Get a random array element index by excluding some of that array's element values

Question

I the following array of all false values

bool[] deniedMoves = new bool[64];

think of a game board with x possible moves. After a move is played, it is recorded, some of the moves become impossible in the next play. A game like Go would fit this description as you cannot play over an intersection that has already been played on.

I want the "AI" to play a random move from a list of possible moves. First move is OK:

Random randMove1 = new Random();
int selectedMove1 = randMove1.Next(1, deniedMoves.Length);

and then that move sets some of the values in my array as true.

For example:

void Reset()
{
    if (mainBoard[0, 0] == 1)
        deniedMoves[0] = true;
    deniedMoves[3] = true;
    deniedMoves[5] = true;
}

The next random move the "AI" will play cannot choose from all 64 possibilities so I don't want it to randomize to 5 for example.

I have tried something like this, but RandomElement doesn't belong...

IEnumerable<bool> queryMoves = deniedMoves.Where(deniedMove => deniedMove !=             
true).RandomElement();

My first question and I am terribly sorry for bad code and general coding ignorance. I have tried.

in your initial example int selectedMove1 = randMove1.Next(1, deniedMoves.Length); why do you skip the first item in the array? — littlerufe
– littlerufe, Commented Feb 6, 2018 at 0:11

Christopher · Accepted Answer · 2018-02-06 00:02:17Z

1

What you are doing is what I call "the Lottery problem": You have a number of options. But as each option is picked, it is removed from teh list of possible options. The same way you literally draw Numbers from a set of lottery balls.

You should store all the options in a List<bool> and remove elements as you draw/use them. Example code done largely in my head:

int[] input = new int[20]();

//Fill array
for(int i = 0; i < numbers.lenght; i++)
  input[i] = i;

//initialise the real work variables
List<int> Drawables = new List<int>(input);
List<int> DrawnNumbers = new List<int>();
Random rng = new Random();

while(Drawables.Count > 0){
  int temp = Random.NextInt(Drawables.Count);
  DrawnNumbers = Drawables[i];
  Drawables.Remove(temp);
}

answered Feb 6, 2018 at 0:02

Christopher

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4 Comments

NetMage Over a year ago

How about var Drawables = Enumerable.Range(0, 20).ToList(); ? Your code has a lot of errors (where is i from at the bottom? What is numbers.lenght? Where is numbers from? You don't have a List<bool> any where?)

Christopher Over a year ago

That works too. In the end whose loop fills the array does not really mater, as there will always be a loop. I would propably use Enumerable.Range just to build the input array as I wanted the explicit array-> List step in there. I think keeping it simple to read and Framework agonostic (Enumerable.Range needs 3.5+) maters more with examples taht I will post a lot.

NetMage Over a year ago

Except your code won't run. Also, the OP specified a move may disqualify multiple moves, making this more complicated than a simple card deal, though your idea is sound.

Christopher Over a year ago

That is why I said "largely done in my head". It is an idea done without a compiler at my call. Just something I scribelled up in Notepad++ half from memory. And I have a really poor syntax memory.

littlerufe · Accepted Answer · 2018-02-06 00:45:49Z

I'm not certain if this is what you're after, and it's very similar to NetMages' answer, but one thing you could do is write a method that returns all the indexes in your array that contain a false value:

private static IEnumerable<int> GetIndexesOfFalseItems(IEnumerable<bool> allItems)
{
    return allItems
        .Select((value, index) => new { value, index })
        .Where(item => item.value == false)
        .Select(item => item.index);
}

Then, to choose a random item from your array whose value is false, you could just do:

private static readonly Random Rnd = new Random();

private static void Main()
{
    // Initialize all moves to false
    bool[] allMoves = new bool[64];
    for (int i = 0; i < allMoves.Length; i++) allMoves[i] = false;

    // Pretend a move was made and set some to true
    allMoves[0] = true;
    allMoves[3] = true;
    allMoves[5] = true;

    // Get ALL indexes of 'false' items (currently everything except 0, 3, and 5)
    var validIndexes = GetIndexesOfFalseItems(allMoves).ToList();

    // Choose a random item from that list
    var randomValidIndex = validIndexes[Rnd.Next(validIndexes.Count)];

    // Now we get our next move from our main list at that index
    var nextMove = allMoves[randomValidIndex];

    // Rest if game code...
}

NetMage · Accepted Answer · 2018-02-06 00:16:03Z

0

I would use some LINQ - this may not be the most efficient answer...

bool[] deniedMoves = new bool[64];

System.Random randMove1 = new System.Random();

First, choose a random move amongst the allowed moves:

int allowedMove = randMove1.Next(0, deniedMoves.Count(d => !d)-1);

Then get the actual move index:

int selectedMove = deniedMoves.Select((d, i) => new { d, i })
                              .Where(di => !di.d)
                              .Skip(allowedMove)
                              .First().i;

I changed the moves to be from 0 - 63 as that matches C# arrays better.

edited Feb 6, 2018 at 0:16

answered Feb 6, 2018 at 0:05

NetMage

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