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Year 10
AQA

Merge sort

I can perform a merge sort to order a list.

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New
New
Year 10
AQA

Merge sort

I can perform a merge sort to order a list.

Download all resources
Share activities with pupils

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Lesson details

Key learning points

  1. Merge sort splits items in lists into individual lists before merging pairs of lists together till all items are sorted.
  2. Merge sort is an efficient “divide and conquer” algorithm that can perform well in real world use.
  3. A merge sort algorithm is usually faster to execute but more complex to write than bubble sort.
  4. Executing a merge sort takes up extra space in memory.

Keywords

  • Merge sort - a sorting algorithm that works by repeatedly splitting data into sublists and merging pairs of sublists, ordering the items as they are merged

  • Splitting - the initial step of dividing an unsorted list into smaller sublists

  • Merging - the process of combining two already sorted lists into a single, larger sorted list

Common misconception

Merge sort is the most efficient sorting algorithm for all cases.

For smaller data sets, simpler algorithms such as insertion sort or bubble sort can be faster due to the overhead in merge sort of having to break apart the data to be sorted.


To help you plan your year 10 computer science lesson on: Merge sort, download all teaching resources for free and adapt to suit your pupils' needs...

The steps of algorithms should be modelled with explanation from the teacher at every step. Pupils can understand processes better when understanding the detail of what is required at each stage.
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Licence

This content is © Oak National Academy Limited (2025), licensed on Open Government Licence version 3.0 except where otherwise stated. See Oak's terms & conditions (Collection 2).

Lesson video

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Starter quiz

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6 Questions

Q1.
What sorting algorithm repeatedly compares pairs of values within a list and swaps them if they are out of order?
Correct Answer: bubble sort, bubble
Q2.
What happens if two adjacent items are in the wrong order in bubble sort?
Correct answer: They are swapped.
They are deleted.
They are ignored.
They are added to the end.
Q3.
What is a "pass" in bubble sort?
a single comparison of two items
a random selection of items
a sorting of half the list
Correct answer: a complete traversal of the list
Q4.
When does bubble sort stop making passes?
after a fixed number of passes
when the largest item is sorted
Correct answer: when no swaps are needed
when the smallest item is sorted
Q5.
Which of these is a disadvantage of bubble sort?
It doesn’t sort lists.
It only works for numbers.
It requires extra memory.
Correct answer: It is slow for large lists.
Q6.
Place the steps of bubble sort in the correct order:
Correct answer: compare adjacent items
swap if needed
repeat until no swaps
list is sorted

Exit quiz

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6 Questions

Q1.
What is the first step in a merge sort?
merging all items
Correct answer: splitting the list into sublists
sorting the largest item
comparing adjacent items
Q2.
Match the terms to their definitions:
Correct Answer:merge sort,a sorting algorithm that splits and merges data

a sorting algorithm that splits and merges data

Correct Answer:splitting,dividing a list into smaller parts

dividing a list into smaller parts

Correct Answer:merging,combining sorted sublists into a single list

combining sorted sublists into a single list

Q3.
What is the purpose of merging in merge sort?
to split the list further
to remove duplicates
to compare adjacent items
Correct answer: to combine sorted sublists into a single sorted list
Q4.
Place the steps of merge sort in the correct order:
1 - split the list
2 - sort the sublists
3 - merge the sublists
4 - create a sorted list
Q5.
What is one disadvantage of merge sort?
Correct answer: It uses extra memory.
It is slow for large data sets.
It cannot handle unsorted data.
It only works with numbers.
Q6.
Which is faster for smaller data sets: merge sort or bubble sort?
Correct answer: bubble sort
merge sort
both are equally fast
neither is fast for small data sets