C Boolean

Summary: in this tutorial, you’ll learn about the C Boolean type and how to use the Boolean values effectively in your program.

Introduction to the C Boolean type

In programming, the Boolean type has two possible values that represents the truth values: true and false.

C doesn’t natively support boolean type. In C, zero means false. And other values are true. For example, all the numbers which are not zero are true.

To make it more convenient to work with the Boolean values, the community employed a number of techniques. And one of them is to define a Boolean type using the enum type;

typedef enum {false, true} bool;
Code language: C++ (cpp)

ANSI C99 added the Boolean type in the stdbool.h header file. To use the standard Boolean type, you need to include the stdbool.h file in your program.

The standard Boolean type uses the bool keyword as the type. It has two values true and false. Internally, true evaluates to 1 and false evaluates to 0.

To declare a Boolean variable, you use the bool keyword as follows:

bool is_running = true;
bool is_shared = false;
Code language: C++ (cpp)

In this example, we declared two variables is_running and is_shared and assigned true and false to each.

When you print out a Boolean value using the printf() function, you still need to use the %d specificier. For example:

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

int main()
{

    bool is_running = true;
    bool is_shared = false;

    printf("is_running: %d\n", is_running); // 1
    printf("is_shared: %d\n", is_shared); // 0


    return 0;
}
Code language: C++ (cpp)

As clearly shown in the output, the boolean values evaluate to 1 and 0 instead of true and false.

If you want to output true and false for a boolean value, you can use an if...else statement or a ternary operator. For example:

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


int main()
{

    bool success = false;
    puts(success ? "true" : "false"); // false

    success = true;
    puts(success  ? "true" : "false"); // true

    return 0;
}
Code language: C++ (cpp)

The ?: is called the ternary operator in C. It has the following syntax:

expression ? value_if_true : value_if_false;
Code language: C++ (cpp)

The ternary operator (?:) evalutates the expression. If the result is true, it returns the value that follows the ?. Otherwise, it returns the value that follows the :.

Note that you’ll learn more about the ternary operator in the ternary operator tutorial.

In this example, the program first outputs false because the success is false. After that, the success is true, therefore, the program displays true.

Summary

• The Boolean type has two possible truth values: true and false.
• C doesn’t support the Boolean type natively. Instead, it uses 0 as false and other values as true.
• Include the stdbool.h file in the program and use the bool keyword for declaring the boolean variables.