The conditional operator is the only ternary operator in C++(www.cppentry.com). It allows us to embed simple if-else tests inside an expression. The conditional operator has the following syntactic form

cond   expr1 : expr2; 

where cond is an expression that is used as a condition (Section 1.4.1, p. 12). The operator executes by eva luating cond. If cond eva luates to 0, then the condition is false; any other value is true. cond is always eva luated. If it is true, then expr1 is eva luated; otherwise, expr2 is eva luated. Like the logical AND and OR (&& and ||) operators, the conditional operator guarantees this order of eva luation for its operands. Only one of expr1 or expr2 is eva luated. The following program
illustrates use of the conditional operator:

int i = 10, j = 20, k = 30;  
// if i > j then maxVal = i else maxVal = j  
int maxVal = i > j   i : j; 

注意：不要写出 (a > b) true : false 这种多余的条件操作符，直接写 (a > b) 即可。

Avoid Deep Nesting of the Conditional Operator

We could use a set of nested conditional expressions to set max to the largest of three variables:

int max = i > j  
  i > k   i : k  
: j > k   j : k; 

We could do the equivalent comparison in the following longer but simpler way:

int max = i;  
if (j > max)  
max = j;  
if (k > max)  
max = k; 

Using a Conditional Operator in an Output Expression

The conditional operator has fairly low precedence. When we embed a conditional expression in a larger expression, we usually must parenthesize the conditional subexpression. For example, the conditional operator is often used to print one or another value, depending on the result of a condition. Incompletely parenthesized uses of the conditional operator in an output expression can have surprising results:

cout << (i < j   i : j); // ok: prints larger of i and j  
cout << (i < j)   i : j; // prints 1 or 0!  
cout << i < j   i : j; // error: compares cout to int 

The second expression is the most interesting: It treats the comparison between i and j as the operand to the << operator. The value 1 or 0 is printed, depending on whetheri < jis true or false. The << operator returns cout, which is tested as the condition for the conditional operator. That is, the second expression is equivalent to

cout << (i < j); // prints 1 or 0  
cout   i : j; // test cout and then eva luate i or j  
// depending on whether cout eva luates to true or false 

EXERCISES SECTION 5.7

Exercise 5.20: Write a program to prompt the user for a pair of numbers and report which is smaller.

Exercise 5.21: Write a program to process the elements of a vector<int>. Replace each element with an odd value by twice that value.