Substitute random items in a Javascript array

Question

In Javascript, am trying to randomly substitute half (in this case, 3 out of 6) of the items from an array with different ones (all of the same type), and I need the original items' position to be kept. So for instance, if I have:

var my_array = [a, b, c, d, e, f]

I would want to select three random ones to be substituted with a 0, while the others to keep their initial position. So let's say a, c, and d are the ones the random selector will make go away on one instance, then my array would become:

my_array = [0, b, 0, 0, e, f]

On a different run, the random selector would perhaps pick b, c, and f and so I'd have:

my_array = [a, 0, 0, d, e, 0]

And so on. Thank you so much for your help!

@NinaScholz I am just new to JavaScript and I am not sure where to start from! I've been using a shuffle function and then select the first three items to get a random selection so far, but that doesn't keep the original positions in place — gimi
– gimi, Commented Aug 15, 2020 at 9:51

Nina Scholz · Accepted Answer · 2020-08-15 09:57:01Z

2

You could take a closure over the array and wanted zero counts and return a function which generates random integer and map the array with zeros or values.

function getRandom(array, count) {
    return function () {
        const indices = new Set();
        do {
            indices.add(Math.floor(Math.random() * array.length));
        } while (indices.size < count)
        return array.map((v, i) => indices.has(i) ? 0 : v);
    };
}

var myArray = ['a', 'b', 'c', 'd', 'e', 'f'],
    getArrayWithZeros = getRandom(myArray, 3);


console.log(...getArrayWithZeros());
console.log(...getArrayWithZeros());
console.log(...getArrayWithZeros());
console.log(...getArrayWithZeros());
console.log(...getArrayWithZeros());

answered Aug 15, 2020 at 9:57

Nina Scholz

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Comments

tarkh · Accepted Answer · 2020-08-15 10:16:18Z

1

Another option

const myArray = ['a', 'b', 'c', 'd', 'e', 'f'];

// Randomizer function
const rand = ([...arr], len, rep) => {
  let ins = {};
  while(Object.keys(ins).length < len) {
    let r = ~~(Math.random() * arr.length);
    if(!ins[r]) ins[r] = true;
  }
  for(let i = 0; i < arr.length; i++) if(ins[i]) arr[i] = rep;
  return arr;
}

// Array, number of elements to replace, replace with
// Here we transform toString for better readability
console.log(rand(myArray, 3, 0).toString());
console.log(rand(myArray, 3, 0).toString());
console.log(rand(myArray, 3, 0).toString());
console.log(rand(myArray, 3, 0).toString());
console.log(rand(myArray, 3, 0).toString());

answered Aug 15, 2020 at 10:16

tarkh

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Comments

Majed Badawi · Accepted Answer · 2020-08-15 09:59:10Z

0

You need to calculate half of the length of the array, generate random indexes of this amount, and then change them:

var my_array = ['a', 'b', 'c', 'd', 'e', 'f'];
function alterHalfWithZero(arr){
     let my_array = [...arr];
     let indexList = {};
     let len = Math.floor(my_array.length / 2)
     while(Object.values(indexList).length != len){
          let random_index = Math.floor(Math.random() * my_array.length);
          indexList[random_index] = random_index;
     }
     indexList = Object.values(indexList);
     for(let i = 0; i < indexList.length; i++)
           my_array[indexList[i]] = 0;
     return my_array;
}

console.log(...alterHalfWithZero(my_array));
console.log(...alterHalfWithZero(my_array));
console.log(...alterHalfWithZero(my_array));

edited Aug 15, 2020 at 9:59

answered Aug 15, 2020 at 9:53

Majed Badawi

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Comments

Siva Kondapi Venkata · Accepted Answer · 2020-08-15 11:04:50Z

0

First get unique random indexes. Loop over the indexes and make them 0.

var my_array = ["a", "b", "c", "d", "e", "f"];

// Get unique random indexes
const random = (num, count) => {
  const set = new Set();
  while (set.size < count) {
    set.add(Math.floor(Math.random() * num));
  }
  return [...set];
};

const alter = (arr, count = 3) => {
  const output = [...arr];
  random(arr.length, count).forEach((index) => (output[index] = 0));
  return output;
};

console.log(alter(my_array));
console.log(alter(my_array));

answered Aug 15, 2020 at 11:04

Siva Kondapi Venkata

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Nathan · Accepted Answer · 2020-08-15 11:47:12Z

0

Here is one solution.

var my_array = ["a", "b", "c", "d", "e", "f"];

const substitute = array => {
  const indexes = Array.from(Array(array.length).keys())
    .sort(() => Math.random() - 0.5)
    .slice(0, array.length / 2);
  return array.map((value, index) =>
    indexes.some(i => i === index) ? value : 0
  );
};

console.log(substitute(my_array));

answered Aug 15, 2020 at 11:47

Nathan

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