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!

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.

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