C++ Ternary Operator

One expression, two outcomes

The ternary operator is written condition ? a : b. C++ evaluates the condition. If it is true, the whole expression becomes a. If it is false, the expression becomes b. You get one value. You do not get a block of statements.

That is the difference from if. An if chooses which code runs. A ternary chooses whichvalue to use. Put it on the right of an assignment, inside cout, or as a return value.

#include <iostream>
#include <string>
using namespace std;

int main() {
  int n = 8;
  string kind = (n % 2 == 0) ? "even" : "odd";
  cout << n << " is " << kind << endl;
  return 0;
}

Same choice as a short if/else

This program is equivalent to an if with an else that each assign one string. The ternary version keeps the decision next to the variable. Use it when both arms are short and both produce the same kind of value.

#include <iostream>
#include <string>
using namespace std;

int main() {
  int score = 72;
  string result = (score >= 50) ? "pass" : "fail";
  cout << result << endl;
  return 0;
}

Parentheses around the condition are not always required, but they make the test easy to see. This tutorial keeps them. The colon is not optional: every ? has a matching :.

Use it where a value is needed

Because the ternary is an expression, you can print it directly. You do not have to store it first. This program prints the larger of two integers. Swap the numbers and compile again.

#include <iostream>
using namespace std;

int main() {
  int a = 9;
  int b = 14;
  cout << "larger: " << ((a > b) ? a : b) << endl;
  return 0;
}

Extra parentheses around the ternary keep cout from misreading the << chain. When the operator sits inside a larger expression, wrap it.

Both arms need a common type

a and b must make sense as one type. Two int values are fine. Twostring values are fine. Mixing "yes" with the integer 1 is not a value the compiler can pick cleanly. Match the arms.

The condition itself is a boolean test: comparison, !, &&, ||. Do not write if inside the parentheses. The test is an expression, the same kind you already use in an if header.

Nesting is legal, not kind

You can put a ternary in an arm of another ternary. The compiler accepts it. A person reading the line has to count question marks. Prefer else if once you have three or more outcomes.

#include <iostream>
#include <string>
using namespace std;

int main() {
  int score = 82;
  string grade = (score >= 90) ? "A" : (score >= 80) ? "B" : "C";
  cout << grade << endl;
  return 0;
}

It is not a block

Each arm is one expression. You cannot put two statements in an arm, and you should not hide acout plus an assignment in one ternary. If the true path needs two lines, write anif.

Do not use the operator for side effects such as incrementing in one arm and not the other. Readers expect a value, not a mutation. Keep the arms boring: literals, variables, or a short call that returns something.

When if is the better tool

Stay with if when you need braces, more than one statement, or a chain of ranges. Use the ternary when you are filling one variable or printing one chosen value. Next: nested loops, where one foris not enough to walk a table.