Chapter 7 - VB.Net by Schneider1 Chapter 7 – Arrays 7.1 Creating and Accessing Arrays 7.2 Using Arrays 7.3 Control Arrays Skip Structures (7.3), 7.4 and.

Chapter 7 - VB.Net by Schneider 1

Chapter 7 – Arrays7.1 Creating and Accessing Arrays7.2 Using Arrays7.3 Control Arrays

Skip Structures (7.3), 7.4 and 7.5


7.1 Creating and Accessing Arrays

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


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.


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.


Example• Suppose that you want to evaluate the exam

grades for 30 students and to display the names of the students whose scores are above average.

Private Sub btnDisplay_Click(...) Handles btnDisplay.Click

Dim student1 As String, score1 As DoubleDim student2 As String, score2 As DoubleDim student3 As String, score3 As Double


Using Arrays

Dim student(30) As StringDim score(30) As Double

Array name

Upper bound of subscriptsin the array

Data type


Putting Values into an Arraystudent(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.


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


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

array’s name followed by the element’s subscript


One-dimensional Array

Nebraska New Jersey New Mexico Tennessee Texas

NebraskaNew JerseyNew MexicoTennesseeTexas

Demo Provinces


Example 1Private 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


Output Example 1


Example 2Dim teamName(3) As StringPrivate 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


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"}


Initializing an Array by Reading from a File

Dim student (30) As String, count as integerDim sr as IO.streamreader = IO.File.OpenText …Open “STUDENTS.TXT” For Input As #1For count = 1 To 30 student ( count ) = sr.readlineNextsr.close

Demo Provinces2


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)


The Average Program Revisited!For count = 1 To 30 student (count) = sr.readline grade (count) = Cdbl(sr.readline)Nextsum = 0For count = 1 To 30 sum += grade(count)Nextaverage = sum/30


Unknown Array Size

• What do I do if I don’t know the size of a file – and thus the size of the array??


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


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


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


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.


Preserve• To keep any data that has already been

stored in the array when resizing it, useReDim Preserve arrayName(m)

•Demo Provinces3


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 >= "A") And (letter <= "Z") Then index = Asc(letter) - 64 'The ANSI value of "A" is 65 charCount(index) += 1 End If Next


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) > 0 Then lstCount.Items.Add(letter & " " & _ charCount(index)) End If NextEnd Sub


Example 4 Output


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)


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.


Exercises p. 312In-class P. 319, #36


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


Ordered Arrays• An array is ordered if the elements are in

ascending or descending order.

• Why would we want to order an array??


Ordered Arrays - Example 1Dim 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) >= name2Find) Or (n = 10) If nom(n) = name2Find Then txtResult.Text = "Found." Else txtResult.Text = "Not found." End IfEnd Sub


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


Merging Two Ordered Arrays• To consolidate two lists into a single ordered list:1. Compare the two names (numbers) at the top of the first

and second lists.a) If one name (number) precedes the other, copy it onto

the third list and cross it off its original list.b) 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.

2. Repeat Step 1 with the current top names (numbers) until you reach the end of either list.

3. Copy the names from the remaining list onto the third list.

•Demo 7-2-3 modified


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


Example 4Private 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 totalEnd Function


Comments• Searching successive elements of an

ordered list beginning with the first element is called a sequential search.


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 SubPrivate Function Triple(ByVal x As Integer) As Integer Return 3 * xEnd Function


7.3 Some Additional Types of Arrays

• Control Arrays• Array of Structures (OMIT)


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• orDim arrayName() As ControlType


Control Arrays continued• The following statements declare control

arrays.Dim lblTitle(10) As LabelDim txtNumber(8) As TextBoxDim btnAmount() As Button

•Demo 7-3-1modified


Example 1Dim lblDept(5) As LabelDim txtDept(5) As TextBoxPrivate 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

Array of controls

Placing controlsInto arrays


Example 1 continued For depNum = 1 To 5 lblDept(depNum).Text = "Department " & depNum txtDept(depNum).Clear() NextEnd SubPrivate 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


Example 1 Output


• Exercises p. 345


ListBoxes• Adding to a listbox

• Listboxes as arrays

• Events associated with listboxes


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


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))


Currently Highlighted Item in a List Boxes

The currently highlighted item can be obtained by:

lstBox.Items(lstBox.SelectedIndex)OrlstBox.TextOrlstBox.SelectedItem


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


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 < n

then the following generates a whole number between m and n (including m, but excluding n)

randomNum.Next(m, n)Demo – Random

Chapter 7 - VB.Net by Schneider1 Chapter 7 – Arrays 7.1 Creating and Accessing Arrays 7.2 Using Arrays 7.3 Control Arrays Skip Structures (7.3), 7.4 and.

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