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#include <stdio.h>
#include <ctype.h>
/* The C Programming Language, 2nd Edition
*
* Exercise 2-3: Write the 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.
*
* Answer: ctype.h has some cool functions in it. Using just two functions from
* that header, my function's length is much shorter than it would be if I
* were to wing it alone with C primitives.
*
* Anyway, the idea is to make sure you're working with a hex digit,
* homogenize the case of the letters, and ignore x's. Since hex is base-16
* instead of base-10, plug a 16 in and perform some clever math based on
* ASCII knowledge.
*
* Since the function can be objectively proven correct, I included test cases
* to ensure that its output is correct.
*
* `man ascii` for more info, if you're using an OS that's actually useful.
*/
int htoi(char s[]) {
int i, val;
for (i = val = 0; isxdigit(s[i]) || toupper(s[i]) == 'X'; ++i) {
if (toupper(s[i]) == 'X') {
continue;
}
if (s[i] > '9') {
val = 16 * val + (toupper(s[i]) - '7');
// The 7 is because 'A' is 7 higher than '9' in ASCII and thus only needs
// to be knocked down by that much to fall in line with the normal integer
// conversion
} else {
val = 16 * val + (s[i] - '0');
}
}
return val;
}
int main() {
printf(" HEX | DECIMAL\n---------------\n 1 = %3d\n 0xf = %3d\n0X64 = %3d\n fF = %3d\n 093 = %3d\n", htoi("1"), htoi("0xf"), htoi("0X64"), htoi("fF"), htoi("093"));
return 0;
}
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