What is the difference between a variable and a constant in C?

A variable stores a value which can change. A constant stores a value which cannot change after it is set.

#include <stdio.h>
#define DAYS 7

int main(void) {
    int count = 5;
    const float PI = 3.14f;

    count = 10;          // allowed
    // PI = 3.15f;       // error: constant cannot change
    printf("%d %d %.2f\n", count, DAYS, PI);
    return 0;
}

Output:

See Variables and Constants.

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