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Enterprise Development Using Visual Basic 6.0

Autumn 2002

Tirgul #2

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Short Quiz

• What is a property?

• What is the difference between Sub and Function?

• Write a sub that gets 2 integers and perform an Add operation

• Write the same as function

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Objective• Variables types• In a nutshell

– If - Then - Else– Case– Loops

• Write general sub procedures• Write user-defined functions• Examine some String manipulations

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Declaring Variables • Private , public – In a general module• Dim (Inside a sub/form)

– New memory location to a variable

• Static(Inside a sub/form)– retains its value between procedure calls– uses same memory location and keeps old value– Use static for variables whose values are

persistent (totals, counts, etc.)

• Const – for constants Sub test() static i As Integer i= i + 1End Sub

Sub test() dim i As Integer i= i + 1End Sub

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Declaring Variables 2

dim counter As Integer

private middleName As String

Static Index as Integer

Public const ARRAY_SIZE= 10

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Declaring Variables 3Data Types

– Boolean - True or false

– Date - From Jan 1, 100 to Dec 31, 9999

– Integer - Numbers without a decimal point

– Long - Long integer

– Single - Numbers with a decimal point

– Double - Long Single

– Currency - Dollar amounts

– String - Character and alphanumeric data

– Object - Any object reference such as Word document

– Variant - default, can hold any data type

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Data conversion

• Int to String? Use STR

• String to Int? Use Cint

• String to Numeric? Use Val – (Generic)

Str(num)

Cint(Str)

Val(Str)

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Decisions in VB• Used to alter the flow of a

program while program is running

• based on TRUE/FALSE

If condition Then

statements to be executed if condition is true

End If

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Decisions in VB 2• Complementary condition

• Use Else to perform both cases

If condition Then

statements to be executed if condition is TRUE

Else

statements to be executed if condition is FALSE

If

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ExampleIf index > 10 Then

print “Index > 10”

End If

If index > 10 Then

print “Index > 10”

Else

print ?

End If

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Indentation!!!Indentation!!!

• I rest my case…

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More IF Examples

If index > 10 AND printFlag = true Then

print “Index > 10”

End If

If index > 10 OR printFlag = true Then

print ?

End If

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Using IF in actual VB programming

TRUE

TRUE

FALSE

FALSE

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Code:

If opt1.Value = True Then

print “Option 1 selected”

End If

If opt1.Value = Checked Then

print “Option 1 selected”

Else

print “Option 2 selected”

End If

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Case Structure• Case Can replaces IF • Code readability – Important issue!• valueList options

– Case 1– Case 2 to 5– Case 6, 9– Case Is >= 10– Case “text”

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ExampleSelect Case textValue

Case “Print”

print

Case “send”

sendMail

Case Else

defaultAction

End Select

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Loops• The group of repeated instructions is called a

loop• a single execution of the statements in the

loop is called an iteration• All loops must have a mechanism to control

the number of iterations• Breaking point!

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For / Next Loops• Format:

For loopIndex = initialValue to testValue [Step increment]

body of loopNext loopIndex

• Items enclosed in [ ] are optional

• loopIndex index is declared before• loopIndex must be a numeric variable, testValue may

be any numeric expression (e.g. function) • Use For when you know the number of iterations.

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ExampleDim index as integer

For index = 1 to 10

print index

Next index

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Procedures

• Event procedures -

– associated with control events

– bounded by Sub and End Sub

• General procedures -

– not associated with events

– consist of statements that are grouped together because they perform a specific task

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Example Event Procedures

Private Sub cmdExit_Click()

closeConnection

End Sub

Private Sub cmdDisplay_Click()refreshData

End Sub

Private Sub Form_Load()openConnection

End Sub

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General procedures

• written in the General code section, but in a different window from the declarations

• two types:– Sub procedure - performs an action

– Function procedure - performs an action and returns a value. Format is based on mathematical notation: f(x), g(x, y)• May be user-defined or built in to VB (intrinsic)

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VB Intrinsic Functions

• Operate on 0 or more variables, and return exactly 1 value

• Functions we have already used:– Val(numeric string)

• returns a numeric value– Format(number, format string)

• returns a string in a specific form– Conversion functions

Review

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More intrinsic functions

• General math

– sqr( ), abs( ), exp( ), log( ), rnd( )

• Trigonometric functions

– sin( ), cos( ), tan( )

• Financial functions

• String Functions

Review

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String Functions

• Len - returns the length of a string

• Left, Mid, Right - returns the left, right, or middle part of a string

• Instr - returns the position of one string within another, or 0 if not found

• also note that “+” or “&” concatenates 2 strings together

Review

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Example 1

• Sub procedure PrintMessage is called from within the Click event procedure

• No arguments

Private Sub PrintMessage( )Print “Hello”

End Sub

Private Sub cmdDisplay_Click( )

PrintMessageEnd Sub

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Example 2

• Sub procedure Add_And_Print is called from within the Click event procedure

• Two arguments - Score1 and Score2

Sub Add_And_Print (x as Single, y as Single)

picOut.Print x + y

End Sub

Private Sub cmdDisplay_Click( )Add_And_Print (Score1, Score2)

End Sub

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Parameter passing• When variables are used as inputs (parameters)

– Parameters type must be consistent

– order of Parameters

– Optional - [ ]

Sub Output (stName as String, iNum as Integer)Print stName, iNum

End Sub

‘in another SubCall Output (“Joe”, 31) ‘ NOT Call Output(31, “Joe”)Call Output (stFirst, sAge)

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User Defined Functions

• starts with the word FUNCTION

• returns EXACTLY ONE value - as a given type

• called by using its name on the right side of an assignment statement, returns to a variable on the left side of an assignment statement

Function Add_Two (x as Integer, y as Integer) as IntegerAdd_Two = x + y

End Function

‘ in calling subsum = Add_Two(5, 6)

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Sample Functions

Function NewName (stFirst as String, stSec as String) as StringNewName = stFirst + “ “ + stSec

End Function

Function Celsius (ByVal fahrentemp As Single) as SingleCelsius = 5/9*(fahrentemp - 32)

End Function

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Example

Find whatever a given integer is a perfect number.

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Example(2) • Factorial Example

Factorial(10) ?

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String Manipulation • Usefull String manipulations:

str = “welcome

mid(str,4,4) = ? (watch index!!!)

str1 = “welcome”

str2 = “come”

InStr(1,str1,str2) = ?

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String Manipulation (2)

• StrComp function return variant (Integer) indicating the result of a string comparison.

• Syntax :

- StrComp(string1, string2, [compare])

dim MyStr1 , MyStr2 As String

dim MyComp As Integer

MyComp = StrComp (MyStr1, MyStr2, vbDatabaseCompare) : returns 0

MyComp = StrComp (MyStr1, MyStr2, vbBinaryCompare) : Returns - 1

MyComp = StrComp (MyStr2, MyStr1) : Returns 1

0vbBinaryCompare - Performs a binary comparison.

1vbTextCompare Performs a textual comparison.

2vbDatabaseCompare For Microsoft Access performs a comparison based on information contained in your database.