not selecting random number again

I'm working on the game battleship on c++ and everything works out great except for the part that its the AI turn to take a shot. I got it to work with the AI picking a random number, but it sometimes picks the same number twice. so I was working on the code to check if the coordinates had already been picked before. according to me the code below should work, but if doesn't it stills picks the same coordinates twice.

What its supposed to do is pick to random numbers. then it checks it with the other previous coordinates. if they match it sets attack to false and the while loop repeats. then it stores the coordinates in a vector.
this is a member function of a class. Row and column are declared private in the class.

Code:

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vector<int> prevAttacksY(1, -1);
    vector<int> prevAttacksX(1, -1);
    bool attack;
    int count = 0;
    while(!attack){
        row = rand() % 8;
        column = rand() % 8;
        attack = true;
        for(count = 0; count < prevAttacksY.size(); count++){
                if (row == prevAttacksY.at(count) && column == prevAttacksX.at(count))
                    attack = false;
        }
    }
    prevAttacksY.push_back(row);
    prevAttacksX.push_back(column);

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Quote:

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Originally Posted by diego

picking a random number, but it sometimes picks the same number twice.

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Well, that is possibility of random numbers. Here's a tip about rand: if you are going to use it, seed the RNG when the program first starts with srand(time(NULL)). Only do that *once*. If you call srand() repeatedly, you may end up with the same series of numbers repeating.

That's not the problem. Calling srand() will give you a difference sequence of random numbers each time you start the program. What the OP is trying to do here is prevent the same 2D coordinate from being generated multiple times.

You never set attack to false to begin with. The code might run right now, because in some debug builds variables are initialized to zero, but it's undefined behaviour. Your while loop might never get entered.

Assuming your board has a fixed size, it would probably be way easier and certainly more efficient -- O(1) instead of O(n) -- to use something like this.

Code:

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bool picked[WIDTH][HEIGHT] = {};  // automatically init to all false

int x, y;
do {
    x = rand() % WIDTH;
    y = rand() % HEIGHT;
} while(picked[x][y]);
picked[x][y] = true;

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Quote:

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Originally Posted by dwks

That's not the problem. Calling srand() will give you a difference sequence of random numbers each time you start the program. What the OP is trying to do here is prevent the same 2D coordinate from being generated multiple times.

You never set attack to false to begin with. The code might run right now, because in some debug builds variables are initialized to zero, but it's undefined behaviour. Your while loop might never get entered.

Assuming your board has a fixed size, it would probably be way easier and certainly more efficient -- O(1) instead of O(n) -- to use something like this.

Code:

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bool picked[WIDTH][HEIGHT] = {};  // automatically init to all false

int x, y;
do {
    x = rand() % WIDTH;
    y = rand() % HEIGHT;
} while(picked[x][y]);
picked[x][y] = true;

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not setting attack to false was a big mistake. your code makes perfect sense and is much better but I tried it and it is still giving repeating coordinates.

Hmm, it shouldn't be. :) Did you make the picked[][] array static or global? If you just declared it as a local array it would be reset to all false every time the picking function was executed.

If it's still not working, you may want to post some of your code so that we can look at it.

Quote:

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Originally Posted by dwks

Hmm, it shouldn't be. :) Did you make the picked[][] array static or global? If you just declared it as a local array it would be reset to all false every time the picking function was executed.

If it's still not working, you may want to post some of your code so that we can look at it.

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that got it working. I had it declared in the function. I changed it to global and it worked. thanks a lot. I couldn't figure out the problem.