Chapter 8

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# Chapter 8 - PowerPoint PPT Presentation

Chapter 8. Arrays, Timers, and More. 8.1 Introduction. Chapter 8 Topics. Arrays are like groups of variables that allow you to store sets of data A single dimension array is useful for storing and working with a single set of data

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### Chapter 8

Arrays, Timers, and More

### 8.1Introduction

Chapter 8 Topics
• Arrays are like groups of variables that allow you to store sets of data
• A single dimension array is useful for storing and working with a single set of data
• A multidimensional array can be used to store and work with multiple sets of data
• Array programming techniques covered
• Summing and averaging all the elements in an array
• Summing all the columns in a two-dimensional array
• Searching an array for a specific value
• Using parallel arrays

### 8.2Arrays

An Array Is Like a Group of Variables With One Name

You Store and Work With Values in an Array by Using a Subscript

Array Characteristics
• An array is a like group of variables with a single name
• All of the variables within an array are called elements and are of the same data type
• You access the individual variables in an array through a subscript
Subscript Characteristics
• A subscript, also known as an index, is a number that identifies a specific element within an array
• Subscript numbering begins at 0, so the subscript of the first element in an array is 0
• The subscript of the last element in an array is one less than the total number of elements
Declaring an Array
• ArrayName is the name of the array
• UpperSubscript is the value of the array\'s highest subscript
• It must be a positive whole number
• DataType is a Visual Basic .NET data type

Dim ArrayName(UpperSubscript) As DataType

Example of an Array

Dim hours(5) As Integer

hours(0) hours(1) hours(2) hours(3) hours(4) hours(5)

Default Initialization
• All of the elements of an Integer array are initialized with the same value as a normal integer, zero (0)
• Same for other types of arrays
• Float, zero (0.0)
• String, Nothing
Explicit Array Initialization
• Example:
• No upper subscript value is given
• Visual Basic .NET will automatically make the array long enough to hold the values

Dim numbers As Integer = {2, 4, 5, 7, 9, 12}

Named Constants As Array\'s Highest Subscript Value
• In place of a whole number, a named constant may be used as an array\'s highest subscript:

Const upperSub As Integer = 100

Dim array(upperSub) As Integer

Working With Array Elements
• An Array element (e.g., numbers(2)) is used just like an ordinary variable:

numbers(0) = 100

numbers(1) = 200

numbers(2) = 300

numbers(3) = 400

numbers(4) = 500

numbers(5) = 600

pay = hours(3) * rate

tallies(0) += 1

MessageBox.Show(grossPay(5).ToString)

Arrays and Loops
• Frequently, arrays will be processed in loops

Dim count As Integer

For count = 0 To 9

series(count) = Val(InputBox("Enter a number."))

Next count

Dim count As Integer

For count = 0 To 999

names(count) = ""

Next count

Array Bounds Checking
• Visual Basic .NET will check each subscript value of each array reference used at run time
• If a subscript is used that is either negative or greater than the highest allowed, a dialog box will appear giving the option of Debugging or Stopping the application
• (Note that no bounds checking is done at design time)
For Each … Next Statement
• This statement is similar to a For…Next

Dim employees As String() = {"Jim", "Sally", _

"Henry", "Jean", "Renee" }

Dim name As String

For Each name In employees

MessageBox.Show(name)

Next name

There Are Many Uses of Arrays and Many Programming Techniques That Involve Them

Arrays May Be Used to Total Values and Search for Data

Related Information May Be Stored in Multiple Parallel Arrays

In Addition, Arrays May Be Resized at Run Time

Determining the Number ofElements in an Array
• Array\'s have a Length property that holds the number of elements in the array
• Note that the length is the number of elements on the array, not the largest subscript

Dim values(25) As Integer

For count = 0 to (values.Length – 1)

MessageBox.Show(values(count).ToString)

Next count

How to Total the Values in an Array
• The values are summed up in a variable that is initialized to zero beforehand:

Dim total As Integer = 0 \' Initialize accumulator

For count = 0 To (units.Length – 1)

total += units(count)

Next count

Getting the Average of theValues in a Numeric Array
• This algorithm first totals the values, then calculates the average:

Dim total As Integer = 0 \' Initialize accumulator

Dim average As Single

For count = 0 To (units.Length – 1)

total += units(count)

Next count

average = total / units.Length

Finding the Highest and LowestValues in a Numeric Array
• Pick the first element as the highest, then look through the rest of the array for even higher ones, always saving the highest value
• Proceed analogously to find the lowest value

highest = numbers(0)

For count = 1 To (numbers.Length - 1)

If numbers(count) > highest Then

highest = numbers(count)

