C Booleans

How C represents true and false

C did not always have a dedicated boolean type. Under the hood, C treats the number 0 as false and any non-zero number as true. That rule is simple but powerful: an integer variable holding 5, -3, or 1 all count as true, while only 0 counts as false.

• 0 means false.
• Any value that is not 0 (like 1, 42, or -7) means true.
• Comparisons such as 10 > 9 produce the value 1 for true and 0 for false.

#include <stdio.h>

int main() {
    printf("%d\n", 10 > 9);   // 1 (true)
    printf("%d\n", 10 == 9);  // 0 (false)
    printf("%d\n", 10 < 9);   // 0 (false)
    return 0;
}

The bool type from stdbool.h

Since C99, you can include the header <stdbool.h> to use the keyword bool along with the words true and false. This makes your code much easier to read, because the intent is spelled out clearly instead of relying on 1 and 0.

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

int main() {
    bool isLearning = true;
    bool isTired = false;

    printf("%d\n", isLearning);  // 1
    printf("%d\n", isTired);     // 0
    return 0;
}

Booleans from comparisons

The most common way to get a boolean is to compare two values. The result of a comparison can be stored in a bool variable and used later in your program, which is the foundation of decision making.

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

int main() {
    int myAge = 25;
    int voteAge = 18;

    bool canVote = myAge >= voteAge;
    printf("Can vote: %d\n", canVote);  // 1
    return 0;
}