Thread: Chess problem Horse

^^^ ROFL!

I thought something like this was enough to give him a start:

It still needs to be finished, but I know the logic has a bug in it, already.

If the OP finishes the knights possible moves and gets the logic fixed, he'll be good to go, I believe.


/* knight's tour by Adak May 1, 2009

status: just started

In the main() function:
1. Declare a 8x8 two dimensional array (int chess[8][8] to represent a chess
board and set all its elements to -1 (i.e., no cells (squares) are visited, yet).

2. Ask user to enter i and j as the row and column numbers of the initial cell. (Since
you are using C, make sure that user enters a number between 0 and 7 (inclusive)
for row and column numbers).

3. Set chess[i][j] to 1 means that the horse visited that cell first.

4. Implement and call a function to determine in which order the horse visits the legal
squares once according to our random selection strategy described below and
returns the number of squares visited
numofsquares = random_horse(chess, i, j);
print(“Number of squares visited = %d\n”, numofsquares);

5. Implement and call a function print_board(chess); to print the values in chess
board showing in which order the cells are visited

In the int random_horse(int chess[][8], int i, int j) function:
while(1) {
a. Identify the legal but not-yet-visited squares
b. If there is no such squares return chess[i][j];
c. Pick one of these cells randomly (say you picked chess[x][y])
d. Update order number (chess[x][y] = chess[i][j] + 1
e. Move the horse to the selected cell (i=x; j=y
} /* end of while */

*/

Code:

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

void print_board(int chess[][8]);
int random_horse(int chess[][8], int i, int j);

int main(void) {

   int r,c, i, j, numofsquares;
   int chess[8][8];
   for(r = 0; r < 8; r++)
      for(c = 0; c < 8; c++)
         chess[r][c] = =1;
 
   i = -1;
   while(i < 0 || i > 7) {
      printf("\nEnter your starting row [0-7] ");
      scanf("%d", &i);
   }
   j = -1;
   while(j < 0 || j > 7) {
      printf("\nEnter your starting column [0-7] ");
      scanf("%d", &j);
   }

   chess[i][j] = 1;
   numofsquares = random_horse(chess, i, j);
   print_board(chess);

   printf("Number of squares visited = %d\n", numofsquares);

   printf("\n\n\t\t\t     press enter when ready ");
   i = getchar();
   return 0;
}

int random_horse(int chess[][8], int i, int j) {
   int r = i;
   int c = j;
   int k, kmoves, moves, validmoves, n, sqr;
   int valid[8] = { 0 };
   moves = kmoves = 0;
/* 
Log:
-1 = sqr has not been visited by the knight
>0= sqr has been visited by the knight
-2= sqr is a legal next move by the knight

array rows are in ascending order on the screen
   0
   1
   2
*/
   while(1) {
      //check the 8 (max) possible knight moves
      if((r > 1) && (c < 7) && (chess[r-2][c+1] == -1))  //1:30 o'clock move ok?
         chess[r-2][c+1] = -2; 
      if((r > 0) && (c < 6) && (chess[r-1][c+2] == -1)) //2:30 move
         chess[r-1][c+2] = -2;
      if((r < 7) && (c < 6) && (chess[r+1][c+2] == -1)) //3:30 move
         chess[r+1][c+2] = -2;
      if((r < 6) && (c < 7) && (chess[r+2][c+1] == -1)) //5:30 move
         chess[r+2][c+1] = -2;

     //etc., for all 8 possible moves.

      for(r = 0, k = 0; r < 8; r++) {
         for(c = 0; c < 8; c++) { 
            if(chess[r][c] == -2) { 
               validmoves++;
               if(!r) sqr = c;
               else sqr = r * 10 + c;
               valid[k] = sqr;
               ++k;
            }
         }
      }
      if(!moves) {
         return chess[i][j];
      }
      n = rand() % validmoves; 
      if(valid[n] < 8) {
         r = 0;
         c = valid[n];
      }
      else {
         r = valid[n] / 10;
         c = valid[n] % 10;
      }
      chess[r][c] = kmoves;  
      ++kmoves;
      print_board(chess);
      moves = getchar();

      //resets
      moves = 0;
      for(i = 0; i < 8; i++)
         valid[i] = 0;
     validmoves = 0;
            

   }

}
void print_board(int chess[][8]) {
   int i, r, c;
   printf("\n\n");

   for(r = 0; r < 8; r++) {
      for(i = 0; i < 9; i++) {
         if(i < 8)
            printf("|---");
         else
            printf("|");
      }
      printf("\n");

      for(c = 0; c < 8; c++) {
         printf("| %d ", chess[r][c]);
         if(c % 7 == 0 && c > 0)
            printf("|\n");
      }
   }
   for(i = 0; i < 9; i++) {
      if(i < 8)
         printf("|---");
      else
         printf("|");
   }
   printf("\n");
}

Edit:
A bit of smoothing out was needed, even for a rough idea.

No need to post it here, but the above is on the right track.

The random choice does not allow it to ever complete a full knights tour of all 64 squares, unless you add
more logic, as noted in my posts earlier in the thread.

^^^ Does that qualify as a Public Service?


I tried to steer the code toward the assignment, and keep it clear. There is a simpler way to test for a knights move, but I couldn't remember it. If he tried to grok it, the OP would really just get nauseous, anyway.

I modified my own version to check all 64 starting squares. The highest number of moves it found was only 53.

I also looked more into Warndorff's algorithm, and apparently it only works 100% of the time, if you use the Ira Pohl innovation, that when you have two moves with identical subsequent move possibilities, you must choose the move which, if it were made, would give the knight on the subsequent move, the fewest possible moves.

(so, check it just one ply deep, in cases where there's a tie).

Coincidentally, Ira Pohl was one of the authors of my very first book on C.