quine

[from the name of the logician Willard van Orman Quine, via Douglas Hofstadter] A program that generates a copy of its own source text as its complete output. Devising the shortest possible quine in some given programming language is a common hackish amusement. (We ignore some variants of BASIC in which a program consisting of a single empty string literal reproduces itself trivially.) Here is one classic quine:

 ((lambda (x) (list x (list (quote quote) x))) (quote (lambda (x) (list x (list (quote quote) x))))) 

This one works in LISP or Scheme. It's relatively easy to write quines in other languages such as Postscript which readily handle programs as data; much harder (and thus more challenging!) in languages like C which do not. Here is a classic C quine for ASCII machines:

 char*f="char*f=%c%s%c;main() {printf(f,34,f,34,10);}%c"; main(){printf(f,34,f,34,10);} 

For excruciatingly exact quinishness, remove the interior line breaks. Here is another elegant quine in ANSI C:

 #define q(k)main(){return!puts(#k"\nq("#k")");} q(#define q(k)main(){return!puts(#k"\nq("#k")");}) 

Some infamous Obfuscated C Contest entries have been quines that reproduced in exotic ways. There is an amusing Quine Home Page.