Chapter 7

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Chapter 7 Arrays

• 7.1 Creating and Accessing Arrays
• 7.2 Using Arrays
• 7.3 Control Arrays
• Skip Structures (7.3), 7.4 and 7.5

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7.1 Creating and Accessing Arrays

• Array Creation
• ReDim Statement
• Using an Array as a Frequency Table

3
Simple variables

• A variable (or simple variable) is a name to
  which VB.NET can assign a single value.
• An array variable is a collection of simple
  variables of the same type to which VB.NET can
  efficiently assign a list of values.

4
Elements of an Array

• Array Name A valid variable name for the
  structure.
• Subscript or Index A value that refers to a
  particular array element.
• Element An individual data item within an array.

5
Example

• Suppose that you want to evaluate the examgrades
  for 30 students and to display the names of the
  students whose scores are above average.
• Private Sub btnDisplay_Click(...)
• Handles btnDisplay.ClickDim student1 As
  String, score1 As DoubleDim student2 As String,
  score2 As DoubleDim student3 As String, score3
  As Double

6
Using Arrays
Upper bound of subscripts in the array

• Dim student(30) As String
• Dim score(30) As Double

Array name
Data type
7
Putting Values into an Array

• student(1) "Tom Brown"

subscript
Read "student sub one equals Tom Brown" Which
means that the string "Tom Brown" is being stored
at the second location in the array called
student because all arrays begin counting at 0.
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Array Terminology

• Dim arrayName(n) As DataType
• 0 is the "lower bound" of the array
• n is the "upper bound" of the array the last
  available subscript in this array
• The number of elements is the size of the array

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Variable Arrays

• Can have one or two dimensions
• A one-dimensional array is simply a row (or
  column) of variables
• Each element in an array is identified by a
  subscript
• You refer to an array element by the arrays name
  followed by the elements subscript

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One-dimensional Array
State(3)
State(4)
State(5)
State(1)
State(2)
State(0)
Tennessee
Nebraska
New Jersey
New Mexico
Texas
State(0)
Nebraska
State(1)
New Jersey
State(2)
New Mexico
State(3)
Tennessee
State(4)
Demo Provinces
State(5)
Texas
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Example 1

• Private Sub btnWhoWon_Click(...) _
• Handles btnWhoWon.Click
• Dim teamName(4) As String
• Dim n As Integer
• 'Place Super Bowl Winners into the array
• teamName(1) "Packers"
• teamName(2) "Packers"
• teamName(3) "Jets"
• teamName(4) "Chiefs"
• 'Access array
• n CInt(txtNumber.Text)
• txtWinner.Text teamName(n)
• End Sub

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Output Example 1
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Example 2

• Dim teamName(3) As String
• Private Sub btnWhoWon_Click(...)Handles
  btnWhoWon.Click
• Dim n As Integer
• n CInt(txtNumber.Text)
• txtWinner.Text teamName(n)
• End Sub
• Private Sub Form1_Load(...) Handles MyBase.Load
• 'Place Super Bowl Winners into the array
• teamName(1) "Packers"
• teamName(2) "Packers"
• teamName(3) "Jets"
• End Sub

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Initializing Arrays

• Arrays may be initialized when they are created
• Dim arrayName() As varType value0, _
• value1, value2, ..., valueN
• declares an array having upper bound N and
  assigns value0 to arrayName(0), value1 to
  arrayName(1), value2 to arrayName(2), ..., and
  valueN to arrayName(N).
• Dim teamName() As String "", "Packers", _
• "Packers", "Jets", "Chiefs"

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Initializing an Array by Reading from a File

• Dim student (30) As String, count as integer
• Dim sr as IO.streamreader IO.File.OpenText
• Open STUDENTS.TXT For Input As 1
• For count 1 To 30
• student ( count ) sr.readline
• Next
• sr.close
• Demo Provinces2

16
Parallel Arrays

• Two arrays are referred to as parallel if
  subscripted variables having the same subscript
  (elements) are related.
• i.e. student(30) and grade(30)

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The Average Program Revisited!

• For count 1 To 30
• student (count) sr.readline
• grade (count) Cdbl(sr.readline)
• Next
• sum 0
• For count 1 To 30
• sum grade(count)
• Next
• average sum/30

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Unknown Array Size

• What do I do if I dont know the size of a file
  and thus the size of the array??

