Count number of digits - which method is most efficient?

Question

There are more than one solution for finding the-number-of-digits in a given number.

For example:

Method-1:

int findn(int num) {     char snum[100];     sprintf(snum, "%d", num);     return strlen(snum); } 

Method-2:

int findn(int num) {     if (num == 0) return 1;     int n = 0;     while(num) {         num /= 10;         n++;     }     return n; } 

Method-3:

int findn(int num) {     /* math.h included */     return (int) log10(num) + 1; } 

The question is - what is the most efficient method? I know method-2 is O(n) but what about method-1 and method-3? How do I find the run-time complexity of library functions?

Answer 1

The following is even more efficient:

int findn(int num) {    if ( num < 10 )       return 1;    if ( num < 100 )       return 2;    //continue until max int } 

You could optimize this even further by doing a binary search, but that would be overkill.