110 N-1

Motivation: to deal with large amounts of data

e.g sorting values:Input: 10, 15, 4, 15, 17, 3, 12, 36, 48, 32, 9, 21Want the Output: 3, 4, 9, 10, 12, 15, 17, 21, 25, 32, 36, 48

e.g Compute the average of 7 grades:Dim dblTotal, dblG1,dblG2, dblG3 As DoubleDim dblG4, dblG5, dblG6, dblG7 As Double'Initialize the grades…dblTotal = dblG1+dblG2+dblG3+dblG4 + dblG5+dblG6 MessageBox.Show(“Avg” & FormatNumber(dblTotal/7.0))

Is there a better way?

Arrays 1

110 N-2

Arrays 2• List or series of values OF THE SAME

TYPE all referenced by the same name• Similar to list of values for list boxes and

combo boxes - without the box• Use an array to keep a series of values for

later processing such as – Reordering– Calculating– Printing

110 N-3

Array Terms• Element

– Individual item in the array• Index (or subscript)

– Zero based number used to reference the specific elements in the array

– Must be an integer• Boundaries

– Lower Subscript, 0 by default– Upper Subscript

110 N-4

Simple Array ExamplestrName Array

(0)(1)(2)(3)(4)(5)(6)(7)(8)(9)

Janet BakerGeorge LeeSue LiSamuel HoosierSandra WeeksWilliam MacyAndy HarrisonKen FordDenny FranksShawn James

110 N-5

Defining Arrays • Use Dim statement to declare• Specify the number of elements in the array

as the UpperSubscriptGeneral form:Dim ArrayName(UpperSubscript) as DatatypeExample:Dim dblGrades(7) as Double

•Each element of the array will be assigned a default value

–Numeric ==> 0–String ==> empty string, 0 characters

110 N-6

Defining Arrays - Alternate Form• You cannot declare the Upper Subscript.

The number of elements will be determined by your entry.

• Optionally, the elements in the array may be assigned values in the Dim statement (initialized at declaration time)

General form:Dim ArrayName( ) as Datatype = {InitialValueList}Example:Dim dblGrades() as Double={3.4, 4.0,3.8}

110 N-7

Dim Statement for Arrays Examples - Default Values

Dim strName(3) as StringResults in an array of 4 elements:

strName(0), strName(1), strName(2), strName(3)

All initialized to an empty string

Dim decBalance(99) as DecimalResults in an array of 100 elements:

decBalance(0), . . . , decBalance(99) All initialized to an zero

NOTE:3 is the upper index Value. 0 is the lower index Value. There are 4 elements

110 N-8

Dim Statement for Arrays Examples - Assigned Values

Dim strDept( ) as String = {"ACT", "MKT", "HR"}Dim intActCode( ) as Integer = {10, 20, 30, 40}

110 N-9

What does VB do with the array?

• When the DIM statement for the array is processed VB sets aside room for it in memory.

• Ex: Dim strName(3) as String– VB sets aside a memory location for 4 strings

strName(0)(1)(2)(3)

110 N-10

Referencing Array Elements

• Use the Index(s) of the Element

strName(row)(0)(1)(2)(3)

Sam SmithJill CreechPaul FryRich Wells

strName(0) : "Sam Smith"strName(1) : "Jill Creech"strName(2) : "Paul Fry"strName(3) : "Rich Wells"

110 N-11

Example:Dim dblGrade(7) As Double...dblGrade(intVar1+3+intVar2) = 3.0

OK if 0 iVar1+3+iVar2 7 Can have complicated expressions:

dblGrade( CInt(3.1*2.71828*Math.Sin(2.*3.14)) )

An array index must evaluate to anInteger between the lower and upper bounds of the array. If the index is not anInteger, VB rounds it to an Integer. No Exceptions

Index rule: dblGrade(i)Some Rules ! (1)

110 N-12

lblGrade.Text = FormatNumber (dblGrade(i),0)

dblGrade(i)=2^2 *4.5

Example

Element rule:

An array element can be used wherever a simple variable of the same type can be used

No Exceptions

Some Rules ! (2)dblGrade(i)

110 N-13

Working with Arrays

• Use Loops to reference each element in the array– For / Next – For Each / Next

110 N-14

For Each Loop General Form

For Each ElementName In ArrayNameStatements to execute

Next [ElementName]

110 N-15

For Each / Next

• VB references EACH element of the array• VB assigns its value to ElementName

– The variable used for ElementName must be same datatype as the array elements or an Object datatype

• Makes one pass through the loop per element• Use Exit For statement within loop to exit

early

110 N-16

For Each / Next Examples

' Assumes array strName previously dimensionedDim strOneName As String ‘same type as the arrayFor Each strOneName In strName

Messagebox.Show(strOneName) ' Write one array elementNext strOneName

' Assumes array intTotal previously dimensioned Dim intOneTotal As IntegerFor Each intOneTotal In intTotal

intOneTotal=0 ' reinitialize the arrayNext intOneTotal

Name of the Array

110 N-17

Goal:• Get the grades of a class (the class size may vary)• Display the average grade

How?• Have an array • Ask for the size of the class: intClass

• Get the grades (loop)

• compute the average (loop)

Program Example(1)

• Dimension the Array to be big enough Dim dblGrades(intClass-1) as Double

110 N-18

110

Displays a prompt in a dialog box, waits for the user to input text or click a button, and returns a String containing the content of the text box.strSize = InputBox("Enter class Size","Input")

strSize contains 110

strSize contains ""

Program Example(2)-Input Box

110 N-19

Code for the example(3)

Dim strSize As String

Dim intCounter As Integer

Dim intClassSize As Integer

'ask user size of the class (number of students)

Do

strSize = InputBox("Please Enter Size", "Input")

If Not IsNumeric(strSize) Then

MessageBox.Show("enter a numeric value")

End If

Loop Until IsNumeric(strSize)

intClassSize = CInt(strSize)

 

110 N-20

Code for the example (4) 'now create the Array. All elements in the array are initialized to zero

Dim dblGrades(intClassSize - 1) As Double

'Use a loop to get all of the grades

Dim dblGrade As Double

For intCounter = 0 To dblGrades.Length - 1

'ask the user for the grades and place them in the array

Try

dblGrade = CDbl(InputBox("Enter Grade"))

dblGrades(intCounter) = dblGrade

Catch

MessageBox.Show("enter a numeric grade")

'Decrement to use the same index value at the next iteration

intCounter = intCounter - 1

End Try

Next

110 N-21

Code for example (5)'now we have the array all populated with data we can

calculate the average ‘we are going to use a For Each Next Loop Dim dblGrade As Double Dim dblTotalSum As Double Dim dblAverage As Double For Each dblGrade In dblGrades dblTotalSum = dblTotalSum + dblGrade Next dblGrade dblAverage = CDbl(dblTotalSum / dblGrades.Length) MessageBox.Show("Average: " & _

FormatNumber(dblAverage, 2)) 'If you wanted to use a normal For Next LoopFor intCounter = 0 To _ dblGrades.Length - 1 dblTotalSum = _ dblTotalSum + _dblGrades(intCounter) Next

110 N-22

Dim dblArray1(intSIZE) As DoubleDim dblArray2(intSIZE) As Double

some constant previously defined

Suppose dblArray1 has been initialized:Cannot do: dblArray2 = dblArray1;

Cannot do: If dblArray1 = dblArray2 ...

But can do this on array elements

Write your own functions

Pitfalls of Arrays