Quine

A Quine is a program that produces its complete source code as its only output. Programs that take input are not considered quines.

A quine that contains no code is ruled out as trivial; in many programming languages executing such a program will output the code (i.e. nothing). Such an empty program once won the “worst abuse of the rules” prize in the Obfuscated C contest.

Quines are named after philosopher Willard Van Orman Quine, who made an extensive study of indirect self-reference. He coined, among others, the following paradox-producing expression, known as Quine’s paradox:

“‘Yields falsehood when preceded by its quotation’ yields falsehood when preceded by its quotation.”

A quine exists in any programming language that has the ability to output any computable string, as a direct consequence of Kleene’s recursion theorem.

An example in colloquial English would be.

“quoted and followed by itself is a quine.” quoted and followed by itself is a quine.

An example in C Programming language follows.

/* A simple quine (self-printing program), in standard C. */
/* Note: in designing this quine, we have tried to make the code clear
* and readable, not concise and obscure as many quines are, so that
* the general principle can be made clear at the expense of length.
* In a nutshell: use the same data structure (called "progdata"
* below) to output the program code (which it represents) and its own
* textual representation. */

#include </code>

void quote(const char *s)
/* This function takes a character string s and prints the
* textual representation of s as it might appear formatted
* in C code. */
{
int i;

printf(" \"");
for (i=0; s[i]; ++i) {
/* Certain characters are quoted. */
if (s[i] == '\\')
printf("\\\\");
else if (s[i] == '"')
printf("\\\"");
else if (s[i] == '\n')
printf("\\n");
/* Others are just printed as such. */
else
printf("%c", s[i]);
/* Also insert occasional line breaks. */
if (i % 48 == 47)
printf("\"\n \"");
}
printf("\"");
}

/* What follows is a string representation of the program code,
* from beginning to end (formatted as per the quote() function
* above), except that the string _itself_ is coded as two
* consecutive '@' characters. */
const char progdata[] =
"/* A simple quine (self-printing program), in st"
"andard C. */\n\n/* Note: in designing this quine, "
"we have tried to make the code clear\n * and read"
"able, not concise and obscure as many quines are"
", so that\n * the general principle can be made c"
"lear at the expense of length.\n * In a nutshell:"
" use the same data structure (called \"progdata\"\n"
" * below) to output the program code (which it r"
"epresents) and its own\n * textual representation"
". */\n\n#include \n\nvoid quote(const char "</code>
"*s)\n /* This function takes a character stri"
"ng s and prints the\n * textual representati"
"on of s as it might appear formatted\n * in "
"C code. */\n{\n int i;\n\n printf(\" \\\"\");\n "
" for (i=0; s[i]; ++i) {\n /* Certain cha"
"racters are quoted. */\n if (s[i] == '\\\\')"
"\n printf(\"\\\\\\\\\");\n else if (s["
"i] == '\"')\n printf(\"\\\\\\\"\");\n e"
"lse if (s[i] == '\\n')\n printf(\"\\\\n\");"
"\n /* Others are just printed as such. */\n"
" else\n printf(\"%c\", s[i]);\n "
" /* Also insert occasional line breaks. */\n "
" if (i % 48 == 47)\n printf(\"\\\"\\"
"n \\\"\");\n }\n printf(\"\\\"\");\n}\n\n/* What fo"
"llows is a string representation of the program "
"code,\n * from beginning to end (formatted as per"
" the quote() function\n * above), except that the"
" string _itself_ is coded as two\n * consecutive "
"'@' characters. */\nconst char progdata[] =\n@@;\n\n"
"int main(void)\n /* The program itself... */\n"
"{\n int i;\n\n /* Print the program code, cha"
"racter by character. */\n for (i=0; progdata[i"
"]; ++i) {\n if (progdata[i] == '@' && prog"
"data[i+1] == '@')\n /* We encounter tw"
"o '@' signs, so we must print the quoted\n "
" * form of the program code. */\n {\n "
" quote(progdata); /* Quote all. */\n"
" i++; /* Skip second '"
"@'. */\n } else\n printf(\"%c\", p"
"rogdata[i]); /* Print character. */\n }\n r"
"eturn 0;\n}\n";

int main(void)
/* The program itself... */
{
int i;

/* Print the program code, character by character. */
for (i=0; progdata[i]; ++i) {
if (progdata[i] == '@' && progdata[i+1] == '@')
/* We encounter two '@' signs, so we must print the quoted
* form of the program code. */
{
quote(progdata); /* Quote all. */
i++; /* Skip second '@'. */
} else
printf("%c", progdata[i]); /* Print character. */
}
return 0;
}