how to get random element of array which is not null?

Question

Let's say i haven an array of let arr = [null, null, null, null, null, null, null, null, null] containing 9 items.

And i have 9 <button data-index> with each having an data-index=0 to data-index=9

If i click a button, let's say <button data-index=4>, then we assign 'Cookie' to the corresponding index of arr: arr[4] = 'Cookie'

arr becomes [null, null, null, null, 'Cookie', null, null, null, null]

How can i select a random element/index of arr which is null without it selecting arr[4] since it already contains Cookie?.

It's for the AI move of a tic-tac-toe game.

Alberto Trindade Tavares · Accepted Answer · 2017-07-02 15:25:11Z

3

An ES6 solution with filter:

var arr = [null, null, null, null, 'Cookie', null, null, null, null];

var indexes = Array.from(Array(arr.length).keys());
var availableIndexes = indexes.filter((index) => arr[index] == null);
var selectedIndex = availableIndexes[Math.floor(Math.random()* availableIndexes.length)];

console.log(availableIndexes);
console.log(selectedIndex);

edited Jul 2, 2017 at 15:25

answered Jul 2, 2017 at 15:18

Alberto Trindade Tavares

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Prathik Rajendran M · Accepted Answer · 2017-07-02 15:16:20Z

2

Do a fisher-yates shuffle to get a random order of the indices of the array and then pick them one by one.

Idea is to set the random index to the buttons at the start. And not compute the random index on click.

answered Jul 2, 2017 at 15:16

Prathik Rajendran M

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Junaid · Accepted Answer · 2017-07-02 15:34:14Z

2

var randomIndex = 0;
while ( arr[randomIndex] != null ) {
    randomIndex = Math.floor( Math.random() * (arr.length - 1) );
}

Let me know if this helps.

edited Jul 2, 2017 at 15:34

answered Jul 2, 2017 at 15:21

Junaid

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AlexW · Accepted Answer · 2017-07-02 15:32:09Z

1

Another solution:

var arr = [null, null, null, null, 'Cookie', null, null, null, null];
var indexes = [];
for (var i =0; i<arr.length; i++){
    if(arr[i] == null)
        indexes.push(i);
}
var index = indexes[parseInt(Math.random() * (indexes.length - 1))];

alert(index);

answered Jul 2, 2017 at 15:32

AlexW

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