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First Technique Unknown Size

• Dim Array with no upper bound
• Use a loop and counter to count the number of
  items in the file
• ReDim the array based on the counter
• Use a second loop to then read the known file size

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Second Technique Unknown Size

• Dim Array with a large upper bound
• Use a loop and sr.peek to see if additional data
  exists in the file
• Use ReDim Preserve to increase the upper bound of
  the array by one
• Read one piece of data into the new array element
  just created

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Third Technique Unknown Size

• Dim an Array with a large upper bound (large
  enough to easily hold all data)
• Read from the file into the array using a loop
  with a counter
• Keep track of the number of pieces of data in the
  array with the counter
• So in effect, you only use part of a large array

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ReDim Statement

• The size of an array may be changed after it is
  created
• ReDim arrayName(m)
• where arrayName is the name of the already
  declared array and m is an Integer
  literal,variable, or expression.

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Preserve

• To keep any data that has already been stored in
  the array when resizing it, use
• ReDim Preserve arrayName(m)

• Demo Provinces3

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Using an Array as a Frequency Table

• Private Sub btnAnalyze_Click(...) Handles
  btnAnalyze.Click
• 'Count occurrences of the various letters in a
  sentence
• Dim index, letterNum As Integer
• Dim sentence, letter As String
• Dim charCount(26) As Integer
• 'Examine and tally each letter of the sentence
• sentence (txtSentence.Text).ToUpper
• For letterNum 1 To sentence.Length
• letter sentence.Substring(letterNum - 1, 1)
• If (letter gt "A") And (letter lt "Z") Then
• index Asc(letter) - 64 'The ANSI value of
  "A" is 65
• charCount(index) 1
• End If
• Next

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Example 4 continued

• 'List the tally for each letter of alphabet
• lstCount.Items.Clear()
• For index 1 To 26
• letter Chr(index 64)
• If charCount(index) gt 0 Then
• lstCount.Items.Add(letter " " _
• charCount(index))
• End If
• Next
• End Sub

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Example 4 Output
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Out of Bounds Error

• The following sets up an array, and then
  references an element that doesn't exist. This
  will cause an error.
• Dim trees() As String "", "Sequoia", _
• "Redwood", "Spruce"
• txtBox.Text trees(5)

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Copying Arrays

• If arrayOne() and arrayTwo() have been declared
  with the same data type, then the statement
• arrayOne arrayTwo
• makes arrayOne() an exact duplicate of
  arrayTwo(). It will have the same size and
  contain the same information.

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Exercises p. 312In-class P. 319, 36
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7.2 Using Arrays

• Ordered Arrays
• Using Part of an Array
• Merging Two Ordered Arrays
• Passing Arrays to Procedures

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Ordered Arrays

• An array is ordered if the elements are in
  ascending or descending order.
• Why would we want to order an array??

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Ordered Arrays - Example 1

• Dim nom() As String "", "AL", "BOB", "CARL",
  "DON","ERIC", _
• "FRED", "GREG", "HERB", "IRA", "JACK"
• Private Sub btnSearch_Click(...) Handles
  btnSearch.Click
• Dim name, name2Find As String
• Dim n As Integer 'Subscript of the array
• name2Find txtName.Text.ToUpper
• Do
• n 1 'Add 1 to n
• Loop Until (nom(n) gt name2Find) Or (n 10)
• If nom(n) name2Find Then
• txtResult.Text "Found."
• Else
• txtResult.Text "Not found."
• End If
• End Sub

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Ordered Arrays

• Answer It makes searching them much more
  efficient
• On average, you only have to search half an array
  that is ordered whereas you have to search the
  whole array if unordered

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Merging Two Ordered Arrays

• To consolidate two lists into a single ordered
  list
• Compare the two names (numbers) at the top of
  the first and second lists.
• If one name (number) precedes the other, copy it
  onto the third list and cross it off its original
  list.
• If the names (numbers) are the same, copy the
  name (number) onto the third list and cross out
  the name (number) from the first and second
  lists.
• Repeat Step 1 with the current top names
  (numbers) until you reach the end of either list.
• Copy the names from the remaining list onto the
  third list.

