# Quine

Quine is a nonempty [program](program.md) which prints its own source code. It takes no input, just prints out the [source code](source_code.md) when run (without [cheating](cheating.md) such as reading the source code file). Quine is basically a [self-replicating](self_replication.md) program, just as [in real world](irl.md) we may construct robots capable of creating copies of themselves (afterall we humans are such robots). The name *quine* refers to the philosopher Willard Quine and his paradox that shows a structure similar to self-replicating programs. Quine is one of the standard/[fun](fun.md)/[interesting](interesting.md) programs such as [hello world](hello_world.md), [compiler bomb](compiler_bomb.md), [99 bottles of beer](99_bottles.md) or [fizzbuzz](fizzbuzz.md).

From [mathematical](math.md) point of view quine is a fixed point of a [function](function.md) (not to be confused with [fixed_point arithmetic](fixed_point.md)) represented by the [programming language](programming_language.md). I.e. if we see the programming language as a function f(x), where *x* is source code and the function's output is the program's output, quine is such *x* that *f(x) = x*.

Similar efforts include e.g. making self matching [regular expressions](regex.md) (for this task to be non-trivial the regex has to e.g. be enclosed between `/`s).

Quine can be written in any [Turing complete](turing_completeness.md) [language](programming_language.md) (according to [Wikipedia](wikipedia.md)), the challenge is in the [self reference](self_reference.md) -- normally we cannot just single-line print a string literal containing the source because that string literal would have to contain itself, making it [infinite](infinity.md) in length. The idea commonly used to solve this problem is following:

1. On first line start a definition of string *S*, later copy-paste to it the string on the second line.
2. On second line put a command that prints the first line, assigning to *S* the string in *S* itself, and then prints *S* (the second line itself).

This is a quine in [C](c.md):

```
#include <stdio.h>
char s[] = "#include <stdio.h>%cchar s[] = %c%s%c;%cint main(void) { printf(s,10,34,s,34,10,10); return 0; }";
int main(void) { printf(s,10,34,s,34,10,10); return 0; }
```

This is a quine in [Python](python.md):

```
s="print(str().join([chr(115),chr(61),chr(34)]) + s + str().join([chr(34),chr(10)]) + s)"
print(str().join([chr(115),chr(61),chr(34)]) + s + str().join([chr(34),chr(10)]) + s)
```

This is a quine in [comun](comun.md):

```
0 46 32 34 S 34 32 58 83 S --> S: "0 46 32 34 S 34 32 58 83 S --> " .
```

TODO: more langs?

Yet a stronger quine is so called *radiation hardened quine*, a quine that remains quine even after any one character from the program has been deleted (found here in [Ruby](ruby.md): https://github.com/mame/radiation-hardened-quine). Other plays on the theme of quine include e.g. a program that produces a bigger program which will again produce yet bigger program etc.

In the [Text](plaintext.md) [esoteric programming language](esolang.md) every program is a quine (and so also a radiation hardened one).

## See Also

- [polyglot](polyglot.md)