What is a trace table in computer science?

A trace table is a technique used to test algorithms to ensure that no logic errors occur. It tracks the values of variables line-by-line as an algorithm executes (known as dry-running).

How do you handle arrays in a Cambridge trace table?

In Cambridge exams, each individual array element gets its own column (e.g., Numbers[1], Numbers[2], Numbers[3]). Remember that Cambridge pseudocode arrays are 1-indexed by default.

Where does output go in a trace table?

The final column of a Cambridge trace table is always labeled 'OUTPUT'. Any value produced by an OUTPUT statement is written in this column on the line execution occurs.

Trace Tables in Pseudocode — IGCSE & A-Level Guide 2026

1. What is a Trace Table?

A Trace Table (also called a dry-run table) is a structured matrix used to track the values of variables as an algorithm executes line by line.

In Cambridge IGCSE Computer Science (0478/0984) Paper 2 and A-Level (9618) Paper 2 & 4, trace tables account for up to 15–20% of your total exam mark. Examiners use trace tables to test whether you truly understand loop boundaries, logic conditions, and variable state updates.

2. Official Cambridge Column Rules

When completing a trace table in exam conditions, follow these strict rules:

1. Scalar Variables Column

Every scalar variable declared or modified (e.g., Count, Total, Flag) receives its own dedicated column in order of declaration.

2. Array Index Columns

If an array is declared, every index must have its own separate column header: List[1], List[2], List[3]. Cambridge pseudocode arrays are 1-indexed!

3. The OUTPUT Column

The final rightmost column is ALWAYS labeled OUTPUT. Whenever the code executes OUTPUT ..., record the exact text or number printed.

3. Step-by-Step Worked Example

Consider the following pseudocode script that calculates the sum of even numbers from 1 to 4:

                        DECLARE Count : INTEGER

                        DECLARE Sum : INTEGER

                        Sum ← 0

                        FOR Count ← 1 TO 4

                            IF Count MOD 2 = 0 THEN

                                Sum ← Sum + Count

                            ENDIF

                        NEXT Count

                        OUTPUT "Final Sum is: ", Sum

Here is the exact Cambridge Trace Table for this script:

Count	Sum	OUTPUT
-	0
1
2	2
3
4	6
-	-	Final Sum is: 6

4. Tracing 1D and 2D Arrays

When tracing an array algorithm (like a Bubble Sort or Linear Search), each element must be recorded in its respective column when modified.

                    DECLARE Scores : ARRAY[1:3] OF INTEGER

                    Scores[1] ← 15

                    Scores[2] ← 25

                    Scores[3] ← Scores[1] + Scores[2]

                    OUTPUT Scores[3]

Scores[1]	Scores[2]	Scores[3]	OUTPUT
15
	25
		40
			40

5. Common Exam Pitfalls & Mistakes

❌ Writing values when nothing changed: Do not repeat unchanged variable values on every row! Only record when a value changes.
❌ Forgetting 1-based indexing: Cambridge arrays start at index 1, not 0.
❌ Missing quotes or text in OUTPUT: Always output the exact printed string including variable values concatenated.
❌ Stopping early on FOR loops: Make sure to complete all loop iterations through NEXT Count.

6. Interactive Trace Table Practice

Ready to test your trace table skills? Use our dedicated Trace Table Practice environment to dry-run exam pseudocode with instant answer checking!

Practice Trace Tables Now

Solve exam-style trace tables, fill matrix values row-by-row, and get real-time feedback with solution explanations.

📊 Start Trace Table Practice ➔ 💻 General Practice Exercises ➔

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