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Decision Making in Visual Basic

Discussion in 'Visual Basic [VB]' started by Sanskruti, Mar 31, 2007.

  1. Sanskruti

    Sanskruti New Member

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    All comparison operators produce true or false results. In other words, the comparison is either true or the comparison is false. The mathematical operators produce numeric values, whereas the comparison operators produce only true or false values. The rest of the program can use the true or false comparison operator result to make decisions. Comparison operators are operators that compare data values against each other and produce true or false results.

    The If Statement



    Perhaps the most important statement in a program is the If statement and then its statements. If performs one of two possible code actions, depending on the result of the comparison. In other words, If uses comparison operator results to test data. If might execute one or more lines of subsequent code, depending on the result of a comparison.

    Before If, the code you wrote executed sequentially, one statement after another. If lets your program be more decisive and execute only parts of the program if the data warrants partial execution. For example, suppose you were writing an invoicing system. In such a system, no sales tax should be computed for tax-exempt organizations, so your program would skip over the tax computation code when processing such organizations. If makes decisions. If a condition is true, the body of the If statement executes.

    Here is one format of If:
    Code:
    If Condition Then
    	One or more Visual Basic statements
    End If
    End If tells Visual Basic where the body of the If statement ends.

    Example:
    Code:
    Dim a As Integer 
    Dim b As Integer 
    a = 3 
    b = 4 
    If a > b Then 
    	MsgBox ("a is greater then b") 
    Else 
    	MsgBox ("b is greater then a") 
    End If 
    The parentheses are not required around the condition in an If, but they help separate the test from the rest of the code. In addition, the indentation helps illustrate the code that appears inside the If statement's body.

    There is a shortcut form of If that you might run across. The single-line If statement has a format that looks like this:
    Code:
    If condition Then VBStatement
    The single-line If doesn't require an End If statement because the condition and the body of the If reside on the same line. Single-line If statements don't provide for easy program maintenance. If you decide that you want to add to the body of the If, you must convert the single-line If to a multiple-line If, and you might forget to then add End If. Therefore, even if the body of an If statement takes only one line, code the If as a multiple-line If...End If statement to make the program more maintainable.

    If Statement's Else



    Whereas If executes code based on the condition's true condition, the Else statement executes code based on the condition's false condition. Else is an optional part of the If statement. Else specifies the code that executes if the condition is false.

    The complete format of the If statement with Else is as follows:

    Code:
    If condition Then
      One or more Visual Basic statements
    Else
      One or more Visual Basic statements
    End If
    Example:

    Code:
    num = Int(Text1.Text) 'Input
    
    If (num > 0) Then
    	strReply = "You have a car!"
    Else
    	strReply = "You don't have a car!"
    End If
    
    If (num > 1) Then strReply = "Are you rich!"
    
    'display our comment
    Label1.Caption = strReply
    

    Select Case



    If you have a lot of conditional statements, using If..Then..Else could be very messy. For multiple conditional statements, it is better to use Select Case the format is :
    Code:
    Select Case expression 
       Case value1 
            Block of one or more VB statements 
       Case value2 
            Block of one or more VB Statements 
       Case value3 
            Block of one or more VB statements 
       Case value4 
            . 
            . 
            . 
       Case Else 
            Block of one or more VB Statements 
    End Select   
    The data type specified in expression must match that of Case values.

    Example:

    Code:
    ' Examination Grades 
    Dim grade As String 
    Private Sub Compute_Click( ) 
    	grade=txtgrade.Text 
    	Select Case grade 
    	  Case  "A" 
    		   result.Caption="High Distinction" 
    	  Case "A-" 
    		  result.Caption="Distinction" 
    	  Case "B" 
    			result.Caption="Credit" 
    	  Case "C" 
    			result.Caption="Pass" 
    	  Case Else 
    			result.Caption="Fail" 
    	  End Select 
    End Sub
    Please note that grade is a string, so all the case values such as "A" are of String data type.

    Example:
    Code:
    Dim mark As Single 
    Private Sub Compute_Click() 
    	'Examination Marks 
    	 mark = mrk.Text 
    	Select Case mark 
    	Case Is >= 85 
    		comment.Caption = "Excellence" 
    	Case Is >= 70 
    		comment.Caption = "Good" 
    	Case Is >= 60 
    		comment.Caption = "Above Average" 
    	Case Is >= 50 
    		comment.Caption = "Average" 
    	Case Else 
    		comment.Caption = "Need to work harder" 
    	End Select 
    End Sub 
    Note we use the keyword Is here to impose the conditions. This is generally used for numeric data.
     
  2. Steel9561

    Steel9561 New Member

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    I think that decisions are used in all programs and this is taught in introduction to programming books. Basically you make decisions based on what you think the user will do in the program. Depending on the user selections you then take a specific action that you want the program to do.

    Luis Lazo
     
    Last edited by a moderator: May 1, 2008
  3. faizulhaque

    faizulhaque New Member

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    Any one tell me who two condition is used in if statement
    as in Turbo C++ we use

    if (statement && statement)
    {
    statement
    else
    statement
    }
    operator are allowed in VB
    like NOT, AND, OR
     
  4. shabbir

    shabbir Administrator Staff Member

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    Using the And Operator
     
  5. faizulhaque

    faizulhaque New Member

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    thanks u Shabbir for your assistance.

    I got and operator (gate) ' Or ' from a boxx mastring VB 6.0.

    I got my problem solved.
    Thanks a lot
     
  6. shabbir

    shabbir Administrator Staff Member

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    My pleasure.
     
  7. faizulhaque

    faizulhaque New Member

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    MSDN for VB?

    pl any one tell frm where i got that.
     

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