Quine

In reference to http://cboard.cprogramming.com/showt...232#post841232:

Quote:

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Originally Posted by Snafuist

Or better: write a program in C that prints a program in Perl that prints the original C program. Once done, this is easily extended to an arbitrary number of steps.

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Ok, so here it is: the following C program prints Perl source which, when executed, prints the source of the original C program. q[0] contains header and footer for C, q[1] for Perl respectively.

The function main() simply prints q[1][0] (Perl header), prints q itself in Perl syntax, and q[1][1] (Perl footer). The resulting Perl program does the same analogously.

Code:

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#include <stdio.h>
#include <string.h>

static char *q[3][1024] = {
        {"\x23\x69\x6e\x63\x6c\x75\x64\x65\x20\x3c\x73\x74\x64\x69\x6f\x2e\x68\x3e\x0a\x23\x69\x6e\x63\x6c\x75\x64\x65\x20\x3c\x73\x74\x72\x69\x6e\x67\x2e\x68\x3e\x0a\x0a\x73\x74\x61\x74\x69\x63\x20\x63\x68\x61\x72\x20\x2a\x71\x5b\x33\x5d\x5b\x31\x30\x32\x34\x5d\x20\x3d\x20\x7b\x0a", "\x09\x7b\x4e\x55\x4c\x4c\x2c\x20\x4e\x55\x4c\x4c\x7d\x0a\x7d\x3b\x0a\x0a\x69\x6e\x74\x20\x6d\x61\x69\x6e\x28\x29\x0a\x7b\x0a\x09\x69\x6e\x74\x20\x69\x2c\x20\x6a\x2c\x20\x6b\x3b\x0a\x0a\x09\x70\x72\x69\x6e\x74\x66\x28\x22\x25\x73\x22\x2c\x20\x71\x5b\x31\x5d\x5b\x30\x5d\x29\x3b\x0a\x09\x66\x6f\x72\x28\x69\x20\x3d\x20\x30\x3b\x20\x71\x5b\x69\x5d\x5b\x30\x5d\x20\x21\x3d\x20\x4e\x55\x4c\x4c\x3b\x20\x69\x2b\x2b\x29\x20\x7b\x0a\x09\x09\x70\x72\x69\x6e\x74\x66\x28\x22\x5c\x74\x5b\x22\x29\x3b\x0a\x09\x09\x66\x6f\x72\x28\x6a\x20\x3d\x20\x30\x3b\x20\x6a\x20\x3c\x20\x32\x3b\x20\x6a\x2b\x2b\x29\x20\x7b\x0a\x09\x09\x09\x70\x72\x69\x6e\x74\x66\x28\x22\x5c\x22\x22\x29\x3b\x0a\x09\x09\x09\x66\x6f\x72\x28\x6b\x20\x3d\x20\x30\x3b\x20\x6b\x20\x3c\x20\x73\x74\x72\x6c\x65\x6e\x28\x71\x5b\x69\x5d\x5b\x6a\x5d\x29\x3b\x20\x6b\x2b\x2b\x29\x20\x7b\x0a\x09\x09\x09\x09\x70\x72\x69\x6e\x74\x66\x28\x22\x5c\x5c\x78\x25\x30\x32\x78\x22\x2c\x20\x71\x5b\x69\x5d\x5b\x6a\x5d\x5b\x6b\x5d\x29\x3b\x0a\x09\x09\x09\x7d\x0a\x09\x09\x09\x69\x66\x28\x6a\x3d\x3d\x30\x29\x20\x70\x72\x69\x6e\x74\x66\x28\x22\x5c\x22\x2c\x20\x22\x29\x3b\x20\x65\x6c\x73\x65\x20\x70\x72\x69\x6e\x74\x66\x28\x22\x5c\x22\x22\x29\x3b\x0a\x09\x09\x7d\x0a\x09\x09\x70\x72\x69\x6e\x74\x66\x28\x22\x5d\x2c\x5c\x6e\x22\x29\x3b\x0a\x09\x7d\x0a\x09\x70\x72\x69\x6e\x74\x66\x28\x22\x25\x73\x22\x2c\x20\x71\x5b\x31\x5d\x5b\x31\x5d\x29\x3b\x0a\x0a\x09\x72\x65\x74\x75\x72\x6e\x20\x30\x3b\x0a\x7d\x0a\x0a"},
        {"\x23\x21\x2f\x75\x73\x72\x2f\x62\x69\x6e\x2f\x70\x65\x72\x6c\x0a\x0a\x40\x71\x20\x3d\x20\x28\x0a", "\x09\x5b\x22\x22\x2c\x20\x22\x22\x5d\x0a\x29\x3b\x0a\x0a\x70\x72\x69\x6e\x74\x20\x24\x71\x5b\x30\x5d\x2d\x3e\x5b\x30\x5d\x3b\x0a\x66\x6f\x72\x28\x24\x69\x20\x3d\x20\x30\x3b\x20\x24\x69\x20\x3c\x20\x40\x71\x2d\x31\x3b\x20\x24\x69\x2b\x2b\x29\x20\x7b\x0a\x09\x70\x72\x69\x6e\x74\x28\x22\x5c\x74\x7b\x22\x29\x3b\x0a\x09\x66\x6f\x72\x28\x24\x6a\x20\x3d\x20\x30\x3b\x20\x24\x6a\x20\x3c\x20\x32\x3b\x20\x24\x6a\x2b\x2b\x29\x20\x7b\x0a\x09\x09\x70\x72\x69\x6e\x74\x28\x22\x5c\x22\x22\x29\x3b\x0a\x09\x09\x66\x6f\x72\x65\x61\x63\x68\x20\x24\x63\x20\x28\x73\x70\x6c\x69\x74\x20\x2f\x2f\x2c\x20\x24\x71\x5b\x24\x69\x5d\x2d\x3e\x5b\x24\x6a\x5d\x29\x20\x7b\x0a\x09\x09\x09\x70\x72\x69\x6e\x74\x66\x28\x22\x5c\x5c\x78\x25\x30\x32\x78\x22\x2c\x20\x6f\x72\x64\x28\x24\x63\x29\x29\x3b\x0a\x09\x09\x7d\x0a\x09\x09\x69\x66\x28\x24\x6a\x3d\x3d\x30\x29\x20\x7b\x20\x70\x72\x69\x6e\x74\x28\x22\x5c\x22\x2c\x20\x22\x29\x3b\x20\x7d\x0a\x09\x7d\x0a\x09\x70\x72\x69\x6e\x74\x28\x22\x5c\x22\x7d\x2c\x5c\x6e\x22\x29\x3b\x0a\x7d\x0a\x70\x72\x69\x6e\x74\x20\x24\x71\x5b\x30\x5d\x2d\x3e\x5b\x31\x5d\x3b\x0a\x0a"},
        {NULL, NULL}
};

int main()
{
        int i, j, k;

        printf("%s", q[1][0]);
        for(i = 0; q[i][0] != NULL; i++) {
                printf("\t[");
                for(j = 0; j < 2; j++) {
                        printf("\"");
                        for(k = 0; k < strlen(q[i][j]); k++) {
                                printf("\\x%02x", q[i][j][k]);
                        }
                        if(j==0) printf("\", "); else printf("\"");
                }
                printf("],\n");
        }
        printf("%s", q[1][1]);

        return 0;
}

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Note that there is a single blank line at the end of the source. Also note that line endings are '\n', so this might not work properly on native Windows or Mac.

This concept can easily be extended to increase the number of iterations until the original program appears again, possibly displaying quines in other languages as well.

Quiz question: how do you build a universal Turing machine that prints its own Gödel number?

Greets,
Philip