C Ternary Operator

One expression, two outcomes

The ternary operator is written condition ? a : b. C evaluates the condition. If it is true (non-zero), the whole expression becomes a. If it is false (zero), the expression becomesb. 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 printf, or as a return value.

#include <stdio.h>

int main(void) {
  int n = 8;
  const char *kind = (n % 2 == 0) ? "even" : "odd";
  printf("%d is %s\n", n, kind);
  return 0;
}

Same choice as a short if/else

This program is equivalent to an if with an else that each assign one pointer. 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 <stdio.h>

int main(void) {
  int score = 72;
  const char *result = (score >= 50) ? "pass" : "fail";
  printf("%s\n", result);
  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 <stdio.h>

int main(void) {
  int a = 9;
  int b = 14;
  printf("larger: %d\n", (a > b) ? a : b);
  return 0;
}

Extra parentheses around the ternary keep the arguments to printf easy to read. 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. Two string literals (type const char *) are fine. Mixing "yes" with the integer1 is not a value the compiler can pick cleanly. Match the arms.

The condition itself is a scalar 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 <stdio.h>

int main(void) {
  int score = 82;
  const char *grade = (score >= 90) ? "A" : (score >= 80) ? "B" : "C";
  printf("%s\n", grade);
  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 aprintf plus an assignment in one ternary. If the true path needs two lines, write anif. Stay with if when you need braces { }, more than one statement, or a chain of ranges. Next: nested loops, where one for is not enough to walk a table.