Section 5.7. The Conditional Operator

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.