Can I use array.filter() to get sample elements from array?

Question

I am learning JavaScript and trying to apply it in statistical sampling. The function below does not work but assumes:

array = population to study
sample = sample drawn from population
element = each observation from population

const generateSample = array => {
    var sample = []
    array.filter(element => {
            (element in sample) ? return null : sample = [...sample, element]
    })
}

No. The function does not work. I am trying to use the filter method as it seems less verbose and descriptive. I also realized i need to add sample size to the equation — , Apr 04 '20 at 18:06
Why you want to use `filter` for such cases, use `forEach` or any loop, filter should be used without any side-effects — Code Maniac, Apr 04 '20 at 18:07
I'm unsure of what you're trying to do here. [filter()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/filter) takes in a predicate function that's run over every element in the array. It returns an array made of each element for which the predicate results in `true`. If you're trying to achieve something similar to Python's slice notation, look at [slice()](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/slice). — OSH, Apr 04 '20 at 18:08
If array = [1,2,3,4,5] and size = 2, the result could be [3,1] or [2,4], and not [2,2]. I thought filter checks a condition of whether the randomly selected element is in the sample — , Apr 04 '20 at 18:12
Does this answer your question? [Is it correct to use JavaScript Array.sort() method for shuffling?](https://stackoverflow.com/questions/962802/is-it-correct-to-use-javascript-array-sort-method-for-shuffling) — TheMaster, Apr 04 '20 at 18:32
Yes. I tested Array.sort() but it seems to mutate the original array. I should probably read on Fisher-Yates shuffle and try to write it in js. — , Apr 04 '20 at 18:41

score 1 · Accepted Answer · answered Apr 04 '20 at 22:53

After doing some research, this is what i found when retrieving a sample of size N from an array

Shuffle the array
Slice the shuffled array to get sample of size N

NOTE: The shuffleArray function was created with the help of this question and answer

Below are the two functions that solved the problem

// Function that shuffles an array
const shuffleArray = array => {
  if (array.length === 1) return array

  const randomIndex = Math.floor(Math.random() * array.length);
  const randomElement = array[randomIndex]

  array = array.filter((_, i) => i !== randomIndex)

  return [randomElement, ...shuffleArray(array)];
};

// Function that returns sub array from array
const subArray = (array, n) => array.slice(0, n)

const population = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
const shuffled = shuffleArray(population)
console.log( shuffled )
console.log(subArray(shuffled, 4))

domondo · Answer 2 · 2020-04-04T18:54:25.643

Array.filter expects a function that returns true or false (whether to keep the element in the array). The function you give to filter returns null... or doesn't return at all. (element in array) will check if array.length >= element as the in operator checks if a property is in an object or its prototype chain. To check if an element is in an array use .includes() or .indexOf() >= 0. Also the ternary operator condition ? expr : expr expects expressions, not statements like return null or sample =

Maybe something like?

let data = [1, 2, 3, 4, 5];

let sample = data.filter(() => Math.round(Math.random()));

console.log(sample);

Ok. I edited my code and meant (element in sample) to check if randomly selected element is in sample array — , Apr 04 '20 at 18:56

Can I use array.filter() to get sample elements from array?

2 Answers2