Sams Teach Yourself C++ in 21 Days

Creating Expressions and Statements 93

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It might be that the programmer wanted this expression to evaluate true if both xand y

are greater than 5 or if zis greater than 5. On the other hand, the programmer might have

wanted this expression to evaluate true only if xis greater than 5 and if it is also true that

either yis greater than 5 or zis greater than 5.

If xis 3, and yand zare both 10, the first interpretation is true (zis greater than 5 ,so

ignore xand y), but the second is false (it isn’t true that xis greater than 5 , and thus it

doesn’t matter what is on the right side of the &&symbol because both sides must be

true).

Although precedence determines which relation is evaluated first, parentheses can both

change the order and make the statement clearer:

if ( (x > 5) && (y > 5 || z > 5) )

Using the values from earlier, this statement is false. Because it is not true that xis

greater than 5 , the left side of the ANDstatement fails, and thus the entire statement is

false. Remember that an ANDstatement requires that both sides be true—something isn’t

both “good tasting”AND“good for you” if it isn’t good tasting.

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 to understand. Using parentheses help to clarify your intent and

avoid errors that come from misunderstanding operator precedence.

TIP

More About Truth and Falsehood ..........................................................................

In C++, zero evaluates to false, and all other values evaluate to true. Because an expres-

sion always has a value, many C++ programmers take advantage of this feature in their

ifstatements. A statement such as

if (x) // if x is true (nonzero)

x = 0;

can be read as “If xhas a nonzero value, set it to 0 .” This is a bit of a cheat; it would be

clearer if written

if (x != 0) // if x is not zero

x = 0;

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.