Is there a better way of selecting a random element from an array of variable size than this?

Question

I'm very new to C++ and was wondering if if there is a better way of doing this. It's going to run on an Arduino so I can't use ArrayLists or anything.

byte GetFreeCell(short x, short y)
{
    byte possibleMoves[4] = {0,0,0,0};
    if (y - 2 >= 0 && _grid[y - 2][x] == 0)
        possibleMoves[0] = 1;
    if (x + 2 < WIDTH && _grid[y][x + 2] == 0)
        possibleMoves[1] = 2;
    if (y + 2 < HEIGHT && _grid[y + 2][x] == 0)
        possibleMoves[2] = 3;
    if (x - 2 >= 0 && _grid[y][x - 2] == 0)
        possibleMoves[3] = 4;

    if (possibleMoves[0] == 0 && possibleMoves[1] == 0 && possibleMoves[2] == 0 && possibleMoves[3] == 0) {
        return 0;
    }

    byte move = 0;
    while(move == 0){
        move = possibleMoves[random(4)];
    }
    return move;
}

Thanks,

Joe

Define "better"? Do you want a more concise way of doing it? — André Caron
– André Caron, Commented Jan 25, 2013 at 20:08

Anton Kovalenko · Accepted Answer · 2013-01-25 20:24:15Z

4

byte GetFreeCell(short x, short y)
{
    byte possibleMoves[4];
    byte index = 0;
    if (y - 2 >= 0 && _grid[y - 2][x] == 0)
        possibleMoves[index++] = 1;
    if (x + 2 < WIDTH && _grid[y][x + 2] == 0)
        possibleMoves[index++] = 2;
    if (y + 2 < HEIGHT && _grid[y + 2][x] == 0)
        possibleMoves[index++] = 3;
    if (x - 2 >= 0 && _grid[y][x - 2] == 0)
        possibleMoves[index++] = 4;

    return index ? possibleMoves[random(index)] : 0;
}

edited Jan 25, 2013 at 20:24

answered Jan 25, 2013 at 20:09

Anton Kovalenko

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5 Comments

us2012 Over a year ago

+1, beat me to it. Maybe make index a byte, it's an unsigned 8bit type on arduino (vs int, which is signed 16bit).

jrok Over a year ago

I was just going to suggest what you edited in, but it's not really neccesary (it does help readability, though) +1

Anton Kovalenko Over a year ago

@billz read carefully, OP's code has return 0; earlier, for the same condition that mine.

JoeS Over a year ago

Thanks, makes perfect sense. Not sure why I didn't think of that, I'm a little embarrased now.

JoeS Over a year ago

us2012: The last ++ operator acts as the index when setting the array and then increments meaning that the random(index) is random(4) which can be 0,1,2 or 3.

Nikos C. · Accepted Answer · 2013-01-25 20:09:21Z

0

You can do yourself a favor and use this:

https://github.com/maniacbug/StandardCplusplus/#readme

Then you can sanitize your code by using standard containers.

Also, there's no ArrayList in C++. That's Java. With the above library, you can use std::vector instead.

answered Jan 25, 2013 at 20:09

Nikos C.

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