How to Answer Trace Table Questions

How do I answer an OCR A Level Computer Science trace table question?

What is a trace table?

• Trace tables are used to follow the values of variables as a program runs
• These questions test your ability to simulate code line by line and accurately record outputs

Examiner Tips and Tricks

• You must read the code carefully and simulate it exactly as a computer would
• Use a pencil and annotate each step
• The most common mistake is skipping or misreading a line

What you need to know

• You may be asked to complete a trace table, explain a trace table, or spot errors in output
• These questions are normally worth 3–6 marks
• Most commonly appear in questions involving iteration, selection, and arrays/lists
• In most cases, the structure of the table is provided, your task is to complete the missing values

Command words to look out for

Command word	What it means
Complete	Fill in the table with correct values
State	Give the value/output/result (usually a single word or number)
Explain	Say why something happens or changes in the program
Identify	Spot where something goes wrong or doesn’t match expectations

How to tackle a trace table question

Follow these steps to answer trace table questions accurately:

1. Read the code carefully

   Understand what each line does

   Look out for loops, IF statements, and variable changes

2. Use the trace table provided

   Use the headings and row count to guide how many iterations or steps are needed

3. Work line by line

   Simulate the program in your head or on paper

   Update each variable after every change

4. Watch out for variable resets

   Variables declared inside loops often reset every time, don’t carry the wrong value across iterations

5. Show the final output clearly

   If you’re asked for the output, make sure it’s written exactly as it would appear on screen

Worked example

Question

This algorithm checks whether a list of numbers contains a duplicate:

01 numList ← [4, 7, 2, 7]
02 foundDuplicate ← False
03 FOR i ← 0 TO LENGTH(numList) - 2
04     FOR j ← i + 1 TO LENGTH(numList) - 1
05         IF numList[i] = numList[j] THEN
06             foundDuplicate ← True
07         ENDIF
08     NEXT j
09 NEXT i
10 OUTPUT foundDuplicate

Complete the trace table below when the algorithm is run

You may not need to use all the rows in the table

Line number	i	j	numList[i]	numList[j]	foundDuplicate	Output

1. Read the code carefully

   • Line 1: Sets up a list of numbers: [4, 7, 2, 7]
   • Line 2: A flag called foundDuplicate starts as False
   • Lines 3–9: Nested loops compare every pair of numbers in the list
     • If a match is found, foundDuplicate becomes True
   • Line 10: The final result is printed True if a duplicate was found, otherwise False

2. Use the trace table provided

   • The trace table has six rows, showing all the important iterations of the loop and when foundDuplicate changes
   • Some rows may have repeated values if no changes happen
   • If you reach the end and only used 2 or 3 rows, go back, you’ve likely missed a loop iteration

3. Work line by line

   05 IF numList[i] = numList[j] THEN

   4 ≠ 7, so no change

   05 IF numList[i] = numList[j] THEN

   4 ≠ 2, so no change

   05 IF numList[i] = numList[j] THEN

   4 ≠ 7, so no change

   05 IF numList[i] = numList[j] THEN

   4 ≠ 7, so no change

   05 IF numList[i] = numList[j] THEN

   7 = 7 → set foundDuplicate = True

   10 OUTPUT foundDuplicate

   End of loops → output result

4. Watch out for variable resets

   • Each loop changes both i and j
     • i increases from 0 to 2
     • j starts at i + 1 and goes up to 3

5. Show the final output clearly

   10 OUTPUT foundDuplicate

   True

   

Examiner Tips and Tricks

• You don’t need to fill in Line 06 (where the assignment happens), just show the change in foundDuplicate
• Use a dash (–) when a column isn’t relevant (like i, j after the loops finish)
• Add a comment column in the margin if you’re writing by hand, it helps show your thought process if you make a mistake