csci 3131.01 chapter 8 arrays instructor: bindra shrestha university of houston – clear lake

CSCI 3131.01

Chapter 8 Arrays

Instructor: Bindra Shrestha

University of Houston – Clear Lake

Acknowledgement

Dr. Xinhua Chen

And

Starting Out with Visual Basic 2010 by Tony Gaddis and Kip Irvine

Chapter 8 Arrays, Timers, and More

Topics•Declare and initialize a one-dimensional array•Search a one-dimensional array•Compute the sum and average of a one-dimensional array•Sort a one-dimensional array•Copy between arrays•Using For Each … Next with array•Create and manipulate parallel one-dimensional arrays•Create and initialize a two-dimensional array•Enabled property•Timer•Random number generation

Introduction

• Arrays are like groups of variables that allow you to store sets of similar data– A single dimension array is useful for storing and working

with a single set of data, like a column or a row of data.– A multidimensional array can be used to store and work

with multiple sets of data. A two-dimensional array deals with data in a table.

• 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

Array Characteristics

• An array stores multiple values of same type• Like a group of variables with a single name• For example, the days of the week might be:

– a set of 7 string variables

– with a maximum length of 9 characters

• All variables within an array are called elements and must be of the same data type

• You access the elements in an array through a subscript

Subscript Characteristics

• A subscript, also called an index, is a number that identifies a specific element within an array

• Subscript numbering works like a list box index:– Subscript numbering begins at 0

– 1st element in an array is always subscript 0

– Last element is total number of elements – 1

• An array with 7 elements refers to the 1st element as subscript 0 and the last element as subscript 6

Declaring a One-Dimensional Array

• ArrayName is the variable name of the array• UpperSubscript is the value of the array's highest

subscript– Must be a positive whole number

• DataType may be any Visual Basic data type• VB knows an array is declared when name is followed

by parentheses with number of elements

Dim ArrayName(UpperSubscript) As DataType

Array Declaration Example

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

Dim intHours(5) As Integer

• Note that 6 elements are available when an array with an upper subscript of 5 is declared

• Each element is an integer type

Default Initialization

• All elements of an Integer array are initialized to zero– Same initialization as an integer variable

• Each array element is initialized exactly the same as a simple variable of that data type– A Single is initialized to zero (0.0)– A String is initialized to Nothing

Implicit Array Sizing & Initialization

• An array can be initialized when declared• Example:

• This array is implicitly sized– Upper subscript value is left blank

– Number of elements implied from initialization

– Upper subscript of 5 implied by this example

– This results in a 6 element array

• Elements are assigned the values shown

Dim intNumbers() As Integer = {2, 4, 5, 7, 9, 12}

Named Constant as Upper Subscript

• A named constant may be used as an array's highest subscript instead of a number

• This is a common use for named constants– Highest subscript is often used multiple times– If highest subscript changes, use of a named

constant allows it to be changed in one place

Const UPPER_SUB As Integer = 100Dim array(UPPER_SUB) As Integer

Working With Array Elements

• Simply include the subscript to use an array element like an ordinary variable

• intNumbers(2) refers to 3rd element of arrayintNumbers(0) = 100intNumbers(1) = 200intNumbers(2) = 300intNumbers(3) = 400intNumbers(4) = 500intNumbers(5) = 600

decPay = intHours(3) * decRate

intNumbers(0) += 1

MessageBox.Show(decPay(5).ToString())

Arrays and Loops

• Loops are frequently used to process arrays

• Use a loop to prompt for 10 values

• Use a loop to reset values in the strNames array to an empty string

Dim intNbrs(9) as IntegersDim intCount As IntegerFor intCount = 0 To 9 intNbrs(intCount) = CInt(InputBox("Enter number"))Next intCount

Dim strNames(999) as StringDim intCount As IntegerFor intCount = 0 To 999

strNames(intCount) = ""Next intCount

Array Bounds Checking

• Visual Basic checks each subscript value of each array reference used at run time

