Constants and variables

Constants are fixed values that do not change during execution. They are of types integer (25), real (3.14), character ('A') and string ("Hello"). They can be named with #define PI 3.14 or const int DAYS = 7;.

Variables are named memory locations that store data which can change during execution. They must be declared with a type before use: int count = 0; float avg;. Variable names follow the rules of identifiers.

Example:

#include <stdio.h>
#define PI 3.14159
int main(void)
{
    const int DAYS = 7;       /* constant */
    int count = 0;            /* variable */
    float radius = 2.0f;
    count = count + DAYS;
    printf("%d %.2f\n", count, PI * radius * radius);
    return 0;
}

Output:

7 12.57