A Simple ROT13 Macro

Originally published on: Wed, 22 Jul 2009 23:26:56 +0000

Here's a simple C routine written as a preprocessor macro that will perform ROT13 encoding on a single character.

#define rot13(x) isalpha(x) ? tolower(x)>'m' ? x-13 : x+13 : x

If your C compiler's version of tolower expands to a nested ternary expression, then the above macro may generate some tricky-to-read code out of the preprocessor phase.

Here's a simple program to read a string from stdin and ROT13 encode it using the above macro.

//
// A simple ROT13 encoder macro
//
// License: MIT / X11
// Copyright (c) 2009 by James K. Lawless
// jimbo@radiks.net http://www.radiks.net/~jimbo
// http://www.mailsend-online.com
//
// Permission is hereby granted, free of charge, to any person
// obtaining a copy of this software and associated documentation
// files (the "Software"), to deal in the Software without
// restriction, including without limitation the rights to use,
// copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the
// Software is furnished to do so, subject to the following
// conditions:
//
// The above copyright notice and this permission notice shall be
// included in all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
// EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
// OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
// NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
// HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
// WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
// OTHER DEALINGS IN THE SOFTWARE.

// sorry about the includes below. Something in the Wordpress
// source code widget is garbling up the
// source. Please be sure to put angle brackets around the
// header file names
#include stdio.h
#include ctype.h

#define rot13(x) isalpha(x) ? tolower(x)>'m' ? x-13 : x+13 : x

int main() {
   int c;
   while((c=fgetc(stdin))!=-1)
      fputc(rot13(c),stdout);
   return 0;
}

Unless otherwise noted, all code and text entries are Copyright Š2009 by James K. Lawless

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