Sorting algorithms: bubble & merge sort – GCSE Computer Science exam questions with mark scheme

1Grade 4–54 marks

A bubble sort is used to put this list into ascending order:

[7, 3, 9, 1, 5]

(a) Show the list after the first pass of the bubble sort. [2]

(b) State how many swaps are made during the first pass. [1]

(a) 9 is in the last position(1)
(a) Whole list correct: [3, 7, 1, 5, 9](1)
(b) 3(1)
(c) A complete pass is made with no swaps (accept: after n − 1 passes, where n is the number of items)(1)
Answer: (a) [3, 7, 1, 5, 9] (b) 3 (c) a pass is completed with no swaps
Examiner tip: A pass compares every adjacent pair once. Do not stop after the first swap.

2Grade 6–76 marks

A merge sort is used to sort this list into ascending order:

[6, 2, 8, 4, 1, 7, 3, 5]

(a) Show the stages of the merge sort. [4]

(b) Give one advantage of a merge sort compared with a bubble sort and one advantage of a bubble sort compared with a merge sort. [2]

(a) List split (repeatedly) into single items: [6] [2] [8] [4] [1] [7] [3] [5](1)
(a) Merged into ordered pairs: [2, 6] [4, 8] [1, 7] [3, 5](1)
(a) Merged into ordered groups of four: [2, 4, 6, 8] [1, 3, 5, 7](1)
(a) Final merge: [1, 2, 3, 4, 5, 6, 7, 8](1)
(b) Merge sort: usually much quicker / more time-efficient (especially for large lists)(1)
(b) Bubble sort: uses less memory (sorts in place / no extra lists needed) / simpler to code. Accept: can be quicker on a list that is already (almost) sorted(1)
Answer: Split to singles → [2,6][4,8][1,7][3,5] → [2,4,6,8][1,3,5,7] → [1,2,3,4,5,6,7,8]
Examiner tip: Show the splitting AND every merge level; jumping straight to the sorted list scores only the final mark.

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