Visual Basic Comparison and Logic
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In the previous chapter (Visual Basic Arithmetic) we looked at how expressions are constructed in Visual Basic and looked in some detail at how to create arithmetic expressions. In this chapter we will look at constructing comparison and logic expressions in Visual Basic.
 Visual Comparison Operators
Comparison operators provide the ability to compare one value against another and return either a True or False result depending on the status of the match. For example, you might use a comparison operator to check if a variable value matches a particular number, or whether one string is identical to another. Visual Basic provides a number of different operators for just about every comparison need.
The comparison operators are used with two operands, one to the left and one to the right of the operator. The following table outlines the Visual Basic comparison operators and provides brief descriptions and examples:
|=||Equal to||Returns True if first operand equals second||Dim intValue = 10
If intValue = 10 Then Exit Sub
|<||Less than||Returns True if the value of the first operand is less than the second||If intValue < 10 Then Exit Sub|
|>||Greater than||Returns True if the value of the first operand is greater than the second||If intValue > 10 Then Exit Sub|
|<=||Less than or equal to||Returns True if the value of the first operand is less than, or equal to, the second||If intValue <= 10 Then Exit Sub|
|>=||Greater than or equal to||Returns True if the value of the first operand is greater than, or equal to, the second||If intValue >= 10 Then Exit Sub|
 Visual Basic Logical Operators
Logical Operators are also known as Boolean Operators because they evaluate parts of an expression and return a True or False value, allowing decisions to be made about how a script should proceed.
The first step to understanding how logical operators work is to construct a sentence rather than to look at a script example right away. Let's assume we need to check some aspect of two variables named intVal1 and intVal2. Our sentence might read:
If intVal1 is less than 25 AND intVal2 is greater than 45 display a message.
Here the logical operator is the "AND" part of the sentence. If we were to express this in Visual Basic we would use the AND logical operator:
Dim intVal1, intVal2 As Integer intVal1 = 10 intVal2 = 45 If intVal1 < 25 And intVal2 > 45 Then MessageBox.Show("OK") End If
Similarly, if our sentence was to read:
If intVal1 is less than 25 OR intVal2 is greater than 45 display a message.
Then we would use the Visual Basic Or operator in our If statement:
Dim intVal1, intVal2 As Integer intVal1 = 10 intVal2 = 45 If intVal1 < 25 Or intVal2 > 45 Then MessageBox.Show("OK") End If
Another useful logical operator is the Exclusive Or (XOR) operator. The XOR operator returns true if only one of the expressions evaluates to be true. For example:
If intVal1 is EITHER less than 25 OR greater than 45 display a message
We represent XOR with the keyword Xor:
Dim intVal1 As Integer intVal1 = 10 If intVal1 < 25 Xor intVal1 > 45 Then MessageBox.Show("OK") End If
The final Logical Operator is the Not operator which simply inverts the result of an expression:
(10 > 1) ' returns ''True'' Not(10 > 1) ' returns ''False'' because we have inverted the result with the logical NOT
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