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

Searching algorithms Optimisation algorithms sorting algorithm Example of insertion sort sorting a list of random numbers. Array О(n2) O(n) О(n2) О(n) total, O(1) auxiliary No Bubble Sort thumb|center|280px|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. center 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 ) ( 1 5 4 2 8 ), Here, algorithm compares the first two elements, and swaps them since 5 > 1 ( 1 5 4 2 8 ) ( 1 4 5 2 8 ), It then compares the second and third items and swaps them since 5 > 4 ( 1 4 5 2 8 ) ( 1 4 2 5 8 ), Swap since 5 > 2 ( 1 4 2 5 8 ) ( 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 ) ( 1 4 2 5 8 ), no swap needed ( 1 4 2 5 8 ) ( 1 2 4 5 8 ), Swap since 4 > 2 ( 1 2 4 5 8 ) ( 1 2 4 5 8 ), no swap needed ( 1 2 4 5 8 ) ( 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 ) ( 1 2 4 5 8 ) ( 1 2 4 5 8 ) ( 1 2 4 5 8 ) ( 1 2 4 5 8 ) ( 1 2 4 5 8 ) ( 1 2 4 5 8 ) ( 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 Construct 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 What does the above code output? 1: 12 2: 54 3: 98 4: 99 Show the following lists after one pass of bubble sort: Sort into alphabetical order: Henry, Cat, George, Mouse Cat, George, Henry, Mouse Sort into alphabetical order: G, C, N, A, P, C C, G, A, N, C, P Sort into numerical order: 12, 56, 0, 23, 10 12, 0, 23, 10, 56 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) 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 Exercise: Bubble Sort (Harder) Draw 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 300px|thumb|center|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: Array prior to the insertion of x becomes Array after the insertion of x with each element greater than x copied to the right as it is compared against x. thumb|center|280px|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 Example: Insertion Sort 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. 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) 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 Exercise: Insertion Sort Describe the process of 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. Show the how insert sort would work on the following unordered array: 9 6 7 1 2 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 Show how the insert sort would work on the following unordered array G K L A J 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 Complete the trace table for the following code: