4.7. The Conditional Operator

The conditional operator (the ?:operator) lets us embed simple if-else logic inside an expression. The conditional operator has the following form:

cond  ? expr1  : expr2;

where cond is an expression that is used as a condition and expr1 and expr2 are expressions of the same type (or types that can be converted to a common type). This operator executes by evaluating cond. If the condition is true, then expr1 is evaluated; otherwise, expr2 is evaluated. As one example, we can use a conditional operator to determine whether a grade is pass or fail:

string finalgrade = (grade < 60) ? "fail" : "pass";

The condition checks whether grade is less than 60. If so, the result of the expression is "fail"; otherwise the result is "pass". Like the logical AND and logical OR (&& and ||) operators, the conditional operator guarantees that only one of expr1 or expr2 is evaluated.

That result of the conditional operator is an lvalue if both expressions are lvalues or if they convert to a common lvalue type. Otherwise the result is an rvalue.

Nesting Conditional Operations

We can nest one conditional operator inside another. That is, the conditional operator can be used as the cond or as one or both of the exprs of another conditional expression. As an example, we’ll use a pair of nested conditionals to perform a three-way test to indicate whether a grade is a high pass, an ordinary pass, or fail:

finalgrade = (grade > 90) ? "high pass"
                          : (grade < 60) ? "fail" : "pass";

The first condition checks whether the grade is above 90. If so, the expression after the ? is evaluated, which yields "high pass". If the condition fails, the : branch is executed, which is itself another conditional expression. This conditional asks whether the grade is less than 60. If so, the ? branch is evaluated and yields "fail". If not, the : branch returns "pass".

The conditional operator is right associative, meaning (as usual) that the operands group right to left. Associativity accounts for the fact that the right-hand conditional—the one that compares grade to 60—forms the : branch of the left-hand conditional expression.

WARNING

Nested conditionals quickly become unreadable. It’s a good idea to nest no more than two or three.

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, we often use the conditional operator to print one or another value, depending on the result of a condition. An incompletely parenthesized conditional operator in an output expression can have surprising results:

cout << ((grade < 60) ?   "fail" : "pass"); // prints pass or  fail
cout << (grade < 60) ?   "fail" : "pass";   // prints 1 or 0!
cout << grade < 60 ?   "fail" : "pass"; //  error: compares cout to 60

The second expression uses the comparison between grade and 60 as the operand to the << operator. The value 1 or 0 is printed, depending on whether grade < 60 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 << (grade < 60);    // prints 1 or 0
cout ?  "fail" : "pass"; // test cout and then yield one of the two literals
                         // depending on whether cout is true or false

The last expression is an error because it is equivalent to

cout << grade;   // less-than has lower precedence than shift, so print grade first
cout < 60 ? "fail" : "pass"; // then compare cout to 60!

INFO

Exercises Section 4.7

Exercise 4.21: Write a program to use a conditional operator to find the elements in a vector<int> that have odd value and double the value of each such element.

Exercise 4.22: Extend the program that assigned high pass, pass, and fail grades to also assign low pass for grades between 60 and 75 inclusive. Write two versions: One version that uses only conditional operators; the other should use one or more if statements. Which version do you think is easier to understand and why?

Exercise 4.23: The following expression fails to compile due to operator precedence. Using Table 4.12 (p. 166), explain why it fails. How would you fix it?

string s = "word";
string pl = s + s[s.size() - 1] == 's' ? "" : "s" ;

Exercise 4.24: Our program that distinguished between high pass, pass, and fail depended on the fact that the conditional operator is right associative. Describe how that operator would be evaluated if the operator were left associative.