• A subscript is invalid if it is– Negative

– Greater than the UpperSubscript declared

• An invalid subscript causes VB to throw a run-time exception

• Bounds checking is not done at design time

For Each Loop Array Processing

• A For…Each loop is similar to a For…Next• Iterates for each element in the array• strName is assigned the value of the next array

element at each iteration

• Tutorial 8-1 demonstrates array processing

Dim strEmployees() As String = {"Jim", "Sally", _"Henry", "Jean", "Renee"}

Dim strName As StringFor Each strName In strEmployees

MessageBox.Show(strName)Next strName

Use For Each To Sum an Array

• Start of iteration 1 intVal = 10 intSum = 0• End of iteration 1 intVal = 10 intSum = 10



• End of iteration 4 intVal = -20 intSum = 120• Start of iteration 4 intVal = -20 intSum = 100

Private intArray() as Integer = {10, 20, 90, -20)Dim intSum as Integer = 0For Each intVal as Integer in intArray intSum = intSum + intValNext

Use For Each To Find Largest Value

• Start of iteration 1 intVal = 10 intLargest = 10• End of iteration 1 intVal = 10 intLargest = 10



• End of iteration 4 intVal = -20 intLargest = 90• Start of iteration 4 intVal = -20 intLargest = 90

Dim intArray() as Integer = {10, 20, 90, -20)Dim intLargest as Integer = intArray(0)For Each intVal as Integer in intArray If intVal > intLargest Then intLargest = intValNext

Determining Number of Elements

• Arrays have a Length property that holds the number of elements in the array

• Note that length is number of array elements, not the upper subscript of the array

• Length property always 1 greater than the upper subscript

Dim intValues(25) As Integer

For intCount = 0 to (intValues.Length – 1)MessageBox.Show(intValues(intCount).ToString)Next intCount

Total the Values in an Array

• Variable to hold sum (intTotal)is initialized to zero prior to loop

• Iterate over all elements, adding each element to intTotal

Dim intUnits(24) as Integer 'Declare arrayDim intTotal As Integer = 0 'Initialize accumulatorDim intCount as Integer 'Declare loop counter

'Find total of all values held in arrayFor intCount = 0 To (intUnits.Length – 1)

intTotal += intUnits(intCount)Next intCount

Average the Values in an Array

• Similar to previous example but then divides total by number of elements to get averageDim intUnits(24) as Integer 'Declare arrayDim intTotal As Integer = 0 'Initialize accumulatorDim dblAverage as Double 'Declare average varDim intCount as Integer 'Declare loop counter

'Find total of all values held in arrayFor intCount = 0 To (intUnits.Length – 1)

intTotal += intUnits(intCount)Next intCount

'Use floating-point division to compute averagedblAverage = intTotal / intUnits.Length

Find Highest & Lowest Array Values

• Pick first element as the highest• Search rest of array for higher values• If a higher value is located, save the value

• Use similar logic to find lowest value

Dim intNumbers(24) as IntegerDim intCount as IntegerDim intHighest as Integer = intNumbers(0)

For intCount = 1 To (intNumbers.Length - 1)If intNumbers(intCount) > intHighest Then

intHighest = intNumbers(intCount)End If

Next intCount

Array Copy Using Loop

• Copy one element at a time

• Note that array copy cannot be done by a simple assignment intNewValues = intOldValues – Causes intNewValues array to reference the same

memory location as intOldValues– Thus any change to intOldValues will affect intNewValues and vice versa

– This is because arrays are object variables

For intCount = 0 To intOldValues.Length -1 intNewValues(intCount) = intOldValues(intCount)Next intCount

Array Copy Using Copy Method of Array

Syntax: Array.Copy(SourceArray, DestArray, length)

Dim a() As Integer = {1, 2, 3, 4, 5, 6, 7, 8}Dim b() As Integer = {2, 3, 4, 5}Array.Copy(b, a, 4) '4 is the number of elements in b

