‘Tirgul’ # 3

Enterprise Development Using Visual Basic 6.0

Autumn 2002

Tirgul #3

‘Tirgul’ # 3

Short Quiz

• Write a simple Function that receives a String and returns its length

• Write a Simple Function that receive an Integer, if it is negative, returns a String with error message, otherwise returns the Integer as a String

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Objectives• Parameters passing• Getting Deeper

– If – Then – Else– Case– Loops

• Arrays– Dimension arrays correctly– Recognize the default values of array elements– Access array items by subscript or all elements in a loop– Use parallel arrays for storage of related lists of items– Use list boxes for displaying choices– Use 2-D arrays

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Review: important concepts• Remember to plan your programs

(algorithm) before starting to write the code! (80:20)

• Use procedures and functions

– parameters must be carefully set

– Use ByRef and ByVal

• Fundamental Structures:

– Sequence, Selection, and Iteration

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Parameter Passing

• By default, variables are passed ByRef

– called procedure uses same memory location for the variable

– assigns a new name for that location, uses that name within the procedure

– contents of this location may be changed

Sub Add_And_Change (ByRef x as Single, ByRef y as Single)x = x + y

End Sub

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Parameter passing

• Parameters can also be passed ByVal– called procedure sets up new memory

locations– value of parameter is copied into the new

locations– changes do not affect original variable

Sub Add_And_Print (ByVal x as Single, ByVal y as Single)x = x + ypicOut.print x

End Sub

Example

What will be printed out?

What if we called

Sample2 (b, a) ?

Sub Sample1 (ByRef x as Single, ByRef y as Single)

x = x - ypicOut x, y

End Sub

Sub Sample2 (ByVal x as Single, ByVal y as Single)

x = x - ypicOut x, y

End Sub

Sub cmdComputeDim a, b as Singlea = 2b = 3Sample2 (a, b)picOut1.Print a, bSample1 (a, b)picOut.Print a, b

End Sub

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Nested IF statements

If condition1 Then

If condition2 Then

statement1

End If

Else

statement2

End if

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ExampleIf Grade >= 90 Then

txtLetterGrade.Text = “A”

Else

If Grade >=80 Then

txtLetterGrade.Text = “B”

Else

txtLetterGrade.Text = “C”

End If

End If

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ExampleIf Income >= 40000 Then

If Status = “Single” Then

TaxRate = 0.33

ElseIf Status = “Married” Then

TaxRate = 0.25

End If

Else

sTaxRate = .15

End If

Less end if’s

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If statement summary• VB is sensitive at design-time and run-

time

• Indentation!

• Use if… Then

• Use end if

• Use elseif

Sub Sample_if () If a = b then … End ifEnd Sub

Sub Sample_if2 () If a = b then … elseif … End ifEnd Sub

Sub Sample_if2 () If a = b then … else if a > b then … end if End ifEnd Sub

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Select Case Structure• Multiple IF statements can be replaced by more

readable Select CASE statements

Select Case selector Case valueList1

action1 Case valueList2

action 2 … Case Else

if no other matchEnd Select

valueList options:Case 1

Case 2 to 5

Case 6, 9

Case “text”

Case Is >= 10

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Example

Select case AccessCode

case is < 1000

message = “Access Denied”

case 1465 To 1689

message = “Technical Personal”

case 999898 , 10000006 To 10000008

message = “Scientific Personal“

case Else

message = “Access Denied”

End Select

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Case statement summary

• Use String/Integer type

• When using String pay attention to content!

• Don’t forget default

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Do / Loops

Format 2:

Do

loop body

Loop {While | Until} Condition

Format 1:

Do {While | Until} Conditionloop body

Loop

•Loop body executes while the condition is true or until the condition is true

•The first form uses a pretest, the second uses a posttest.

•With a pretest, loop may not execute at all.

•With a posttest, loop always executes at least once.

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Do While Example

Do While sTotal < 25

sNum = Val(Inputbox(“Enter a number”))

If sNum > 0 then

sTotal = sTotal + sqr(sNum)

End If

Loop

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Choosing Loop Type

• For..Next loop should be used when you know the number of loops

• Do ..While is used when you do not know in advance the number of iteration.

