Copied 2.3.c for understanding solution

copied_2.3.c

#define YES     1
#define NO      0
#include <stdio.h>

int htoi(char s[]);
int main(){


        char s[] = {'3', '0xA', 'a'};
        htoi(s);
        printf("%x", s);
}




int htoi(char s[])
{
        int hexdigit, i, index, n;

        i=0;
        if(s[i] == '0'){
                ++i;
                if(s[i] == 'x' || s[i] == 'X'){
                        ++i;
                }
        }
        n = 0;
        index = YES;
        //i will be the last value assigned to it when the loop starts.
        for(; index == YES; ++i){
                if(s[i] >= '0' && s[i] <= '9'){
                        hexdigit = s[i] - '0';
                } else if(s[i] >= 'a' && s[i] <= 'f'){
                        hexdigit = s[i] - 'a' + 10;
                } else if(s[i] >= 'A' && s[i] <= 'F'){
                        hexdigit = s[i] - 'A' + 10;
                }else{
                        index = NO;
                }
                if(index == YES){
                        n = 16 * n + hexdigit;
                }
                return n;
        }
}