Problem Solving: Searching and sorting

 ← Pseudo code Searching and sorting

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:

procedure bubbleSort( A : list of sortable items )
do
swapped = false
for each i in 1 to length(A) - 1 inclusive do:
if A[i-1] > A[i] then
swap( A[i-1], A[i] )
swapped = true
end if
end for
while swapped
end procedure


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:

height
1 98
2 12
3 99
4 54
  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, 53


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


Linear Search

The following pseudo code describes a typical variant of linear search, where the result of the search is supposed to be either the location of the list item where the desired value was found; or an invalid location -1, to indicate that the desired element does not occur in the list.

For each item in the list:
if that item has the desired value,
stop the search and return the item's location.
Return ''-1''

dim items() = {"h","g","a","d","w","n","o","q","l","b","c"}
dim searchItem as string

console.write("What are you searching for: ")

For x = 0 to 10
If items(x) = searchItem Then
console.writeline("Found item " & searchItem & " at position " & x)
Exit For
End If
If x = 10 Then
console.writeline(-1)
End If
Loop


Try the code above searching for items "w" and then for item "z":

Code Output

What are you searching for: w
Found item w at position 4

Code Output

What are you searching for: z
-1

Exercise: Linear Search

"Cat","Mouse","Frog","Lion","Panda","Llama","Bee"

For the array above, how many searches would it take to find the following data:

"Panda"

5

"Camel"

7 and still it wouldn't find it!

For an $n$ length list of items, what is the maximum number of searches it would take to see if an item is there or not?

$n$. This is seen as very slow, there is a faster ways of searching called binary search that you will learn about in A2, however, the data must be ordered first.

Make a trace table for the code above, where searchItem = "d"