Question

Is there a way to select a random position in a 2d array of bools (bool[,]) with a negative value without bruteforcing?

Answer 1

Here’s a non-brute-force method, but it involves an initial scan of the entire table:

int[] negOffsets = new int[data.Length];
int dataOffset = 0, count = 0;
foreach(bool x in data)
{
    if(!x) negOffsets[count++] = dataOffset;
    dataOffset++;
}
if(count == 0) {
    // nothing to pick
} else {
    int index = negOffsets[rand.Next(0, count)];

    int x = index / data.GetLength(1),
        y = index % data.GetLength(0);
    // assertion: the following should be false
    bool b = data[x, y];
}

Also, you’d probably want to keep offsets around and re-use it between iterations.