Chapter 7 Notes

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Chapter 7 Notes

• Modified By Evans Adams
• July 2004

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Chapter 7 Arrays

• 7.1 Creating and Accessing Arrays
• 7.2 Using Arrays
• 7.3 Some Additional Types of Arrays
• 7.4 Sorting and Searching (Omit)
• 7.5 Two-Dimensional Arrays

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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.

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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
• Dim student30 As String, score30 As Double
• We would then need to write 60 assignment
  statements to assign values to these 60 variables
  (30 students and 30 scores)
• The resulting program would be long and
  cumbersome

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Defining Arrays
Upper Bound of Array Subscripts, Size of the
Array 29 1 30 Array indices (subscripts)
start with 0

• Dim student(29) As String to hold the names
• Dim score(29) As Double to hold the scores

Array name
Data type
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Putting Values into an Array

• student(0) "Tom Brown"

subscript
Read "student sub zero equals Tom Brown Which
means that the string "Tom Brown" is being stored
into position 0 in the array called student
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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 (n1) is the size of the
  array

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Array Conventions

• Array elements are act like regular variables
  of the assigned data type, except that they
  require a subscript
• Can be assigned values with assignment statements
• Can be displayed in list boxes and text boxes
• Default values are Nothing for Strings and 0 for
  numeric variables
• Scope can be Class Level or Local depending upon
  where the declaration statement is placed
• Subscripts can be either constants, variables or
  expressions, i.e.,
• student(5), student(n), student(n12)

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Example 7.1.1

• Private Sub btnWhoWon_Click(...)Handles
  btnWhoWon.Click
• Dim teamName(3) As String Define or Declare the
  Array variable
• Dim n As Integer
• 'Place Super Bowl Winners into the array
• teamName(0) "Packers"
• teamName(1) "Packers"
• teamName(2) "Jets"
• teamName(3) "Chiefs
• n CInt(txtNumber.Text)
• txtWinner.Text teamName(n-1) 'Access array
  element n 1 No need to ask user for numbers 0
  to 3
• End Sub

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Output Example 1
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Example 7.1.2

• Dim teamName(3) As String Array Name is a
  Class-Level Variable and accessible to all
  procedures
• 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
• The Load Event fires when the form is loaded
  into memory, hence the array is already filled
  when the OnClick Event Procedure executes (use
  Debugger to demonstrate)
• teamName(0) "Packers"
• teamName(1) "Packers"
• teamName(2) "Jets
• teamName(3) Chiefs
• End Sub

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Lab Exercise 7-1

• Demo and discuss While Loop Load and Display
• End of Lecture 1

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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).
• Example
• Dim teamName() As String "Packers", _
• "Packers", "Jets", "Chiefs"

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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)

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Example 7.1.3 (Beginning of FormLoad Event Proc.)
Dim teamName() As String Class Level Array
to hold team names Dim upperBound As Integer
Number of Super Bowls Played Private Sub
Form1_Load () Handles MyBase.Load When the
Form Loads, Read the file, load the array of team
names from a file and set properties of
certain controls Dim sr As IO.StreamReader
IO.File.OpenText("SBWINNERS.TXT") Dim name As
String Dim numLines As Integer 'Count
the number of lines in the file and assign it to
upperBound numLines 0 Do While (sr.Peek
ltgt -1) name sr.ReadLine numLines
1 Loop upperBound numLines - 1
sr.Close()
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Example 7.1.3 (Rest of FormLoad Event Proc.)
'Specify the title bar of the form Me.Text
"First " upperBound 1 " Super Bowls
'Specify the caption of the Next Winner button
btnNextWinner.Text "Add Winner of Game "
upperBound 2 'Specify the size of the array
and fill it with the entries of the file
ReDim teamName(upperBound) re-create Class Level
array of correct size sr
IO.File.OpenText("SBWINNERS.TXT") Open the file
again to read
the team names into the
teamName array For i as Integer 0 To
upperBound can Dim the loop control variable
in the For statement teamName(i)
sr.ReadLine Store a team name into an array
element Next sr.Close() End Sub
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Example 7.1.3 (btnDisplay Event Proc.)
Private Sub btnDisplay_Click () Handles
btnDisplay.Click 'Display the numbers of the
games won by the team in the text box Dim i
As Integer lstGamesWon.Items.Clear() For
i 0 To upperBound If teamName(i)
txtName.Text Then Add the subscript of the
element
with a matching team name in the array to the
list box lstGamesWon.Items.Add(i1) to
convert from array subscript numbers to
numbers that make sense to the user End If
Next End Sub
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Example 7.1.3 (add Winner(s) of latest Super
Bowl(s)
Private Sub btnNextWinner_Click () Handles
btnNextWinner.Click 'Add winner(s) of next
Super Bowl(s) to the array Dim prompt As
String upperBound 1 increment last array
elements subscript 'Add one more element to
the array and preserve its contents ReDim
Preserve teamName(upperBound) 'Request the
name of the next winner prompt "Enter
winner of game " upperBound 1
teamName(upperBound) InputBox(prompt, , "")
'Update the title bar of the form and the
caption of the button Me.Text "First "
upperBound 1 " Super Bowls"
btnNextWinner.Text "Add Winner of Game "
upperBound 2 Note only the array is
changed, the file is not. We will learn how to
add data to files in Chapter 8 End Sub
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Example 7.1.4 First part of event proc.

• 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(25) As Integer
• 'Examine and tally each letter of the sentence
• sentence (txtSentence.Text).ToUpper convert
  to upper case
• For letterNum 1 To sentence.Length
• letter sentence.Substring(letterNum - 1,
  1)select next
• letter of
  the sentence
• If (letter gt "A") And (letter lt "Z") Then
• index Asc(letter) - 65 'The ASCII value
  of "A" is 65
• charCount(index) 1 increment counter
  for the letter
• End If
• Next

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Example 7.1.4 Last part of event proc.
'List the tally for each letter of alphabet
lstCount.Items.Clear() For index 0 To 25
letter Chr(index 65) Generate each letter
from its ASCII value If charCount(index) gt
0 Then lstCount.Items.Add(letter " "
charCount(index)) End If Next End Sub
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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.
• Can use Erase arrayName after an array is no
  longer needed to release all memory cells
  allocated to the array

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