Add a random and not existing item to array from another array

Question

Let's assume we have an array of items:

const arr1 = [22, 54, 67, 11, ...so on]

and empty one:

let arr2 = []

I can add random item with:

arr2 = [...arr2, arr1[Math.floor(Math.random()*arr1.length)]]

BUT how can I add a random item from first array to 2nd and already added should not be added?

Yes, we can keep added indexes in some tmp variable, but it doesnt seems right and I think there should be a different solution.

NOTE: array spreads because I need to add an item with some events. For example user click to button and new item will be added to 2nd array, and this array may contain already added elems

Can the original array have its entries removed as they're added to the second array? If not, could a copy be made of that original array, and the entries for the second array be removed from that? — David Thomas
– David Thomas, Commented Jul 6, 2018 at 13:45
Remove the items from arr1 as you add them to arr2. Use a copy of arr1 is you want to preserve the original — Matt Burland
– Matt Burland, Commented Jul 6, 2018 at 13:45
@NinaScholz: presumably because it's not empty after the first item has been added. — Matt Burland
– Matt Burland, Commented Jul 6, 2018 at 13:45
I would suggest if you want to get random values at once why not just shuffle the original array and use it according to your need. This will be the least memory taking solution. Otherwise, @MattBurland suggestion is the great suggestion. — Ullas Hunka
– Ullas Hunka, Commented Jul 6, 2018 at 13:57

Ankit Agarwal · Accepted Answer · 2018-07-06 14:00:59Z

1

You can create your custom logic to push the random value in arr2 so that the condition is that the numbers must be unique in arr2 and it will have maximum random numbers till the length of arr1:

const arr1 = [27, 54, 67, 11, 15, 22, 26, 58, 45, 87];
let arr2 = [];

function getRandom(){
  var randomNumber = arr1[Math.floor(Math.random()*arr1.length)];
  while(arr2.indexOf(randomNumber) !== -1 && arr2.length !== arr1.length) {
    randomNumber = arr1[Math.floor(Math.random()*arr1.length)];
  }
  arr2.push(randomNumber);
}
getRandom(); getRandom(); getRandom(); getRandom(); getRandom(); getRandom(); getRandom(); getRandom(); getRandom(); getRandom();getRandom();
console.log(arr2);

answered Jul 6, 2018 at 14:00

Ankit Agarwal

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2 Comments

Ullas Hunka Over a year ago

Ain't this same answer as above answers.

Ankit Agarwal Over a year ago

@UllasHunka No. That use recursion and it has if-else block. Mine is simplified.

Niladri Basu · Accepted Answer · 2018-07-06 14:03:53Z

1

I think you are looking for something like this:

const arr1 = [22, 54, 67, 11, 23, 56, 43, 77, 34, 76, 30]

let arr2  = []


function addRandom() {

  if(new Set(arr1).size === arr2.length) return;  //prevention from infinite loop
  // if(arr1.length=== arr2.length) return;  // if arr1 doesn't contain duplicates
  let random = arr1[Math.floor(Math.random()*arr1.length)];
  if (!arr2.includes(random)) {
    arr2.push(random);
  } else {
    addRandom();
  }
}

addRandom();
console.log(arr2);

addRandom();
console.log(arr2);

addRandom();
console.log(arr2);

If you are sure that arr1 wont contain duplicates then replace (new Set(arr1).size === arr2.length) with arr1.length=== arr2.length.

edited Jul 6, 2018 at 14:03

answered Jul 6, 2018 at 13:58

Niladri Basu

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6 Comments

Ullas Hunka Over a year ago

I don't think the answer is far better then @Nicolas it could cause him the infinite loop.

Niladri Basu Over a year ago

@UllasHunka no it wont. See prevention from infinite loop comment

Ullas Hunka Over a year ago

sir that part of the method is not a problem but calling the method from the method is. Let assume if the desired result returned after 10000 times of method ran. Because he is using Math.random() and random can be anything.