End If

Next count

Copying One Array\'sContents to Another
• This is done by copying the elements one at a time, thusly
• Note that newvalues = oldvalues will not work for this use because the value of oldvalues is the storage location for that array (not all of the contents of oldvalues)

For count = 0 To 100

newValues(count) = oldValues(count)

Next count

Parallel Arrays
• Sometimes it is useful to store related data in two or more arrays
• Hence the ith element of one array is related to the ith element of another
• Then the program can access this related information by using the same subscript on both arrays

Dim names(4) As String

Parallel Arrays Example

Dim names(4) As String

For count = 0 To 4

lstPeople.Items.Add( "Name: " & names(count) & _

Next count

Parallelism Between Arrays andList Boxes and Combo Boxes

\' Initialize a List Box with names

\' Initialize an Array with corresponding phone numbers

phoneNumbers(0) = "555-2987"

phoneNumbers(1) = "555-5656"

phoneNumbers(2) = "555-8897"

\' Process a selection

If lstPeople.SelectedIndex > -1 And _

lstPeople.SelectedIndex < phoneNumbers.Length Then

MessageBox.Show(phoneNumbers(lstPeople.SelectedIndex))

Else

MessageBox.Show("That is not a valid selection.")

End If

Searching Arrays, The Search
• This code hunts for the value 100 in the array scores

\' Search for a 100 in the array.

found = False

count = 0

If scores(count) = 100 Then

found = True

position = count

End If

count += 1

Loop

Searching Arrays, Acting on the Result
• This code indicates whether or not the value was found or not

\' Was 100 found in the array?

If found Then

MessageBox.Show( _

"Congratulations! You made a 100 on test " & _

(position + 1).ToString, "Test Results")

Else

MessageBox.Show( _

"You didn’t score a 100, but keep trying!", _

"Test Results")

End If

Sorting an Array
• There is an Array.Sort method that sorts arrays in ascending order, examples:

Dim numbers() As Integer = { 7, 12, 1, 6, 3 }

Array.Sort(numbers)

\' OR

Dim names() As String = { "Sue", "Kim", "Alan", "Bill" }

Array.Sort(names)

Resizing an Array
• ReDim is a new keyword
• If Preserve is specified, the existing contents of the array are preserved
• Arrayname names the existing array
• UpperSubscript specifies the new highest subscript value

ReDim [Preserve] Arrayname(UpperSubscript)

Resizing Example

Dim scores() As Single \' Start array as having "Nothing"

\' Now obtain a size from the user

numScores = Val(InputBox("Enter the number of test scores."))

If numScores > 0 Then

ReDim scores(numScores - 1)

Else

MessageBox.Show("You must enter 1 or greater.")

End If

### 8.4Sub Procedures and Functions That Work With Arrays

You May Pass Arrays As Arguments to Sub Procedures and Functions

You May Also Return an Array From a Function

These Capabilities Allow You to Write Sub Procedures and Functions That Perform General Operations With Arrays

Passing Arrays as Arguments

\' In the calling program

Dim numbers() As Integer = { 2, 4, 7, 9, 8, 12, 10 }

DisplaySum(numbers)

\' The sub procedure

Sub DisplaySum(ByVal a() As Integer)

\' Displays the sum of the elements

Dim total As Integer = 0 \' Accumulator

Dim count As Integer \' Loop counter

For count = 0 To (a.Length - 1) \' << Length obtained

total += a(count)

Next

MessageBox.Show("The total is " & total.ToString)

End Sub

Passing Arrays: ByVal and ByRef
• ByRef when used with an array, allows unrestricted changes to the array
• ByVal with an array
• Does not restrict its values from being changed from within the sub procedure
• It does prevent an array argument from be assigned to another array (see next slide)
ByRef Allows This Sort of ModificationByVal Does Not

Dim numbers() As Integer = { 1, 2, 3, 4, 5 }

ResetValues(numbers)

Sub ResetValues(ByVal a() As Integer)

\' Assign the array argument to a

\' new array. Does this work?

\' NOT when using ByVal

\' It would if ByRef was used instead

Dim newArray() As Integer = {0, 0, 0, 0, 0}

a = newArray

End Sub

Returning an Array From a Function

Function GetNames() As String()

\' Get four names from the user

\' and return them as an array

\' of strings.

