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: VB
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: VB
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: VB
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: vb
If condition Then
One or more Visual Basic statements
Else
One or more Visual Basic statements
End If
Example:
Code: VB
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: VB
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: VB
' 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: VB
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.