• Demo 7-2-3 modified

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Passing Arrays to Procedures

• An array declared in a procedure is local to that
  procedure
• An entire array can be passed to a Sub or
  Function procedure

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Example 4

• Private Sub btnCompute_Click(...) Handles
  btnCompute.Click
• Dim score() As Integer 0, 85, 92, 75, 68,
  84, 86, _
• 94, 74, 79, 88
• txtAverage.Text CStr(Sum(score) / 10)
• End Sub
• Function Sum(ByVal s() As Integer) As Integer
• Dim total, index As Integer
• total 0
• For index 1 To s.GetUpperBound(0) 'The upper
  bound is 10
• total s(index)
• Next
• Return total
• End Function

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Comments

• Searching successive elements of an ordered list
  beginning with the first element is called a
  sequential search.

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Passing an Array Element

• A single element of an array can be passed to a
  procedure just like any ordinary numeric or
  string variable.
• Private Sub btnDisplay_Click(...) Handles
  btnDisplay.Click
• Dim num(20) As Integer
• num(5) 10
• lstOutput.Items.Add(Triple(num(5)))
• End Sub
• Private Function Triple(ByVal x As Integer) As
  Integer
• Return 3 x
• End Function

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7.3 Some Additional Types of Arrays

• Control Arrays
• Array of Structures (OMIT)

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Control Arrays

• Control arrays are arrays of controls, such as
  labels, text boxes, etc.
• They are created in much the same way as any
  other array
• Dim arrayName(n) As ControlType
• or
• Dim arrayName() As ControlType

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Control Arrays continued

• The following statements declare control arrays.
• Dim lblTitle(10) As Label
• Dim txtNumber(8) As TextBox
• Dim btnAmount() As Button

• Demo 7-3-1modified

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Example 1
Array of controls

• Dim lblDept(5) As Label
• Dim txtDept(5) As TextBox
• Private Sub Form1_Load(...) Handles MyBase.Load
• Dim depNum As Integer
• lblDept(1) Label1
• lblDept(2) Label2
• lblDept(3) Label3
• lblDept(4) Label4
• lblDept(5) Label5
• txtDept(1) TextBox1
• txtDept(2) TextBox2
• txtDept(3) TextBox3
• txtDept(4) TextBox4
• txtDept(5) TextBox5

Placing controls Into arrays
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Example 1 continued

• For depNum 1 To 5
• lblDept(depNum).Text "Department " depNum
• txtDept(depNum).Clear()
• Next
• End Sub
• Private Sub btnCompute_Click(...)
• Handles btnCompute.Click
• Dim totalSales As Double 0
• Dim depNum As Integer
• For depNum 1 To 5
• totalSales CDbl(txtDept(depNum).Text)
• Next
• txtTotal.Text FormatCurrency(totalSales)
• End Sub

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Example 1 Output
45

• Exercises p. 345

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ListBoxes

• Adding to a listbox
• Listboxes as arrays
• Events associated with listboxes

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Useful Properties of the List Box

• Items can be placed into a listbox at design time
  or runtime
• lstBox.items.add(item1)
• lstBox.items.add(item2)
• This forms an array with 2 elements
• For the index number of the currently highlighted
  item
• lstBox.SelectedIndex

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More List Box Properties

• lstBox.Items() is the list of items in the list
  box. The value of the item with an index of "n"
  is
• lstBox.Items(n)
• The data type of the elements in the
  lstBox.Items() array is Object. To put the first
  element of lstBox.Items in a text box
• txtBox.Text CStr(lstBox.Items(0))

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Currently Highlighted Item in a List Boxes

• The currently highlighted item can be obtained
  by
• lstBox.Items(lstBox.SelectedIndex)
• Or
• lstBox.Text
• Or
• lstBox.SelectedItem

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List Box Events

• Three main types of events with list boxes
• Click if the user clicks on an item in the list
  box
• SelectedIndexChanged (default event) if the
  user clicks on an item or uses the arrow keys to
  select it
• DoubleClick - if the user double-clicks on an item

Demo - Movies
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The Random Class

• A random number generator declared with the
  statement
• Dim randomNum As New Random()
• If m and n are whole numbers and m lt n then the
  following generates a whole number between m and
  n (including m, but excluding n)
• randomNum.Next(m, n)

Demo Random