How to Answer Trace Table Questions (OCR A Level Computer Science): Revision Note

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

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

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

Final answer