Often you want to ask more than one relational question at a time. "Is it true that x is greater than y, and also true that y is greater than z?" A program might need to determine that both of these conditions are true, or that some other condition is true, in order to take an action.

Imagine a sophisticated alarm system that has this logic: "If the door alarm sounds AND it is after six p.m. AND it is NOT a holiday, OR if it is a weekend, then call the police." C++’s three logical operators are used to make this kind of evaluation. These operators are listed in Table 4.2.

Table 4.2. The Logical Operators.

A logical AND statement evaluates two expressions, and if both expressions are true, the logical AND statement is true as well. If it is true that you are hungry, AND it is true that you have money, THEN it is true that you can buy lunch. Thus,

would evaluate TRUE if both x and y are equal to 5, and it would evaluate FALSE if either one is not equal to 5. Note that both sides must be true for the entire expression to be true.

Note that the logical AND is two && symbols. A single & symbol is a different operator.

A logical OR statement evaluates two expressions. If either one is true, the expression is true. If you have money OR you have a credit card, you can pay the bill. You don’t need both money and a credit card; you need only one, although having both would be fine as well. Thus,

evaluates TRUE if either x or y is equal to 5, or if both are.

Note that the logical OR is two || symbols. A single | symbol is a different operator.

A logical NOT statement evaluates true if the expression being tested is false. Again, if the expression being tested is false, the value of the test is TRUE! Thus

is true only if x is not equal to 5. This is exactly the same as writing

Relational operators and logical operators, being C++ expressions, each return a value: 1 (TRUE) or 0 (FALSE). Like all expressions, they have a precedence order (see Appendix A) that determines which relations are evaluated first. This fact is important when determining the value of the statement

It might be that the programmer wanted this expression to evaluate TRUE if both x and y are greater than 5 or if z is greater than 5. On the other hand, the programmer might have wanted this expression to evaluate TRUE only if x is greater than 5 and if it is also true that either y is greater than 5 or z is greater than 5.

If x is 3, and y and z are both 10, the first interpretation will be true (z is greater than 5, so ignore x and y), but the second will be false (it isn’t true that both x and y are greater than 5 nor is it true that z is greater than 5).

Although precedence will determine which relation is evaluated first, parentheses can both change the order and make the statement clearer:

Using the values from earlier, this statement is false. Because it is not true that x is greater than 5, the left side of the AND statement fails, and thus the entire statement is false. Remember that an AND statement requires that both sides be true—something isn’t both "good tasting" AND "good for you" if it isn’t good tasting.

NOTE: It is often a good idea to use extra parentheses to clarify what you want to group. Remember, the goal is to write programs that work and that are easy to read and understand.

In C++, zero is false, and any other value is true. Because an expression always has a value, many C++ programmers take advantage of this feature in their if statements. A statement such as

can be read as "If x has a nonzero value, set it to 0." This is a bit of a cheat; it would be clearer if written

Both statements are legal, but the latter is clearer. It is good programming practice to reserve the former method for true tests of logic, rather than for testing for nonzero values.

These two statements also are equivalent:

The second statement, however, is somewhat easier to understand and is more explicit.

DO put parentheses around your logical tests to make them clearer and to make the precedence explicit. DO use braces in nested if statements to make the else statements clearer and to avoid bugs. DON’T use if(x) as a synonym for if(x != 0); the latter is clearer. DON’T use if(!x) as a synonym for if(x == 0); the latter is clearer.

NOTE: It is common to define your own enumerated Boolean (logical) type with enum Bool {FALSE, TRUE};. This serves to set FALSE to 0 and TRUE to 1.

The conditional operator (?:) is C++’s only ternary operator; that is, it is the only operator to take three terms.

The conditional operator takes three expressions and returns a value:

This line is read as "If expression1 is true, return the value of expression2; otherwise, return the value of expression3." Typically, this value would be assigned to a variable.

Listing 4.9 shows an if statement rewritten using the conditional operator.

Listing 4.9. A demonstration of the conditional operator.

1: // Listing 4.9 - demonstrates the conditional operator

2: //

3: #include <iostream.h>

4: int main()

5: {

6: int x, y, z;

7: cout << "Enter two numbers.\n";

8: cout << "First: ";

9: cin >> x;

10: cout << "\nSecond: ";

11: cin >> y;

12: cout << "\n";

13:

14: if (x > y)

15: z = x;

16: else

17: z = y;

18:

19: cout << "z: " << z;

20: cout << "\n";

21:

22: z = (x > y) ? x : y;

23:

24: cout << "z: " << z;

25: cout << "\n";

26: return 0;

27: }

Output: 

Enter two numbers.

First: 5

Second: 8

z: 8

z: 8

Analysis: Three integer variables are created: x, y, and z. The first two are given values by the user. The if statement on line 14 tests to see which is larger and assigns the larger value to z. This value is printed on line 19. The conditional operator on line 22 makes the same test and assigns z the larger value. It is read like this: "If x is greater than y, return the value of x; otherwise, return the value of y." The value returned is assigned to z. That value is printed on line 24. As you can see, the conditional statement is a shorter equivalent to the if...else statement.