Niladri Basu Over a year ago

@UllasHunka Yes, but it won't cause an infinite loop.

Ullas Hunka Over a year ago

How can you say it won't?

|

sjahan · Accepted Answer · 2018-07-06 15:01:15Z

1

If this is ok to mutate arr1 (as Matt Burland suggested in comments), try this:

arr2 = [...arr2, ...arr1.splice(Math.floor(Math.random()*arr1.length), 1]);

splice will remove the item from arr1 and return you the item removed, ready to be added in arr2!

If you cannot change it, just make a clone before using this line!

Hoping this will help you :)

edited Jul 6, 2018 at 15:01

answered Jul 6, 2018 at 13:48

sjahan

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Comments

Darkrum · Accepted Answer · 2018-07-06 14:13:22Z

0

If compatibility is not an issue you can use array set.

var arr1 = [44, 55, 100];
var arr2 = [44, 55, 100];

var arr3 = [...new Set(arr1, arr2)];

answered Jul 6, 2018 at 14:13

Darkrum

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Comments

nick · Accepted Answer · 2018-07-06 14:19:09Z

0

Something like this should work

let arr1 = [22,43,67,11,63];
let indexes = [...arr1.keys()];
indexes = indexes.sort(() => Math.Random() - 0.5);
let arr2 = [];
arr2 = [...arr2, arr1[indexes.pop()]]

This lets you preserve the original order of your first array too.

answered Jul 6, 2018 at 14:19

nick

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Comments

Nicolas · Accepted Answer · 2018-07-06 14:35:29Z

0

You could use the include function on your second array and check if the element you are trying to add does not exists.

let randomItem = arr1[Math.floor(Math.random()*arr1.length)];
while(arra2.includes(randomItem)) {
    randomItem = arr1[Math.floor(Math.random()*arr1.length)];
}
arr2.push(randomItem);

edited Jul 6, 2018 at 14:35

answered Jul 6, 2018 at 13:47

Nicolas

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5 Comments

Ullas Hunka Over a year ago

I believe this is more memory taking solution loop may run infinite times to achieve the goal.

WebArtisan Over a year ago

this wont work because it wont be added to new array in case random item is not matches

Nicolas Over a year ago

it will add the item if it is not already in the second array. I don't get what you are trying to do.

WebArtisan Over a year ago

ok behavior -> user clicks -> new random item added, in case this item already added, script should pick another elem, which is not added yet. In your example there no item will be added in case item is already in bundle.

Nicolas Over a year ago

I've added a while loop that will pick an item at random until it found an unique one. then add it.

hygull · Accepted Answer · 2018-07-06 16:19:52Z

@WebArtisan, you can also try the below code.

// Function that returns a randomly selected unique list of items from another 
// array with duplicated items
function getUniqueRandomItems(arr1) {
	let arr2 = [];

	while(arr1.length) {
		// A random number in range [0, arr1.length-1]
		randomIndex = Math.floor((Math.random() * (arr1.length)) + 1) - 1; 

		// Removing 1 item from random index in range
		removedItem = arr1.splice(randomIndex, 1)[0]; 
		
		// If removedItem does not exist then add it to arr1
		if(arr2.indexOf(removedItem) < 0) {
		    arr2.push(removedItem);  
		}
	}

	return arr2 // Return the result array with unique random items
}

/************ TEST 1****************************************/
const arr1 = [22, 54, 67, 11, 54, 22, 67, 11, 88, 90, 54, 2, 3, 2, 11, 54, 0];
const arr2 = getUniqueRandomItems(arr1);
console.log(arr2); // [ 2, 22, 88, 67, 54, 11, 3, 90, 0 ]

/************ TEST 2****************************************/
const arr3 = [1, 5, 2, 5, 1, 3, 3, 6, 3, 1, 9, 8, 10, 9, 7, 4, 3, 4, 2, 1, 10];
const arr4 = getUniqueRandomItems(arr3);
console.log(arr4); // [ 1, 7, 9, 10, 5, 3, 2, 8, 4, 6 ]

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