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