instead of using a switch statement you can use the ASCII value of the character

void ROT13(char *text)
{
      for(;*text;text++)
      {
         if(*text>'A'&&*text<'N'||*text>'a'&&*text<'n')*text+=13;
            else if(*text>'M'&&*text<'Z'||*text>'m'&&*text<'z')*text-=13;
      }
}