With the If statement,simple decision making in Visual Basic takes place, which has the following general format.
A D V E R T I S E M E N T
If condition Then
...statements
End If
Conditional Operators
Using relational operators,conditional expressions are set up to compare values and these operators, which were presented earlier, are repeated in the following table.
Operator
Use
=
Tests whether two values are equal.
<>
Tests whether two values are not equal.
<
Tests whether the first value is less than the second value.
>
Tests whether the first value is greater than the second value.
<=
Tests whether the first value is less than or equal to the second
value.
>=
Tests whether the first value is greater than or equal to the second
value.
A conditional expression is formed by relating values -- either constants or variables -- with these operators. The comparison is evaluated as Boolean True or False, and the script reacts to that condition. The following script presents a simple example of script decision making.
Sub Button_Click (Src As Object, Args As CommandEventArgs)
If Args.CommandName = "Button 1" Then
ButtonResponse.Text = "You clicked Button 1"
End If
End Sub
<asp:Button Text="Click Me" runat="server"
OnCommand="Button_Click" CommandName="Button 1"/>
<asp:Button Text="Click Me" runat="server"
OnCommand="Button_Click" CommandName="Button 2"/>
<asp:Label id="ButtonResponse" EnableViewState="False" runat="server"/>
Logical Operators
Sometimes you need to combine condition tests and that time, you might need to know if a numeric value
is within a particular range; that is, whether the number is greater than one value and less than
another value. Or, you might want to see if the number is outside a range by testing whether
it is less than one value or greater than a second value and these kinds of combination tests are made possible by using two or more relational tests combined with
the logical operators And, Or, and Not to form multiple comparisons.
These operators were presented earlier and are repeated in the table given below.
Logical
Operator
Comparison
And
condition1 And condition2
The condition1 and
condition2 tests both must be true for
the expression to be evaluated as True.
Or
condition1 Or condition2
Either the condition1 or
condition2 test must be true for the
expression to be evaluated as True.
Not
Not condition
The expression result is set to its opposite; a True
condition is set to False and a
False condition is set to
True.
Xor
condition1 Xor condition2
Either condition1 is true or
condition2 is true, but not both, for
the condition test to be evaluated as True.
Below is an example that uses a multiple condition test and after entering a value in the text box, click the "Check" button. The script determines if you entered a number between 1 and 10.If so, confirms this value with a message.
Sub Check_Number (Src As Object, Args As EventArgs)
If Number.Text <> "" Then
If Number.Text >= 1 And Number.Text <= 10 Then
NumberResponse.Text = "The number is between 1 and 10."
End If
End If
End Sub
<asp:TextBox id="Number" Size="1" runat="server"/>
<asp:Button Text="Check" OnClick="Check_Number" runat="server"/>
<asp:Label id="NumberResponse" EnableViewState="False" runat="server"/>
If Number.Text <> "" Then
checks whether a value has been entered into the textbox (it is not equal to empty, or null) and if this condition is true, then the enclosed statements are executed. The enclosed statements form a second If test and in this case the value entered in the textbox is tested so see if it is greater than or equal to 1 and if it is less than or equal to 10. If these two conditions are true, then only the message is displayed
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