ansi_c_pg44_ch02_ex03.c

/*
 * Exercise 2-3.
 * Write a function htoi(s), which converts a string of hexadecimal digits
 * (including an optional 0x or 0X) into its equivalent integer value.
 * The allowable digits are 0 through 9, a through f, and A through F.
 */


#include <stdio.h>


int
h_to_i(const char* s) {
	int i = 0;
	int ret = 0;
	if (s[i] == '0')
		i++;
	if (i && (s[i] == 'x' || s[i] == 'X'))
		i++;
	while (s[i]) {
		if (s[i] >= '0' && s[i] <= '9')
			ret = ret * 16 + s[i] - '0';
		else if (s[i] >= 'a' && s[i] <= 'f')
			ret = ret * 16 + s[i] - 'a' + 10;
		else if (s[i] >= 'A' && s[i] <= 'F')
			ret = ret * 16 + s[i] - 'A' + 10;
		else
			return -1;
		i++;
	}

	return ret;

}

int
main(int argc, char** argv) {

	printf("htoi(%s) = %d\n",argv[1], h_to_i(argv[1]));
	return 0;

}