Notes
Referred from GitBook
Notes
What is a quine in programming?
Understanding quines with a petit list of examples

A quine is a piece of code that upon executing will produce itself as the output. A quine is idempotent as its result will remain exactly the same no matter how much time you execute it. Quine are also known as self-replicating programs or self-copying programs.

Here is a collection of quines in different programming languages that I've collected from the internet.

C implementation

Example 1:

const unsigned char data[] = { /* snip */ };
#include <stdio.h>
int main() {
  printf( "const unsigned char data[] = {" );
  for ( int i=0; i<sizeof(data); i++ ) {
      printf( " %0#4x,", data[i] );
  }
  printf( "\n};\n\n" );
  for ( int j=0; j<sizeof(data); j++) 
      putchar( data[j] );
  return 0;
}

Example 2:

int main(void) {
  char str[]= " int main(void)\
{ char str[]= %c%s%c;\
printf(str, 0x22, str, 0x22);}";
  printf(str, 0x22, str, 0x22);}

Example 3:

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

CoffeeScript implementation

s="s=%j;console.log s,s";console.log s,s

Java implementation

This is a verbose one compared to others.

public class Quine
{
  public static void main(String[] args)
  {
    char q = 34;      // Quotation mark character
    String[] l = {    // Array of source code
    "public class Quine",
    "{",
    "  public static void main(String[] args)",
    "  {",
    "    char q = 34;      // Quotation mark character",
    "    String[] l = {    // Array of source code",
    "    ",
    "    };",
    "    for(int i = 0; i < 6; i++)           // Print opening code",
    "        System.out.println(l[i]);",
    "    for(int i = 0; i < l.length; i++)    // Print string array",
    "        System.out.println(l[6] + q + l[i] + q + ',');",
    "    for(int i = 7; i < l.length; i++)    // Print this code",
    "        System.out.println(l[i]);",
    "  }",
    "}",
    };
    for(int i = 0; i < 6; i++)           // Print opening code
        System.out.println(l[i]);
    for(int i = 0; i < l.length; i++)    // Print string array
        System.out.println(l[6] + q + l[i] + q + ',');
    for(int i = 7; i < l.length; i++)    // Print this code
        System.out.println(l[i]);
  }
}

JavaScript implementation

s="s=%j;console.log(s,s)";console.log(s,s)

(function a(){console.log('('+a+')')})()

code='var q=unescape("%27");console.log("code="+q+code+q+";eval(code)")';eval(code)

Lua implementation

s="s=%qprint(s:format(s))"print(s:format(s))

Perl implementation

Example 1:

$s='$s=%c%s%c;printf($s,39,$s,39);';printf($s,39,$s,39);

Example 2:

$s=q($s=q(%s);printf($s,%s););printf($s,%s);

Python implementation

Example 1:

_='_=%r;print (_%%_)';print (_%_)

Example 2:

s='s=%r;print(s%%s)';print(s%s)

Example 3:

s='s=%r;print(s%%s,sep="")';print(s%s,sep="")

Ruby implementation

Example 1:

s="s=%p;printf s,s";printf s,s

Example 2:

s="s=%p;print s%%s";print s%s

Example 3:

s="s=%p;puts s%%s";puts s%s

Example 4:

s="s=%c%s%c;printf s,34,s,34";printf s,34,s,34

Shell implementation

s='s=\47%s\47;printf "$s" "$s"';printf "$s" "$s"

Scheme implementation

(define x '(
  (display "(define x '(")
  (newline)
  (map (lambda (s) (write s) (newline)) x)
  (display "))")
  (newline)
  (display "(map eval x)")
  (newline)
  ))
(map eval x)

Conclusion

As we can see, the simplest quine have similar format like this s="s=%p;printf s,s";printf s,s, and larger quine requires more complexity and intricacy to produce.

Reference