' This makes array a contain: ' { 2, 3, 4, 5, 5, 6, 7, 8}

Parallel Arrays

• Sometimes it’s useful to store related data in two or more arrays called parallel arrays– Causes the nth element of one array to be related to

the nth element of another

• Allows related data to be accessed using the same subscript on both arrays

Parallel Arrays Example

• Assume the following array declarations

• Element 1 refers to the same person with name in one array and address in the other

Dim strNames(4) As StringDim strAddresses(4) As String

strNames(0) strNames(1) strNames(2) strNames(3) strNames(4)

strAddresses(0) strAddresses(1) strAddresses(2) strAddresses(3) strAddresses(4)

Person #1 Person #2 Person #3 Person #4 Person #5

• Use parallel array processing to add name and address of each person to a list box

• Tutorial 8-2 has an example of parallel arrays

Parallel Arrays Processing

Dim strNames(4) As StringDim strAddresses(4) As String

For intCount = 0 To 4lstPeople.Items.Add("Name: " & strNames(intCount) _

& " Address: " & strAddresses(intCount))Next intCount

Array & List Box Parallel Processing

' Initialize a List Box with nameslstPeople.Items.Add("Jean James") ' Subscript 0lstPeople.Items.Add("Kevin Smith") ' Subscript 1lstPeople.Items.Add("Joe Harrison") ' Subscript 2' Initialize an Array with corresponding phone numbersphoneNumbers(0) = "555-2987"phoneNumbers(1) = "555-5656"phoneNumbers(2) = "555-8897"

' Process a selectionIf lstPeople.SelectedIndex > -1 And _

lstPeople.SelectedIndex < phoneNumbers.Length ThenMessageBox.Show(phoneNumbers(lstPeople.SelectedIndex))

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

End If

• A list box selection used as an array index

Searching Arrays, The Search

• Find 1st instance of value 100 in array intScores• intPosition gives position of this value

Dim blnFound as Boolean = FalseDim intCount as Integer = 0Dim intPosition as Integer = -1

' Search for a 100 in the arrayDo While Not blnFound And intCount < scores.Length

If intScores(intCount) = 100 ThenblnFound = TrueintPosition = intCount

End If intCount += 1Loop

Searching Arrays, The Result

• Indicates whether the value was found

• If found, position is given as a test number

' Was 100 found in the array?If blnFound Then

MessageBox.Show( _"Congratulations! You made a 100 on test " & _(intPosition + 1).ToString, "Test Results")

ElseMessageBox.Show( _

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

End If

Sorting an Array, Ascending

• Arrays have a Sort method • Arranges elements in ascending order (lowest to

highest)• Sort intNumbers in this order: {1, 3, 6, 7, 12}

• Sort strNames in this order: {Alan, Bill, Kim, Sue}

Dim intNumbers() As Integer = { 7, 12, 1, 6, 3 }Array.Sort(intNumbers)

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

Array.Sort(strNames)

Sorting an Array, Descending

• Use the Sort method in conjunction with the Reverse method

• Arranges elements in descending order (highest to lowest)

• Sort intNumbers in this order: {12, 7, 6, 3, 1} Dim intNumbers() As Integer = { 7, 12, 1, 6, 3 } Array.Sort(intNumbers) Array.Reverse()

• Sort strNames in this order: {Sue, Kim, Bill, Alan} Dim strNames() As String = { "Sue", "Kim", _

"Alan", "Bill" } Array.Sort(strNames) Array.Reverse()

Dynamically Sizing an Array

• ReDim is a new keyword– Used to change the number of elements at run time

• If Preserve is specified, the existing contents of the array are preserved

• Arrayname names the existing array• UpperSubscript specifies the new highest

subscript value• Can declare an array with no subscript and state

number of elements later with ReDim

ReDim [Preserve] Arrayname(UpperSubscript)

