Paper 2: Algorithm Tracing and Trace Table Completion

Trace tables are used to track how variables change as an algorithm executes. Completing trace tables is a common exam skill in OCR J277 Paper 2, typically worth 3-6 marks. This lesson teaches you how to trace through code systematically and avoid common errors.

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What Is a Trace Table?

A trace table is a table with a column for each variable and each output. Each row represents one step of the algorithm (usually one line of code or one iteration of a loop). By filling in the table, you can see exactly how the algorithm behaves.

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How to Complete a Trace Table

1. Create columns for every variable mentioned in the code, plus an "Output" column
2. Start with the initial values of variables
3. Execute each line of code in order, updating the relevant column
4. For loops, create a new row for each iteration
5. Only update a variable when the code explicitly changes it
6. Record any output (e.g., print statements) in the Output column

OCR Exam Tip: Cross out each line of code as you trace through it (on the question paper) to keep track of where you are. For loops, use a tally to count iterations.

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Worked Example 1: Simple Loop

x = 1
total = 0
for i = 1 to 4
    total = total + x
    x = x * 2
next i
print(total)

Step	i	x	total	Output
Init	—	1	0
i=1	1	1	1
	1	2	1
i=2	2	2	3
	2	4	3
i=3	3	4	7
	3	8	7
i=4	4	8	15
	4	16	15
End				15

The output is 15.

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Worked Example 2: While Loop with Input

count = 0
total = 0
num = input("Enter a number (0 to stop): ")
while num != 0
    total = total + num
    count = count + 1
    num = input("Enter a number (0 to stop): ")
endwhile
print(total / count)

If the user enters: 10, 20, 30, 0