The ternary operator

Table of contents

The ternary operator

The ternary operator provides a short way to choose between two values.

Its syntax is:

condition ? value_if_true : value_if_false

For example:

int larger = x > y ? x : y;

If x > y is true, larger is assigned the value of x. Otherwise, it is assigned the value of y.

The same code written using if and else would be:

int larger;

if (x > y) {
    larger = x;
} else {
    larger = y;
}

The ternary operator is useful when we need to select one of two simple values.

How it works

The ternary operator contains three parts:

condition ? value_if_true : value_if_false

The condition is evaluated first:

if the condition is true, the expression after ? is used
if the condition is false, the expression after : is used

Only one of the two expressions is evaluated.

For example:

int score = 75;

char result = score >= 40 ? 'P' : 'F';

Since score >= 40 is true, result is assigned 'P'.

Choosing one of two numbers

The ternary operator can select between two numbers:

int x = 10;
int y = 20;

int larger = x > y ? x : y;

Here, larger is assigned 20.

We can also use it as part of a calculation:

int base_score = 50;
bool bonus_available = true;

int total = base_score + (bonus_available ? 10 : 0);

If bonus_available is true, 10 is added to the score. Otherwise, zero is added.

Using the ternary operator with `printf`

A ternary expression can be used directly as an argument to a function:

#include <stdbool.h>
#include <stdio.h>

int main(void)
{
    bool data_ready = true;

    printf("Data ready: %s\n", data_ready ? "yes" : "no" );

    return 0;
}

This prints:

Data ready: yes

If data_ready were false, it would print:

Data ready: no

This is often more useful than printing a Boolean as 0 or 1.

Choosing between enum values

The two possible results can also be enum values:

#include <stdbool.h>

typedef enum {
    LED_OFF,
    LED_ON
} led_state_t;

bool button_pressed = true;

led_state_t state = button_pressed ? LED_ON : LED_OFF;

If button_pressed is true, state becomes LED_ON. Otherwise, it becomes LED_OFF.

Toggling a Boolean

You do not need the ternary operator to toggle a Boolean.

This works:

led_on = led_on ? false : true;

However, the logical NOT operator is clearer:

led_on = !led_on;

Use the ternary operator to choose between two different values. Use ! when you simply want to reverse a Boolean value.

Use parentheses when they help

Parentheses are not required around the condition in this example:

int larger = x > y ? x : y;

However, they can make it easier to see the three parts:

int larger = (x > y) ? x : y;

Parentheses are particularly helpful when the result is used as part of a larger expression:

int total = base_score + (bonus_available ? 10 : 0);

Avoid nested ternary operators

It is possible to put one ternary operator inside another:

char grade =
    score >= 70 ? 'A' :
    score >= 60 ? 'B' :
    score >= 50 ? 'C' :
    score >= 40 ? 'D' :
                  'F';

This is valid C, but god knows if its actually readable! If you need to stop and work out what a ternary expression does, use an if statement instead!

Ternary operator or `if` statement?

Use the ternary operator when selecting one of two values:

int larger = x > y ? x : y;

char result = score >= 40 ? 'P' : 'F';

Use if and else when each branch needs to perform one or more actions:

if (ready) {
    printf("Ready\n");
    start_operation();
} else {
    printf("Waiting\n");
    check_input();
}

The ternary operator should make the code easier to read, not merely shorter.