hex2dec.c

/* converts hex to decimal */
#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <math.h>

/* length for hexa prefix*/
#define HEXA_PREFIX 2 

int htoi(const char *s);

int main(int argc, char **argv)
{
    if(argc != 2) {
        printf("Converts character string from hexadecimal notation to decimal 10\n"
                "Usage:  ./hex2dec STRING\n"
                "Example ./hex2dec 16AB\n"
                "        ./hex2dec 0x16AB\n"
                "        ./hex2dec 0X16AB\n"
              );
        return -1;
    }
    printf("Hex: %s\nDecimal: %d\n", argv[1], htoi(argv[1]));
    return 0;
}


/* converts a string of hexadecimal digits to decimal*/
int htoi(const char *s)
{
    int i;
    int len, result, power;

    len = strlen(s);

    /* if 0X or 0x is present, update the start index */
    if (s[0] == '0' && (s[1] == 'x' || s[1] == 'X')) {
        i = 2;
        power = len - HEXA_PREFIX - 1;
    }
    else {
        i = 0;
        power = len - 1;
    }

    result = 0;
    while (i < len) {
        if(isdigit(s[i])) {
            result += (s[i] - '0') * pow(16, power);
            --power;
        } else {
            switch(s[i]) {
                case 'A':
                case 'a': 
                    result += 10 * pow(16, power);
                    --power;
                    break;
                case 'B':
                case 'b':
                    result += 11 * pow(16, power);
                    --power;
                    break;
                case 'C':
                case 'c':
                    result += 12 * pow(16, power);
                    --power;
                    break;
                case 'D':
                case 'd':
                    result += 13 * pow(16, power);
                    --power;
                    break;
                case 'E':
                case 'e':
                    result += 14 * pow(16, power);
                    --power;
                    break;
                case 'F':
                case 'f':
                    result += 15 * pow(16, power);
                    --power;
                    break;
            }
        }
        ++i;
    }
    return result;
}