with the right columns
• Trace assignments, IF statements and loops
• Fill in a trace table row by row
• Work out an algorithm’s output

Goals tick themselves off as you try the activities below.

Photo: ArnoldReinhold, CC BY-SA 4.0, via Wikimedia Commons. Cropped, student’s name blurred, resized, JPEG q72.

Before computers were close at hand, every program was checked line by line on paper. Exams still ask you to.