A-level Computing/AQA/Paper 1/Fundamentals of algorithms/Sorting algorithms

Bubble Sort

A bubble sort, a sorting algorithm that continuously steps through a list, swapping items until they appear in the correct order.

Bubble sort is a simple sorting algorithm that works by repeatedly stepping through the list to be sorted, comparing each pair and swapping them if they are in the wrong order. The pass through the list is repeated until no swaps are needed, which indicates that the list is sorted. The algorithm gets its name from the way larger elements "bubble" to the top of the list. It is a very slow way of sorting data and rarely used in industry. There are much faster sorting algorithms out there such as insertion sort and quick sort which you will meet in A2.

Step-by-step example

Let us take the array of numbers "5 1 4 2 8", and sort the array from lowest number to greatest number using bubble sort algorithm. In each step, elements written in bold are being compared.

First Pass:
( 5 1 4 2 8 ) $\to$ ( 1 5 4 2 8 ), Here, algorithm compares the first two elements, and swaps them since 5 > 1
( 1 5 4 2 8 ) $\to$ ( 1 4 5 2 8 ), It then compares the second and third items and swaps them since 5 > 4
( 1 4 5 2 8 ) $\to$ ( 1 4 2 5 8 ), Swap since 5 > 2
( 1 4 2 5 8 ) $\to$ ( 1 4 2 5 8 ), Now, since these elements are already in order (8 > 5), algorithm does not swap them.
The algorithm has reached the end of the list of numbers and the largest number, 8, has bubbled to the top. It now starts again.

Second Pass:
( 1 4 2 5 8 ) $\to$ ( 1 4 2 5 8 ), no swap needed
( 1 4 2 5 8 ) $\to$ ( 1 2 4 5 8 ), Swap since 4 > 2
( 1 2 4 5 8 ) $\to$ ( 1 2 4 5 8 ), no swap needed
( 1 2 4 5 8 ) $\to$ ( 1 2 4 5 8 ), no swap needed
Now, the array is already sorted, but our algorithm does not know if it is completed. The algorithm needs one whole pass without any swap to know it is sorted.
Third Pass:
( 1 2 4 5 8 ) $\to$ ( 1 2 4 5 8 )
( 1 2 4 5 8 ) $\to$ ( 1 2 4 5 8 )
( 1 2 4 5 8 ) $\to$ ( 1 2 4 5 8 )
( 1 2 4 5 8 ) $\to$ ( 1 2 4 5 8 )
Finally, the array is sorted, and the algorithm can terminate.

Pseudocode implementation

The algorithm can be expressed as:

 A[i] '''then'''
         swap( A[i-1], A[i] )
         swapped = true
       '''end if'''
     '''end for'''
   '''while''' swapped
 '''end procedure'''

</syntxhighlight>

{{CPTExercise|title=Exercise: Bubble Sort}}

We will now look at an example in Visual Basic using an array of people's heights.  The following data set is being passed:
{| class="wikitable"
|-
! colspan="2" | height
|-
| 1 || 98
|-
| 2 || 12
|-
| 3 || 99
|-
| 4 || 54
|}
<syntaxhighlight lang="vbnet">
  Sub bubbleSort(ByRef height() As integer)
        Dim swapped As Boolean
        Dim temp As integer
        'sort the elements
        Do
            swapped = False
            For Count = 1 To MaxSize - 1

                If height(Count + 1) < height(Count) Then
                    temp = height(Count)
                    height(Count) = height(Count + 1)
                    height(Count + 1) = temp
                    swapped = True
                End If
            Next
        Loop Until swapped = False

        'Print out the elements
        For Count = 1 To MaxSize
            Console.WriteLine(Count & ": " & height(Count))
        Next
    End Sub

Template:CPTQuestionTabConstruct a trace table for the above code:

Swapped	Count	MaxSize	Temp	height
				1	2	3	4
False		4	null	98	12	99	54

Swapped	Count	MaxSize	Temp	height
				1	2	3	4
False		4	null	98	12	99	54
True	1		98	12	98
	2
True	3		99			54	99
False	1
True	2		98		54	98	99
	3
False	1
	2
	3

Template:CPTAnswerTabEnd Template:CPTQuestionTabWhat does the above code output?Template:CPTQuestionTabEnd Template:CPTAnswerTab 1: 12
2: 54
3: 98
4: 99
Template:CPTAnswerTabEnd Template:CPTQuestionTabShow the following lists after one pass of bubble sort:

Sort into alphabetical order:

Henry, Cat, George, Mouse

Cat, George, Henry, Mouse

Template:CPTAnswerTabEnd Template:CPTQuestionTab Sort into alphabetical order:

G, C, N, A, P, CTemplate:CPTQuestionTabEnd

C, G, A, N, C, P

Template:CPTAnswerTabEnd Template:CPTQuestionTab Sort into numerical order:

12, 56, 0, 23, 10Template:CPTQuestionTabEnd

12, 0, 23, 10, 56

Template:CPTAnswerTabEnd Template:CPTQuestionTab Show the following after 2 passes

Sort into alphabetical order:

Emu, Shrike, Gull, Badger 

Emu, Gull, Badger, Shrike (Pass 1)
Emu, Badger, Gull, Shrike (Pass 2)

Template:CPTAnswerTabEnd Template:CPTQuestionTab Sort into numerical order:

