Sorting

1
Sorting

• CS0004
• Chapter 7.4

2
Why?

• Often times we just need to sort
• Improve search
• Recall binary search
• Allow us to prioritize a large list
• What to do next
• Make things pretty
• For the user )

3
Sorting

• Sorting is an algorithm for ordering an array
• Descending
• Ascending
• Many, Many, Many different ways but..
• Bubble Sort
• Shell Sort

4
Swap

• Both bubble and shell use a function called swap
• temp varl
• varl var2
• var2 temp
• The above psuedo-code makes var1 equal to var2
  and var2 equal to var1

5
Using Swap

• Private Sub btnAlphabetize_Click(...) _
• Handles btnAlphabetize.Click
• Dim firstWord, secondWord, temp As String
• firstWord txtFirstWord.Text
• secondWord txtSecondWord.Text
• If (firstWord secondWord) Then
• temp firstWord
• firstWord secondWord
• secondWord temp
• End If
• txtResult.Text firstWord " before " _
• secondWord
• End Sub

6
Bubble Sort - Idea

• Compare the first and second items, if out of
  order then swap them
• Compare the second and third items, if out of
  order then swap them
• Repeat for all other items
• 1/2, 2/3, 3/4, 4/5, 5/6 n-1/n
• Now, the last item is in the proper place, called
  first pass
• Repeat steps 1-3 again ignoring the last element
• Repeat n-1 steps until all are in proper place

7
Example
Pebbles Barney Wilma Fred Dino
First Pass
Barney Pebbles Fred Dino Wilma
Second Pass
8
Example
Barney Fred Dino Pebbles Wilma
Third Pass
Barney Dino Fred Pebbles Wilma
Fourth Pass
9
Write the code

• Loop over the size of the list minus 1
• Then loop from the start of the list to the end
  minus the number of times we have already looped
• If item is out of order with previous
• swap them
• End If
• End Loop
• End Loop

10
Shell Sort

• Much more complex
• General idea
• Compare items at further distance (known as the
  gap) and swap like bubble sort
• Then reduce the gap size by 1 and do it again
• Continue until the gap size is set to 0, meaning
  we are comparing neighbors (as we do in bubble
  sort)
• Not required to understand, details in book