Resizing Example

Dim sngScores() As Single ' Declared with no elementsDim intNumScores as Integer

' Obtain number of elements from the userintNumScores = CInt(InputBox("Enter number of test scores"))If intNumScores > 0 Then

ReDim sngScores(intNumScores - 1)Else

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

• Array sngScores declared with no elements• User prompted for number of elements• ReDim resizes array based on user input

Passing Arrays as Arguments to Procedures

Dim intNumbers() As Integer = { 2, 4, 7, 9, 8, 12 }DisplaySum(intNumbers)

Sub DisplaySum(ByVal intArray() As Integer)Dim intTotal As Integer = 0 ' AccumulatorDim intCount As Integer ' Loop counterFor intCount = 0 To (intArray.Length - 1)

intTotal += intArray(intCount)NextMessageBox.Show("Total is " & intTotal.ToString)

End Sub

• Array intNumbers passed to DisplaySum sub

• Sub computes & shows sum of array elements• Can pass any integer array to DisplaySum

Passing Arrays: ByVal and ByRef

• When passing an array ByVal– Calling procedure “sees” sub procedure changes to element

values• Simple variables don’t work this way

– If sub assigns array argument to another array, no effect on array in calling procedure

• When passing an array ByRef – Calling procedure “sees” sub procedure changes to element

values– If sub assigns array argument to another array, calling

procedure array values affected

Passing Arrays Example

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

Sub ResetValues(ByVal intArray() As Integer) Dim newArray() As Integer = {0, 0, 0, 0, 0} intArray = newArrayEnd Sub

• After ResetValues procedure executes, intNumbers array still contains { 1, 2, 3, 4, 5 }

• If array passed ByRef, intNumbers array will contain { 0, 0, 0, 0, 0 } after procedure runs

An Array Returned From a Function

Function GetNames() As String()' Get four names from user' Return them as an array of strings.Dim strNames(3) As StringDim strInput As StringDim intCount As IntegerFor intCount = 0 To 3 strInput = InputBox("Enter name " & _

(intCount + 1).ToString) strNames(intCount) = strInputNextReturn strNames

End Function

• Return type String() indicates array of strings returned• Thus the function result must be assigned to an array of strings

A Two Dimensional Array Picture

Column 0 Column 1 Column 2 Column 3

Row 0

Row 1

Row 2

• Thus far, arrays have been one-dimensional

• Arrays can also be two-dimensional

• A two-dimensional array looks like a spreadsheet with rows and columns

Two Dimensional Array Syntax

• UpperRow and UpperColumn give the highest subscript for the row and column indices of the array

• The array on the previous slide could be:

– Defines three rows; 0, 1, and 2

– And four columns; 0, 1, 2, and 3

Dim ArrayName(UpperRow, UpperColumn) As DataType

Dim array(2,3) As Single

Two Dimensional Array Subscripts

Dim array(2,3) As Single

Column 0 Column 1 Column 2 Column 3

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



Nested Loops And Two Dimensions

For intRow = 0 To 2For intCol = 0 To 3

intNum = Val(InputBox("Enter a score."))sngScores(intRow, intCol) = intNum

Next intColNext intRow

• Must use two subscripts to indicate a single element in a two dimensional array

• Nested loops are often used to specify subscripts for two-dimensional array processing

• For example, a nested loop below is used to insert a value into every element of array scores

Implicit Sizing and Initialization

• Can be used with multi-dimensional arrays:

• Row 0 values• Row 1 values• Row 2 values

• Initializes array intNumberswith the following values

Dim intNumbers(,) As Integer = _{ {1, 2, 3}, {4, 5, 6}, {7, 8, 9} }

Col 0Col 0 Col 1Col 1 Col 2Col 2

Row 0Row 0 11 22 33

Row 1Row 1 44 55 66

Row 2Row 2 77 88 99

Sum Columns, 2-Dimensional Array

• Code below uses nested loops to compute sum of elements in each column of a 2 dimensional array