Dim names(3) As String

Dim input As String

Dim count As Integer

For count = 0 To 3

input = InputBox("Enter name " & _

(count + 1).ToString)

names(count) = input

Next

Return names

End Function

### 8.5Multidimensional Arrays

You May Create Arrays With More Than Two Subscripts to Hold Complex Sets of Data

A Two Dimensional Array Picture

Column 0 Column 1 Column 2 Column 3

Row 0

Row 1

Row 2

Two Dimensional Array Syntax

Dim ArrayName(UpperRow, UpperColumn) As DataType

• UpperRow and UpperColumn give the highest subscript for the row and column indices of the array
• The array on the previous slide could be:

Dim array(2,3) As Single

A Two Dimensional Array Showing the Subscripts

Dim array(2,3) As Single

Column 0 Column 1 Column 2 Column 3

Row 0 array(0,0) array(0,1) array(0,2) array(0,3)

Row 1 array(1,0) array(1,1) array(1,2) array(1,3)

Row 2 array(2,0) array(2,1) array(2,2) array(2,3)

Two Dimensional Arrays AreOften Processed With Nested Loops

For row = 0 To 2

For col = 0 To 3

num = Val(InputBox("Enter a score."))

scores(row, col) = num

Next col

Next row

Implicit Sizing and Initializationof Two-Dimensional Arrays
• This works for multi-dimensional arrays as well as singly dimensioned ones:

Dim numbers(,) As Integer = {{1, 2, 3}, _

{4, 5, 6}, _

{7, 8, 9}}

For Each Loops andMulti-dimensional Arrays
• A For Each Loop will process all of the elements in an array (without having to have nested loops):

Dim numbers(,) As Integer = {{1, 2, 3}, _

{4, 5, 6}, _

{7, 8, 9}}

For Each element In numbers

total += element

Next element

Summing the Columns of aTwo-dimensional Array

\' Sum the columns.

For col = 0 To 2

\' Initialize the accumulator.

total = 0

\' Sum a column.

For row = 0 To 4

total += values(row, col)

Next row

\' Display the sum of the column.

MessageBox.Show("Sum of column " & col.ToString & _

" is " & total.ToString)

Next col

Three-dimensional Arraysand Beyond
• Visual Basic .NET allows arrays with up to 32 dimensions
• Beyond three dimensions, they are difficult to visualize
• But, all one needs to do is to be consistent in the use of the different indices

### 8.6Enabled Property,Timer Control,Splash Screens

You Disable Controls by Setting Their Enabled Property to False

The Timer Control Allows Your Application to Execute a Procedure at Regular Time Intervals

Splash Screens Are Forms That Appear As an Application Begins Executing

Enabled Property, I
• Most controls have a Boolean property named Enabled
• When a control’s Enabled property is set to false, it is considered disabled, which means:
• It cannot receive the focus and cannot respond to events generated by the user
• In addition, it will appear dimmed, or grayed out
Enabled Property, II
• This property defaults to true (Enabled)
• Under program control this property can be set to whatever the application logic dictates

Timer Control, I
• This control, when placed on a form, generates Tick events on a regular interval
• If there is a corresponding Tick event procedure, it executes at these intervals
• Hence, your form can perform needed operations on a regular interval
Timer Control, II
• The timer control has two important properties:
• Enabled - if set to True, it generates the Tick events, otherwise not
• Interval - holds the interval between Ticks in milliseconds (thousandths of a second)
Splash Screens
• This is a form, typically with the application\'s logo, that is often displayed while an application is loading, it should:
• Have its Topmost property set to True so that it will be seen in preference to the applications other forms
• Disappear shortly (using a Timer)
• Be modeless

### 8.6Anchoring and Docking Controls

Controls Have Two Properties, Anchor and Dock, That Allow You to Determine the Control’s Position on the Form When the Form Is Resized at Run Time

Anchor Property
• A control may be Anchored to two adjacent sides of the form
• This situation will maintain the same distances from those edges whenever the form is resized by the user (control does not change size)
• A control may be anchored to opposite sides of a form
• This situation will again maintain those distances, but by changing the size of the control
Dock Property
• A control can be docked against any side of a form
• Whenever the form is resized, the control will change in size also to fill up the edge that it is docked to

### 8.7Random Numbers

Visual Basic .NET Provides the Tools to Generate Random Numbers and Initialize the Sequence of Random Numbers With a Seed Value

Random Number Generation, Step 1
• Random number generation is initiated with the selection of a "seed" number
• If you wish to repeat the same sequence of random numbers, then use the above statement with the same argument Number
• If you do not want to repeat a sequence, omit the argument

Randomize [Number]

Random Number Generation, Step 2
• Obtain the next random number in the sequence
• Repeatedly use a statement like the above to generate additional random numbers

randomNumber = Rnd

Random Number Range
• Rnd creates random numbers in the range of 0.0 to 1.0
• To scale these to a range between two integers that may be needed in your program:

randomNumber= Int(LowerNumber + Rnd * _

(UpperNumber - LowerNumber))