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Programming Visual Basic .NET, Second Edition by Jesse Liberty

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Logical Operators Within Conditionals

If statements 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 3-3. This table assumes two variables, x and y, in which x has the value 5, and y the value 7.

Table 3-3. Logical operators (assumes x = 5 and y = 7)

Operator

Given this statement:

The expressionevaluates to:

Logic

And

x = 3 And y = 7

False

Both must be true to evaluate true.

Or

x = 3 Or y = 7

True

Either or both must be true to evaluate true.

XOr

X = 5 XOr y = 7

False

True only if one (and only one) statement is true.

Not

Not x = 3

True

Expression must be false to evaluate true.

The And operator tests whether two statements are both true. The first line in Table 3-3 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 3-3:

x = 3 Or y = 7

the entire expression evaluates true because one side (y = 7) is true.

The XOr logical operator (which stands for eXclusive Or) is used to test if one (and only one) of the two statements is correct. Thus, the example from Table 3-3:

x = 5 XOr y = 7

evaluates false because both statements ...

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