AQA 8525 · Section 3.1 · 3.1.1c

Trace
Tables

Dry-running algorithms · Tracking variable values · Spotting errors

CSZone AQA GCSE Computer Science 8525

Learning Objectives

By the end of this lesson you will be able to...

Explain what a trace table is and why it is used

Complete a trace table for a given algorithm, tracking all variable values

Identify the final output of an algorithm by dry-running it

Use trace tables to find bugs in an algorithm

Core Concept

What is a Trace Table?

DEFINITION

A table used to dry-run (manually execute) an algorithm, recording the value of each variable after every step. Used to check whether code produces the correct output without running it on a computer.

COLUMNS

One column per variable + one for output. Header row names each variable.

ROWS

One row per line of code executed (or per iteration of a loop).

Worked Example

Trace This Algorithm

x ← 1
y ← 10
WHILE x < 4
y ← y - x
x ← x + 1
ENDWHILE
OUTPUT y

x	y	OUTPUT
1	10
2	9
3	7
4	4	4

Loop ends when x = 4 (not < 4)

Method

How to Complete a Trace Table

Step 1:Create columns — one for each variable that changes, plus an OUTPUT column

Step 2:Work through the algorithm line by line — only update a cell when that variable changes

Step 3:For loops — add a new row for each iteration, updating the relevant values

Step 4:Only write to the OUTPUT column when an OUTPUT statement is executed

Common Mistakes

Errors to Avoid

Filling every cell — only update a variable's column when IT specifically changes

Off-by-one on loops — check whether the condition uses < or ≤ carefully

Forgetting the final OUTPUT — the output column is only updated on OUTPUT statements

⚡ Exam Tip:Trace tables are worth 3–6 marks on AQA Paper 1. Practice until it feels mechanical.

Exam Practice

Complete the trace table

AQA-style question

Trace the algorithm below. Complete the table showing values of a, b, and OUTPUT.

a ← 3
b ← 1
FOR i ← 1 TO 3
b ← b * a
ENDFOR
OUTPUT b

3 marks

ANSWER: b = 3, 9, 27 → OUTPUT: 27

Each iteration: b = b × 3. After 3 iterations: 1 × 3 × 3 × 3 = 27

Key Takeaways

What to Remember

A trace table manually executes an algorithm and records variable values at each step

Columns = one per variable + OUTPUT. Rows = one per executed line (or loop iteration)

Only update a cell when that specific variable changes

Trace tables help find logic errors — the output tells you if the algorithm is correct