I found this joke on USENET, but the punchline is scrambled. Maybe you can decipher it? According to Wikipedia, ROT13 (http://en.wikipedia.org/wiki/ROT13) is frequently used to obfuscate jokes on USENET.

Hint: For this task you’re only supposed to substitute characters. Not spaces, punctuation, numbers etc.

Test examples:

"EBG13 rknzcyr." -->
 "ROT13 example."

"This is my first ROT13 excercise!" -->
 "Guvf vf zl svefg EBG13 rkprepvfr!"

Solution:

#include <string.h>

char *rot13(char *str_out, const char *str_in)
{
  // write to str_out and return it
  *str_out = '\0';

  char b;
  unsigned long i;
  for (i = 0; i < strlen(str_in); i++)
  {
    b = str_in[i];
    if ((b >= 'A' && b <= 'M') || (b >= 'a' && b <= 'm'))
      b += 13;
    else if ((b >= 'N' && b <= 'Z') || (b >= 'n' && b <= 'z'))
      b -= 13;
    strncat(str_out, &b, 1);
  }
  return str_out;
}

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