99, 45, 32, 56, 12

45, 32, 56, 12, 99 (Pass 1)
32, 45, 12, 56, 99 (Pass 2)

Let's look at a more complicated example, an array of structures, TopScores. The following data set is being passed:

TopScores
	Name	Score
1	Michael	45
2	Dave	78
3	Gerald	23
4	Colin	75

  Sub bubbleSort(ByRef TopScores() As TTopScore)
        Dim swapped As Boolean
        Dim temp As TTopScore
        'sort the elements
        Do
            swapped = False
            For Count = 1 To MaxSize - 1

                If TopScores(Count + 1).Score > TopScores(Count).Score Then
                    temp.Name = TopScores(Count).Name
                    temp.Score = TopScores(Count).Score
                    TopScores(Count).Score = TopScores(Count + 1).Score
                    TopScores(Count).Name = TopScores(Count + 1).Name
                    TopScores(Count + 1).Name = temp.Name
                    TopScores(Count + 1).Score = temp.Score
                    swapped = True
                End If
            Next
        Loop Until swapped = False

        'Print out the elements
        For Count = 1 To MaxSize
            Console.WriteLine(Count & ": " & TopScores(Count).Name & " " & TopScores(Count).Score)
        Next
    End Sub

Template:CPTExercise Template:CPTQuestionTabDraw a trace table to see if it works:

Swapped	Count	MaxSize	Temp		TopScores
			name	score	1		2		3		4
					name	score	name	score	name	score	name	score
False	1	4	null	null	Michael	45	Dave	78	Gerald	23	Colin	75

Swapped	Count	MaxSize	Temp		TopScores
			name	score	1		2		3		4
					name	score	name	score	name	score	name	score
False	1	4	null	null	Michael	45	Dave	78	Gerald	23	Colin	75
True	1	4	Michael	45	Dave	78	Michael	45
True	2	4
True	3	4	Gerald	23					Colin	75	Gerald	23
False	1	4
True	2	4	Michael	45			Colin	75	Michael	45
True	3	4
False	1	4
False	2	4
False	3	4

The output should be:

1: Dave 78
2: Colin 75
3: Michael 45
4: Gerald 23

Insertion Sort

An example on insertion sort. Check each element and put them in the right order in the sorted list.

Unfortunately bubble sort is a very slow way of sorting data and very rarely used in industry. We'll now look at a much faster algorithm, insertion sort.

Insertion sort is a simple sorting algorithm: a comparison sort in which the sorted array (or list) is built one entry at a time. It is much less efficient on large lists than more advanced algorithms such as quicksort, heapsort, or merge sort and you may cover these at university. However, insertion sort provides several advantages:

• simple implementation
• efficient on small data sets
• uses a fixed amount of memory when running

Insertion sort requires the use of two arrays, one ordered, and one unordered. Each repetition of the algorithm moves an item from the unordered list, into a sorted position in the ordered list, until there are no elements left in the unordered list.

Sorting is typically done in-place without needing extra memory. The resulting array after k iterations has the property where the first k + 1 entries are sorted. In each iteration the first remaining entry of the input is removed, inserted into the result at the correct position, thus extending the result:

becomes

with each element greater than x copied to the right as it is compared against x.

Animation of the insertion sort sorting a 30 element array. Notice how the ordered array (left hand side) slowly consumes the unordered array (right hand side), until the entire data set is ordered

Template:ExampleRobox The following table shows the steps for sorting the sequence {5, 7, 0, 3, 4, 2, 6, 1}. For each iteration, the number of positions the inserted element has moved is shown in parentheses. Altogether this amounts to 17 steps.

Template:Center/top 5 7 0 3 4 2 6 1 (0)

5 7 0 3 4 2 6 1 (0)

0 5 7 3 4 2 6 1 (2)

0 3 5 7 4 2 6 1 (2)

0 3 4 5 7 2 6 1 (2)

0 2 3 4 5 7 6 1 (4)

0 2 3 4 5 6 7 1 (1)

0 1 2 3 4 5 6 7 (6) Template:Center/end

 for i ← 1 to i ← length(A)-1
   {
     // A[ i ] is added in the sorted sequence A[0, .. i-1]
     // save A[i] to make a hole at index iHole
     item ← A[i]
     iHole ← i
     // keep moving the hole to next smaller index until A[iHole - 1] is <= item
     while iHole > 0 and A[iHole - 1] > item
       {
         // move hole to next smaller index
         A[iHole] ← A[iHole - 1]
         iHole ← iHole - 1
       }
     // put item in the hole
     A[iHole] ← item
   }

' a procedure to sort an array of integers

Template:Robox/Close Template:CPTExercise Template:CPTQuestion Template:CPTAnswerTabInsertion sort is a simple sorting algorithm: a comparison sort in which the sorted array (or list) is built one entry at a time.Template:CPTAnswerTabEnd Template:CPTQuestion Template:CPTAnswerTab sort left hand side is underlined

9 6 7 1 2
6 9 7 1 2
6 7 9 1 2
1 6 7 9 2
1 2 6 7 9

Template:CPTAnswerTabEnd Template:CPTQuestionTabShow how the insert sort would work on the following unordered array

G K L A J

Template:CPTQuestionTabEnd Template:CPTAnswerTab sort left hand side is underlined

G K L A J
G K L A J
G K L A J
A G K L J
A G J K L