Question

I am making a Battleships-like game and need to place a battleship objects into a battlemap array randomly. Except when I do this, the ships are never placed in the bottom right quadrant (but they are successfully placed in the other 3 quadrants), and I don’t know why. Basically, I get a random integer between 0 and the length and height of the map and a random direction, then check if the ship will fit there, if it can, place it on the map. But it never places them in the bottom right.

void BattleMap::placeRandomly(BattleShip& ship) {
bool correct = true; 
int x_start,y_start,dir;
// size_x, size_y denote the length and height of the array respectively
int length = ship.getLength();      
do{
    correct = true; 
    x_start = abs(rand()%(size_x-length));
    if(x_start+length > size_x) x_start -= length;  
    y_start = abs(rand()%(size_y-length));
    if(y_start+length > size_y) y_start -= length;
    dir = rand()%2; // 0 for vertical, 1 for horizontal;
    for ( int i = 0;  i < length;i++) {
        switch(dir){ // Check if there is already a ship in the candidate squares
          case 0:
            if(this->at(x_start,y_start+i)){
              correct = false;
            }
            break;
          case 1:
            if(this->at(x_start+i,y_start)){
              correct = false;
            }
            break;
        }
      }
   }while(!correct);
   // Place the ships into the array
   ....

}

The at() function is this:

BattleShip*& BattleMap::at(int x, int y){
    if(x > size_x || y > size_y)return 0; 
// error: invalid initialization of non-const reference of type 'BattleShip*&' from a temporary of type 'int'
    return board[x*size_y +y];
}

Answer 1

You are trying too hard to keep the ship from going off the side. Just allow x_start and y_start to be anywhere:

x_start = rand()%size_x;
y_start = rand()%size_y;

And let your at() function return true if it goes off the side:

bool BattleMap::at(int x,int y) const
{
  if (x>=size_x || y>=size_y) return true;
  // regular check for a ship at x,y here
}