Always having the same randomized game of fifteen

Question

I always have the same randomized game of fifteen after randomizing it.

void init(void)
{
    nombre = d*d-1;
    for(int ligne = 0; ligne<d; ligne++){
        for (int colonne= 0;colonne<d;colonne++){
            board[ligne][colonne]=nombre;
            nombre = nombre-1;
        }
    }

        if(d%2==0){
            printf("on a un nombre paire");
            board[d-1][d-3]=1;
            board[d-1][d-2]=2;
        }
    //i will always remain the same    
    int i = d * d - 1;
    while (i > 0) {
      int j = (rand() * i)%((d*d)-1);
      int xi = i % d;
      int yi = i / d;
      int xj = j % d;
      int yj = j / d;

        if(xj<0){
            printf("\nproblème, xj = %i\n",xj);
            printf("i = %i\n",i);
            break;
        }
                if(yj<0){
            printf("\nproblème, yj = %i\n",yj);
                        printf("j = %i\n",j);
                        break;
        }

        swapTiles(xi, yi, xj, yj);
      --i;
      }
}

It always give:

 0  7  6 
 5  4  2 
 3  1  8 

swaptiles(i,j,k,l)is the following

void swapTiles(i, j, k, l) {
    printf("We entered swapTiles\n\n");
    draw();
    printf("\ntiles values:\n");
    printf("xi: %i\n",i);
    printf("yi: %i\n",j);
    printf("xj: %i\n",k);
    printf("yj: %i\n",l);
    int temp = board[i][j];
    printf("board[%i][%i] = %i\n",i,j,board[i][j]);
    board[i][j] = board[k][l];
    printf("board[%i][%i] = %i\n\n",k,l,board[k][l]);
    board[k][l] = temp;
    draw();
    printf("\n\n");
  }

I think the problem comes from rand(). Would using generate we used at the beginning of the program be better? Yet how to use it in a program and not in the command line?

Furthermore I had some difficulties when swapping. Indeed after randomizing given tile positions (xi,yi) and (xj,yj) I am able to get numbers that are below 0. That is why I used conditions and break in my program. Yet, as far as I used ̀ rand() and (d*d)-1 shouldn't I only have positive numbers?

Answer 1

Like most soi-disant "random" functions, rand() isn't really random. It generates pseudorandom numbers using a deterministic algorithm. This algorithm will always produce the same results if it is "seeded" with the same default value. You can obtain a more random-ish pseudorandom number by seeding rand() with a different value, such as a time stamp.

For more info:

man srand

See also this question.

Hope this helps.

Answer 2

In the GNU C library, RAND_MAX, the default maximum return value of rand() is the largest signed integer value. And so, sometimes, if the rand() returns a high enough value and you multiply it by "i", it makes it "overflow" to represent a NEGATIVE value. You should define a lower RAND_MAX value.

As I see, you want to randomly swap tiles, but: u might end up with a table unsolvable! Check out the wikipedia or other materials on n-puzzle solvability.

also you should "debug50" your code with breakpoints, to check how variables and values are assigned, that might help finding why you always get the same result.