I think this might be a classic question but I am not aware of an answer. Can a program output a copy of itself, and, if so, is there a short program that does this?
I do not accept the "empty program" as an answer, and I do not accept programs that have access to there own source code. Rather, I am thinking something like this:
int main(int argc, char** argv){ printf("int main(argc, char** argv){ printf...
but I do not know how to continue...
If you write a quine, be careful that the copies don't also write copies of themselves ad infinitum and end up taking over the world.
In the language invented by Jon Skeet the following operator prints "Hello, world!\n".
h
I can make a modification of this language so that the following program prints "Hello, world!\n":
Hello, world!
So that's the program that prints itself.
Oh, you feel something strange about it, while it has a precise and correct mathematical definition? That's your problem. "I won't accept..." ha! Mathematics does accept, and she's the mistress I serve, so I post this answer.
This is indeed a classic question!
Beyond the existence of specific quines, an important result in computability theory is that for any function you might want to compute, there exists a program that "knows its own program text", i.e. that could print itself if desired. This theorem is called Kleene's second recursion theorem.
I assume you allow interpreted languages. (At some level, all languages are interpreted.) Somebody writes the interpreter, and if you are writing it, you can add to it any built-in functions you like, such as a (lispy) function (foo) that does nothing except print "(foo)".
Or you can add a more complex macro-type function (printMeAndMyArgs ...).
So the trick is in how you define the problem.
The basic idea of most quines is:
You write code that takes a string literal s and prints it, while replacing occurrences (or the occurrence) of a special substring foo in s by the value of s itself.
You take the entire source code of the program so far and use it as the definition for s. but you exclude the definition of s from the string, instead replacing it by foo.
Well, that's the general idea. The rest is string formatting details, really.
Michael Sipser’s “Introduction to the Theory of Computation” explains in one of the chapters how to construct a quine. I have recently written a Java program based on that idea and posted it at : http://bornagainprogrammer.net/2009/11/07/hello-world-from-the-tm-self/
I'd suggest you get hold of that book and try implementing the program yourself in your favorite language. There are lot of other fun theorems in that book.
-kiran