The 8 Queens Puzzle

This is an old puzzle and a popular homework in programming courses. The problem is to place 8 queens on a chessboard, so that no queen is beaten by another. Here is a rather small C-program, which finds all solutions. It contains about 45 lines of code (inclusively comments), but 1/3 are for the output.

+----------------+
|          <>    |
|      <>        |
|            <>  |
|<>              |
|              <>|
|  <>            |
|        <>      |
|    <>          |
+----------------+

It is an ANSI-C program with an ascii-output. So it should run on all operating sytems an should compile with all C-compilers.

Download: damen.c (7747Byte) [The stripped code is 3,5K.]

Here is the program for inspection.

#include <stdio.h> 
#include <stdlib.h> 

int x[8]; /* x[i] = x coordinate of queen in row i. */

/* prints field */
void print ()
{
  int i,j;
  printf ("+----------------+\n");
  for (i=0; i<8; i++) {
    printf ("|");
    for (j=0; j<8; j++)
      if (j==x[i]) printf ("<>");
      else printf ("  ");
    printf ("|\n");
  }
  printf ("+----------------+\n\n");
}

/* tests, whether (ix, iy) is beaten by queens 0...(iy-1) */
int is_free (int ix, int iy)
{
  int i;
  for (i=0; i<iy; i++)
    if ((x[i]==ix) || (abs(x[i]-ix)==abs(i-iy))) return 0;
  return 1;
}

/* tries to place queen n on row n */
void try (int n)
{
  int i;
  if (n==8) print();
  else
    for (i=0; i<8; i++)
      if (is_free(i,n)) {
        x[n]=i;
        try (n+1);
      }
}

int main ()
{
  try (0);
  return 0;
}