68 Part I Introducing Microsoft Visual C# and Microsoft Visual Studio 2008

Microsoft Visual C# provides a data type called bool. A bool variable can hold one of two

values: true or false. For example, the following three statements declare a bool variable called areYouReady, assign true to the variable, and then write its value to the console:

bool areYouReady; areYouReady = true;

Console.WriteLine(areYouReady); // writes True

Using Boolean Operators

A Boolean operator is an operator that performs a calculation whose result is either true or false. C# has several very useful Boolean operators, the simplest of which is the NOT operator, which is represented by the exclamation point (!). The ! operator negates a Boolean val-

ue, yielding the opposite of that value. In the preceding example, if the value of the variable areYouReady is true, the value of the expression !areYouReady is false.

Understanding Equality and Relational Operators

Two Boolean operators that you will frequently use are the equality (==) and inequality (!=) operators. You use these binary operators to find out whether one value is the same as another value of the same type. The following table summarizes how these operators work, using an int variable called age as an example.

Closely related to these two operators are the relational operators. You use these operators to find out whether a value is less than or greater than another value of the same type. The following table shows how to use these operators.

Note Don’t confuse the equality operator == with the assignment operator =. The expression x==y compares x with y and has the value true if the values are the same. The expression x=y

assigns the value of y to x.

Understanding Conditional Logical Operators

C# also provides two other Boolean operators: the logical AND operator, which is represented by the && symbol, and the logical OR operator, which is represented by the || symbol. Collectively, these are known as the conditional logical operators. Their purpose is to combine two Boolean expressions or values into a single Boolean result. These binary operators are similar to the equality and relational operators in that the value of the expressions in which they appear is either true or false, but they differ in that the values on which they operate must be either true or false.

The outcome of the && operator is true if and only if both of the Boolean expressions

it operates on are true. For example, the following statement assigns the value true to validPercentage if and only if the value of percent is greater than or equal to 0 and the value of percent is less than or equal to 100:

bool validPercentage;

validPercentage = (percent >= 0) && (percent <= 100);

Tip A common beginner’s error is to try to combine the two tests by naming the percent variable only once, like this:

percent >= 0 && <= 100 // this statement will not compile

Using parentheses helps avoid this type of mistake and also clarifies the purpose of the expression. For example, compare these two expressions:

validPercentage = percent >= 0 && percent <= 100

and

validPercentage = (percent >= 0) && (percent <= 100)

Both expressions return the same value because the precedence of the && operator is less than that of >= and <=. However, the second expression conveys its purpose in a more readable manner.

The outcome of the || operator is true if either of the Boolean expressions it operates on is true. You use the || operator to determine whether any one of a combination of

Boolean expressions is true. For example, the following statement assigns the value true to invalidPercentage if the value of percent is less than 0 or the value of percent is greater than

100:

bool invalidPercentage;

invalidPercentage = (percent < 0) || (percent > 100);