How to determine how many bytes an integer needs?

Question

I\'m looking for the most efficient way to calculate the minimum number of bytes needed to store an integer without losing precision.

e.g.

int: 10 = 1 byte
         


        

Answer 1

I think this is a portable implementation of the straightforward formula:

#include 
#include 
#include 

int main(void) {
    int i;
    unsigned int values[] = {10, 257, 67898, 140000, INT_MAX, INT_MIN};

    for ( i = 0; i < sizeof(values)/sizeof(values[0]); ++i) {
        printf("%d needs %.0f bytes\n",
                values[i],
                1.0 + floor(log(values[i]) / (M_LN2 * CHAR_BIT))
              );
    }
    return 0;
}

Output:

10 needs 1 bytes
257 needs 2 bytes
67898 needs 3 bytes
140000 needs 3 bytes
2147483647 needs 4 bytes
-2147483648 needs 4 bytes

Whether and how much the lack of speed and the need to link floating point libraries depends on your needs.