• Outer loop controls column subscriptDim intValues(,) As Integer = {{1, 2, 3, 4}, _

{5, 6, 7, 8}, _{9, 10, 11, 12}}

For intCol = 0 to 3intTotal = 0 'Initialize accumulatorFor intRow = 0 to 2 'Sum rows in this column

intTotal += intValues(intRow, intCol)next intRow'Display the sum of the columnMessageBox.Show("Column " & intCol.ToString() _

& " sum is " & intTotal.ToString)Next intCol

Three-Dimensional Arrays & Beyond

• VB allows arrays of up to 32 dimensions• Can think of a 3

dimensional arrayas having multiple pages of two dimensional arrays

• Arrays beyond three dimensions are difficult to visualize

• But, all one needs to do is to be consistent in the use of the different indices

Enabled Property

• Most controls have an Enabled property• If this Boolean property is set to false, the control is

disabled, meaning the control:– Cannot receive the focus– Cannot respond to user generated events– Will appear dimmed, or grayed out

• Default value for this property is true• May be set in code when needed as shown:

btnExample.Enabled = False

Timer Control

• A Timer control generates Tick events– Double-click on Timer tool in Toolbox to add– Appears in component tray– Standard prefix is tmr

• Can perform a process at regular intervals by coding the process in a Tick event procedure

• Two important properties– Enabled must be true to generate Tick events– Interval (milliseconds) decides tick frequency

• Interval of 1000 causes 1 timer tick per second

• Tutorial 8-5 demonstrates the Timer control

Splash Screens

• A form, often with an application logo, that is displayed while an application is loading

• A splash screen usually:– Has Topmost property set to True so it is displayed over the

forms of other applications

– Disappears shortly by using a Timer

– Is modeless so the application continues to load while the splash screen is displayed

• Tutorial 8-6 demonstrates a splash screen

Anchor Property

• Anchor - a property of various form controls• A control anchored to a form edge keeps the same

distance to the form edge• Controls may be anchored to any, all, or none of

the right, left, top, or bottom edges– Default is to anchor to top and left edges– Can just as easily anchor to bottom right– If anchoring to opposing sides, control is resized when

form is resized– Anchoring to all 4 sides causes control to be resized

proportionally to form

Dock Property

• Dock - a property of various form controls• A control docked to a form edge is placed

directly against the form edge • Controls may be docked to

– Any one form edge but not more than one– No edge or not docked (the default)

• Length or width of docked control changes to match length or width of the form edge

Initial Random Number Generation

• Often used for games and simulations• Must create an instance of the Random class to use

random number methods and properties

• May supply a seed number as below so the same set of random numbers are generated every time

• If no seed number is supplied when instantiated:– VB uses its default random number seed

– Causes each sequence to be unique

Private rand as New Random(25)

Private rand as New Random

Generating More Random Numbers

• The Next method returns the next integer in a random number sequence

– Returns integer between 0 and 2,147,483,647

• Repeatedly use a statement such as the following to generate additional random numbers

Dim intNum as Integer = rand.Next()

intNum = rand.Next()

Random Numbers in a Range

• Can set an upper limit for the number generated by specifying the limit as an argument– Following example generates a random number between 0

and 99

• Lower limit of a random number is always 0– Can add or subtract to change this lower limit

– Following example generates a random number between -50 and +49

intNum = rand.Next(100)

intNum = rand.Next(100) - 50

Random Numbers of Type Double

• Use NextDouble method to generate random numbers of type double– Returns floating point number from 0.0 to 1.0

• For a random double between 0.0 and 500.0

• For a random double between 100.0 and 600.0

• Tutorial 8-7 shows an interesting use of random numbers

Dim dblNum as Double = rand.NextDouble()

dblNum = rand.NextDouble() * 500

dblNum = (rand.NextDouble() * 500) + 100

csci 3131.01 chapter 8 arrays instructor: bindra shrestha university of houston – clear lake

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