Arrays

1
Arrays Strings part 2

• More sophisticated algorithms, including sorting

2
Applications in this show

• Palindrome
• Sorting values
• Sort names

3
An aside on using combo box and strings

• Recall the car sales project from VB8.ppt?
• Comboboxes contain their own object type, even if
  you store integers or strings in them.
• If you are trying to figure out which array value
  was selected in a combobox, be a little careful.

4
Using combo box and strings

• Private Sub Button1_Click(ByVal sender As
  System.Object, ByVal e As System.EventArgs)
  Handles btnenter.Click
• Dim name As String
• name txtname.Text
• Dim i As Integer
• Dim j As Integer combo.SelectedIndex
• For i 0 To values.Length - 1
• If values(i) name Then
• list(i) prices(j)
• End If
• Next
• lstdisplay.Items.Clear()
• For i 0 To list.Length - 1
• lstdisplay.Items.Add(values(i) "
  " list(i))
• Next
• txtname.Text ""
• End Sub

5
Using combo box and strings

• Use cbo.selectedindex on the combo box if you
  need to know which one to access a location in
  your array.
• Similarly, you could use cbo.selectedtext or
  cbo.selectedvalue
• Dont use cbo.selectedItem for this purpose, as
  it does not return the correct datatype.

6
An exercise Shift array contents to next
(higher) index

• Move array contents one location farther, so 0th
  value goes in 1st place, 1st place goes into 2nd
  place and so on
• Dim values As Integer() 10, 20, 30, 40, 50
• Dim i As Integer
• For i values.Length - 1 To 1 Step -1
• values(i) values(i - 1)
• Next
• What should we put in position 0?
• Values(0)0

7
Another exercise

• Shift the values to previous location and put a 0
  in the last location instead. So shift
  values(1) into values(0),, values(2) into
  values(1), etc

8
Rotate (up)

• Dim values As Integer() 10, 20, 30, 40, 50
• Dim I,save As Integer
• Savevalues(values.Length - 1 )
• For i values.Length - 1 To 1 Step -1
• values(i) values(i - 1)
• Next
• put old last one into 0th spot
• values(0) save
• What does the array look like now?

9
Rotate right

• Exercise.
• Rotate the array the other direction, so each
  value goes into the previous location and
  values(0) goes into the last (4th) location.

10
Reverse

• Arrays already have a defined method called
  reverse to flip their contents.
• The next few slides cover palindrome checking.
  You could also check for a palindrome by first
  converting the string into a char array

11
Palindrome spells the same forwards or backwards
12
Palindrome

• Palindromes are spelled the same forwards and
  backwards. Why wasnt a man a plan a canal
  panama a palindrome?

13
Palindrome
14
Palindrome

• In button click, go through the string to make
  sure first char matches the last
• Next char matches next-to-last, etc.
• Display result

15
Palindrome

• Harder exercises
• Have your palindrome ignore letter case and
  blanks, so that A man a plan a canal Panama is
  accepted as a palindrome.
• VB has methods for strings to convert the string
  to uppercase or lowercase which simplifies some
  of the palindrome checking.

16
Sorting

• Array contents can be sorted
• There are many ways to sort. Heres a simple
  way, called selection sort
• We could find the largest, put it in position 0.
  (Where should we put current item in position 0?)
• Repeat this, finding 2nd largest, third, and
  positioning them correctly.

17
build this interfaceinclude try catch for format
and indexoutofbounds
18
Sorting from big to small
19
The sort button click

• Dim i, j, last As Integer
• count is a field value declared elsewhere
• lastcount-1 last entry in array named
  values
• For i 0 To last
• j findBigSub(values, i,last)
• swap(values, i, j)
• need to write this sub to swap entries in the
  values array
• Next
• lstvalues.Items.Clear()
• For i 0 To count - 1
• lstvalues.Items.Add(values(i))
• Next

20
The function to find the largest remaining value

• Function findBigSub(ByVal v() As Integer, ByVal
  x As Integer, ByVal c as integer) As Integer
• Dim i, j As Integer
• i x 'assume first is biggest
• For j x1 To c
• If v(j) gt v(i) Then
• i j
• 'if position j holds a bigger value, save
  position
• End If
• Next
• findBigSub i return answer
• End Function

21
Not shown

• A swap subroutine
• Youll need to send the array and the two
  positions.
• Send the array byref, not byval
• Private Sub swap(ByRef v() As Integer, ByVal x
  As Integer, ByVal y As Integer)
• We mostly wrote the 3 lines of code needed in
  class

22
Remember.

• when finding values or displaying array contents
  in the previous example, you need to be careful.
• The entire array may not have been filled.
• Be sure to use count (or count-1), the current
  number of things stored in the array, (as the
  upper bound of your loop), rather than
  array.length since some part of the array may
  contain nothing.

23
Other ways to sort

• Bubble sort
• Go all the way through the array comparing
  elements pair wise, and swapping if they are out
  of order. Youll have to do this many times
  Whats the best case and worst case and how will
  you know when you are done?

24
Other ways to sort

• Insertion sort
• Move elements from the array into a different
  array (or even the same one) as follows
• Put each element in the position it belongs by
  shunting other values backwards.
• For example, of there are 2 elements in the array
  correctly, start comparing the third/new element
  with the 2nd one (in position 1). If it is small,
  shunt the 2nd one back and go on to compare with
  the first element.
• Shunting elements back will make room for the new
  one.

25
How much work is it to find the biggest element?

• In an array has n elements, then to find the
  biggest, we look at all of them n amount of
  work

26
How much work to sort?

• We have to find the biggest of all n elements.
• Then we find the biggest of the remaining
  elements n-1 comparisons.
• And so on.
• The last time, we just compare two.
• How many comparisons is this, for an array of
  size n?

27
Sorting names
28
Sorting names
29
Comparing names

• You shouldnt use code like
• AnameltOthername
• since names are not an ordinal datatype even
  though some of my earlier examples did this.
• Instead, you should use the string method
  compareTo
• Aname.compareTo(Othername)
• which returns an integer value.

30
Use string.compareTo(string)

• Returns 0 for equal
• Returns lt0 for first less than second
• Returns gt0 for first bigger than second
• An example Suppose name is Annie and other is
  Bob
• name.compareTo(other) would return a value less
  than 0 since Annie comes before Bob in
  lexicographical ordering.
• Write code like
• Dim name as StringAnnie
• Dim other as StringBob
• If name.compareTo(other) then
• code to handle less goes here
• Else
• code to handle greater goes here
• endif

31
remember

• Make sure to count your entries and only sort
  the entries, not the empty end of the array.
• Only display the entered portion of your array.