1.2.4Tracing algorithms and trace tables

Edexcel GCSE Computer Science (1CP2) · Computational thinking › Algorithms

Practise Tracing algorithms and trace tables. 12 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.

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Sample questions

Written for this site in the style of Edexcel exam questions. They are not taken from real past papers.

Question 1Easy6 marks

Look at this program.

01 a = 8
02 b = 3
03 temp = a
04 a = b
05 b = temp
06 print(a, b)

(a) Complete the trace table in {table} to show the values of the variables after each line has run.[4]

[object Object]

(b) State the output of the program.[1]

Show the answer and mark scheme

(a) Answer: 02: a = 8, b = 3; 03: temp = 8; 04: a = 3; 05: b = 8 (so a = 3, b = 8, temp = 8 at the end)

line 02: b = 3 (a is still 8)
line 03: temp = 8
line 04: a = 3
line 05: b = 8

(b) Answer: 3 8

3 8

(c) Answer: It keeps a copy of the original value of a. Without it, this value would be lost when line 04 runs, so the values could not be swapped.

to keep a copy of the original value of a (which is overwritten in line 04) so the values can be swapped

Question 2Medium6 marks

A program processes the times, in seconds, taken by five runners in a race.

01 times = [58, 63, 49, 71, 55]
02 fast = 0
03 total = 0
04 for time in times:
05     total = total + time
06     if time < 60:
07         fast = fast + 1
08 average = total / len(times)
09 print(fast, average)

(a) Complete the trace table in {table}. Each row shows the values at the end of one pass through the loop.[3]

[object Object]

(b) State the output of the program.[2]

Show the answer and mark scheme

(a) Answer:

time | total | fast
-----+-------+-----
     | 0     | 0
58   | 58    | 1
63   | 121   | 1
49   | 170   | 2
71   | 241   | 2
55   | 296   | 3

time column: 58, 63, 49, 71, 55
total column: 58, 121, 170, 241, 296
fast column: 1, 1, 2, 2, 3

(b) Answer: 3 59.2

3
59.2

real / float

Question 3Hard6 marks

Look at this program.

01 for row in range(1, 5):
02     line = ""
03     for col in range(row, 5):
04         line = line + str(col)
05     print(line)

(a) State the output of the program.[3]

(b) State the total number of times line 04 is executed.[1]

Show the answer and mark scheme

(a) Answer:

1234 as the first line
234 then 34
4 as the last line, with no other lines

(b) Answer: 10 (4 + 3 + 2 + 1)

1 then 12
123 then 1234

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