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27 lines
1.9 KiB
Markdown
27 lines
1.9 KiB
Markdown
2 years ago
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# Quine
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Quine is a nonempty [program](program.md) which prints its own source code. It takes no input, just prints out the source code when run (without [cheating](cheating.md) such as reading the source code file). Quine is basically a self-replicating 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), [99 bottles of beer](99_bottles.md) or [fizzbuzz](fizzbuzz.md).
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Quine can be written in any [Turing complete](turing_completeness.md) [language](programming_language.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:
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1. On first line start a definition of string *S*, later copy-paste to it the string on the second line.
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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).
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This is a quine in [C](c.md):
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```
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#include <stdio.h>
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char s[] = "#include <stdio.h>%cchar s[] = %c%s%c;%cint main(void) { printf(s,10,34,s,34,10,10); return 0; }";
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int main(void) { printf(s,10,34,s,34,10,10); return 0; }
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```
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This is a quine in [Python](python.md):
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```
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s="print(str().join([chr(115),chr(61),chr(34)]) + s + str().join([chr(34),chr(10)]) + s)"
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print(str().join([chr(115),chr(61),chr(34)]) + s + str().join([chr(34),chr(10)]) + s)
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```
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TODO: more langs?
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In the [Text](plaintext.md) [esoteric programming language](esolang.md) every program is a quine.
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