c - rand() not generating random numbers after modulo operation

Question

I'm taking a C refresher and took on a board game as an exercise. The board game is "Game of the Generals" and is pretty much like chess as it uses pieces with ranks on an 8x8 square.

Basically the implementation of a board is a two-dimensional array of a particular struct. So a square of the board can be accessed through its indices, much like an x-y coordinate system.

Now I decided to randomly distribute the pieces across the board, and the logic is to generate a random x-y coordinate, check if a piece already resides on those coordinates on the board, and populate it with a piece if it is available. If it is not, then another random x-y coordinate is generated. This goes on until all pieces are accounted for.

I'm using rand() to generate random numbers within a specific range (I'm using a modulo operator and a padding number to dictate the range. See code below)

But rand() doesn't seem to give random enough numbers for me to work with. I keep ending up with the same piece distribution over and over again! (But interesting enough, I can generate a different distribution on a Mac, but the distribution is still consistent!)

See code below as to how I'm using rand() to generate a number with a range.

void initPieces(){

    int player, rank_index, population, rand_min, rand_x, rand_y;

    for(player = 1; player <= 2; player++){

       if(player == 1){
            rand_min = 5;
        }else{
           rand_min = 1;
        }

        for(rank_index = 0; ir < sizeof ranking/sizeof ranking[0]; rank_index++){

            for(population = 0; population < getRank(rank_index)->population; population++){

               do{
                   rand_x = (rand() % 8) + 1;
                   rand_y = (rand() % 4) + rand_min;

               }while((getGrid(rand_x,rand_y))->has_piece == 1);

               assignPiecetoGrid(player,rank_index,rand_x,rand_y);

            }

       }

   }

}

Answer 1

You need to realize that rand() is a pseudorandom number generator, and it is specifically engineered to return the same sequence of numbers for a given seed. The seed is set with the srand() function.

srand(0);
printf("the first rand() with seed 0 is %d
", rand());
srand(1);
printf("the first rand() with seed 1 is %d
", rand());
srand(0);
printf("the first rand() with seed 0 is still %d
", rand());

So, the way to make it less predictable is generally to re-seed it from something a bit more random, or at least from something that is not the same every time you run the program:

srand(time(NULL));