1 /*
2  * 2-3. Write the function htoi(s), which converts a string of hexadecimal
3  * digits (including an optional 0x or 0X) into its equivalent integer
4  * value. The allowable digits are 0 through 9, a through f, and A through F.
5  */
6 #include <stdio.h>
7 
8 unsigned int htoi(const char s[]);
9 
10 main () {
11 	printf("0 %u\n", htoi("0x00000000"));
12 	printf("1 %u\n", htoi("0x00000001"));
13 	printf("273 %u\n", htoi("0x00000111"));
14 	printf("2730 %u\n", htoi("0x00000aaA"));
15 	printf("2872373078 %u\n", htoi("0xab34ef56"));
16 	printf("4291686144 %u\n", htoi("0XFFcDeF00"));
17 	printf("4294967295 %u\n", htoi("0XfffFFFff"));
18 	printf("2687823702 %u\n", htoi("a034eF56"));
19 	printf("4294967295 %u\n", htoi("fffFFFff"));
20 }
21 
22 unsigned int htoi(const char s[]) {
23 	int i = 0;
24 	int val = 0;
25 	if (s[0] == '0' && (s[1] == 'x' || s[1] == 'X')) {
26 		i = 2;
27 	}
28 	for (; s[i] != '\0'; ++i) {
29 		if (s[i] >= '0' && s[i] <= '9') {
30 			val = 16*val + s[i] - '0';
31 		} else if (s[i] >= 'a' && s[i] <= 'f') {
32 			val = 16*val + s[i] - 'a' + 10;
33 		} else if (s[i] >= 'A' && s[i] <= 'F') {
34 			val = 16*val + s[i] - 'A' + 10;
35 		}
36 	}
37 	return val;
38 }