7.4 Logical Operators Within ConditionalsIf statements (discussed in Chapter 6) test whether a condition is true. Often you will want to test whether two conditions are both true, or only one is true, or neither is true. VB.NET provides a set of logical operators for this, as shown in Table 7-2. This table assumes two variables , x and y, in which x has the value 5, and y has the value 7. Table 7-2. Logical operators
The And operator tests whether two statements are both True. The first line in Table 7-2 includes an example that illustrates the use of the And operator: x = 3 And y = 7 The entire expression evaluates false because one side ( x = 3 ) is false. (Remember that x=5 and y=7.) With the Or operator, either or both sides must be true; the expression is false only if both sides are false. So, in the case of the example in Table 7-2: x = 3 Or y = 7 the entire expression evaluates true because one side ( y = 7 ) is true. The XOr logical operator (which stands for eXtreme Or) is used to test if one (and only one) of the two statements is correct. Thus, the example from Table 7-2: X = 5 XOr y = 7 evaluates false because both statements are true. (The XOr statement is false if both statements are true, or if both statements are false; it is true only if one, and only one, statement is true.) With the Not operator, the statement is true if the expression is false, and vice versa. So, in the accompanying example: Not x = 3 the entire expression is true because the tested expression ( x = 3 ) is false. (The logic is: "it is true that it is not true that x is equal to 3.") All of these examples appear in context in Example 7-4. Example 7-4. The logical operatorsOption Strict On Imports System Module Module1 Sub Main( ) Dim x As Integer = 5 Dim y As Integer = 7 Dim andValue As Boolean Dim orValue As Boolean Dim xorValue As Boolean Dim notValue As Boolean andValue = x = 3 And y = 7 orValue = x = 3 Or y = 7 xorValue = x = 3 Xor y = 7 notValue = Not x = 3 Console.WriteLine("x = 3 And y = 7. {0}", andValue) Console.WriteLine("x = 3 Or y = 7. {0}", orValue) Console.WriteLine("x = 3 Xor y = 7. {0}", xorValue) Console.WriteLine("Not x = 3. {0}", notValue) End Sub 'Main End Module Output: x = 3 And y = 7. False x = 3 Or y = 7. True x = 3 Xor y = 7. True Not x = 3. True |