C用嵌套函数生成随机数

Question

I can generate random number with srand, rand (by seeding). If I just need to run in 1 particular function, it is okay. But, I have code like this.

int generateBattleShipX() {
      srand ( time(NULL) );
    return rand() % 14;
}

int generateBattleShipY() {
     srand ( time(NULL) );
    return rand() % 14;
}

int tryToFillBattleField(int X, int Y) {

if ((X >= 1 && X <= 13) && (Y >= 1 && Y <= 13)) //battle ship is in the field
{

    int lowerX = X - 1; 
    int upperX = X + 1;

    int lowerY = Y - 1; 
    int upperY = Y + 1;

    // printf("lowerX = %d upperX = %d lowerY = %d upperY = %d", lowerX, upperX, lowerY, upperY);
    int left = my_data[Y][lowerX];
    int top = my_data[lowerY][X];
    int right = my_data[Y][upperX];
    int bottom = my_data[upperY][X];

    if (left == 0 && top == 0 && right == 0 && bottom == 0) {
        my_data[Y][lowerX] = track;
        my_data[lowerY][X] = track;
        my_data[Y][upperX] = track;
        my_data[upperY][X] = track;
        my_data[Y][X] = track;
        track++;

        if (track > 10) {
            return 1; //stop
        }
        else {
              int x = generateBattleShipX();
              int y = generateBattleShipY();

            // printf("x = %d y = %d \n",x,y );
         tryToFillBattleField(x, y); //filling next battle ship
        }
    }
    else {
              int x = generateBattleShipX();
              int y = generateBattleShipY();

        tryToFillBattleField(x, y);//redoing because it overlap
    }
    return 0;
}
else {
    // redoing because battle ship is out of field..
              int x = generateBattleShipX();
              int y = generateBattleShipY();
            // printf("x = %d y = %d \n",x,y );

        tryToFillBattleField(x, y);
    return 0;
}
    return 0;
}

tryToFillBattleField is called by its method again and again about 10 times. Problem is it can generate random number only for 1st time. For the rest, it is always the same. Is there a particular method I need to write for nested function to generate random number?

Answer 1

The way it is now, each time you call generateBattleShipX or generateBattleShipY you re-seed the random number generator. Since time returns the time in seconds, multiple successive calls will return the same value, so the random number generator gets seeded with the same value, resulting in the same "random" number each time.

srand should only be called once at the start of your program to seed the random number generator. Once you do that, you'll get more random results.

Answer 2

dbush is correct, however a better pseudo random number would be to use the encryption library (if you OS supports one) as the "randomness" of the random number generator is better than using time as your seed.

As it stands, with the corrections, this would be easy to predict the numbers and defeat the "game."