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| -rw-r--r-- | ch2/2-03_hex-to-int.c | 48 |
1 files changed, 48 insertions, 0 deletions
diff --git a/ch2/2-03_hex-to-int.c b/ch2/2-03_hex-to-int.c new file mode 100644 index 0000000..566bd75 --- /dev/null +++ b/ch2/2-03_hex-to-int.c @@ -0,0 +1,48 @@ +#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; +} |