30-seconds-of-code/snippets/weightedSample.md

title, tags

title	tags
weightedSample	array,random,advanced

Returns a random element from an array, using the provided weights as the probabilities for each element.

• Use Array.prototype.reduce() to create an array of partial sums for each value in weights.
• Use Math.random() to generate a random number and Array.prototype.findIndex() to find the correct index based on the array previously produced.
• Finally, return the element of arr with the produced index.

const weightedSample = (arr, weights) => {
  let roll = Math.random();
  return arr[
    weights
      .reduce((acc, w, i) => (i === 0 ? [w] : [...acc, acc[acc.length - 1] + w]), [])
      .findIndex((v, i, s) => roll >= (i === 0 ? 0 : s[i - 1]) && roll < v)
  ];
};

weightedSample([3, 7, 9, 11], [0.1, 0.2, 0.6, 0.1]); // 9