5.7. The Conditional Operator

The conditional operator is the only ternary operator in C++. 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 evaluating cond. If cond evaluates to 0, then the condition is false; any other value is true. cond is always evaluated. If it is TRue, then expr1 is evaluated; otherwise, expr2 is evaluated. Like the logical AND and OR (&& and ||) operators, the conditional operator guarantees this order of evaluation for its operands. Only one of expr1 or expr2 is evaluated. 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;

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 whether i < j is 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 evaluate i or j
                      // depending on whether cout evaluates 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.