– Examples• You want to get user input until she hits

the escape button• You want your server to keep listening

for request until it receives a shut down message.

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Loops summary

• For – next

• While – wend

• Do – while

• Common mistakes

– Stop condition

– Increment

• Exit loop

Do For I = 1 To 1000 MyNum = Int(Rnd * 100) Select Case MyNum Case 7: Exit For Case 29: Exit Do Case 54: Exit Sub End Select Next I Loop

While true … index = index +1Wend

Danger

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Arrays

• Array – Set of elements of the same type indexed in a data structure

• Each array object is called an element

• Each element is identified with an Index

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Why use arrays?• Often, all data must be read in and stored for

future use.• Not always possible to get each value and process

it, it is better to get all data, store it in an array and process it

• Use of data structures is preferred and unavoidable.

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Declaration• Array name - identifier

• Array type – type of elements in the array

• Array range – how many elements

class(1) class(30)

Dim class(1 to 10) as students

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Subscripts

• Recall – Array starts at 1 BUT:

• Array of size 25

• subscripts may be constants, variables, or numeric expressions

• Use Constants!!!

Dim GradeArr(MAX_STUDENTS) as Single

Dim GradeArr(0 to 29) as Single

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Array Value setDim GradeArr(1 to MAX_STUDENTS) as Single

GradeArr(1) = 95

for index = 2 to 10

GradeArr(index) = 100

next index

GradeArr(11) = (sGrade(1) + sGrade(2) )/2

GradeArr(30) = ?

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Array bounds• Ubound, Lbound will return the array

declared size (NOT elements)

• To prevent errors, check Ubound for highest index

• Common use to iterate the array:

for index = Lbound(GradeArr) to Ubound(GradeArr)

…

next index

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Declaring Dynamic Array• An Array declaration at the module level – No

memory location yet:

• Memory allocation at sub/function level:

Dim DBConnections() as Connection

Public DBRecordSet() as RecordSet

Sub openConnection ()

. . .

ReDim DBConnections(1 To MAX_CONNECTIONS)

End sub

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Redim• Resizing an existing array• Procedure level allocation• Deallocate memory (free memory)

• Array values are lost at Redim!• Use preserve to keep old values

ReDim preserve newArray(MAX_ARRAY_SIZE + 1)

Redim array(0)

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For Each…Next Statement• Use For Each…Next Iteration to iterate al

array elements

• Use variant type variable if you don’t know element type.

Dim day as Variant

For each day in week

print day

Next day

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Split - Join• Split – Namely split a String to the array

• Join – Namely create a String from array

dim wordsArr()as String

dim Sentence as string

Sentence = “good morning”

wordsArr = Split(Sentence,“ “ )

Sentence = join(wordsArr,“ “)

1

2

Good

Morning

wordsArr

Sentence

“good morning”

Delimiter

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Two-Dimensional Arrays

• Think of a table structure

• use two indexes

– row index

– column index

• all elements must be of the same type

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Examples

• Dim iArray (1 to 2, 1 to 5) as Integer

• array of 2 rows and 5 columns

Col

Row

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Accessing Array Elements

• iArray (1, 2) = 10

• iArray (2, 5) = iArray(1, 4) + iArray(1, 5)

• I = 7 : J = 5

sGrades (I, J) = 93

• x = 1 : y = 2

iArray (1, x + y) = 15

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Practical - Combo Boxes• Combo box represent a list of items to select

from.• Items can be specified during Design/Run

time• Uses:

– Inset Item– Delete Item– Get selected Item

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Code sample for Combo Box

• ListIndex represents the selected Index• Text represents the item String value

• Remove selected item

• Add items to list

cmbDays.RemoveItem cmbDays.ListIndex

cmbDays.AddItem “Sunday”

Debug.print cmbDays.text

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Using Combo Boxes As an Array

• Combo box is an array controlled by VB

• Count – combobox.ListCount

• Add/Remove operations

• Lists start at 0, array at 1

Fill

Clear

Count

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Controls array• In a from, you can group elements

– Tab, Option button…

• Useful when having lots of controls

• Notation:

• Design time

– Copy/Paste in the form

– setting index

Option(0), Option(1)