Find duplicates of an array and replace it with its number

Question

Say I have an array:

arr = [25, 25, 25, 20, 15, 10, 10, 5];

and I want to count up the number of duplicates (ie. three 25s and 2 10s) and make a new array that becomes:

newArr = ['25 * 3', 20, 15, '10 * 2', 5];

How should I go about doing this? Thanks!

I think even though he use reduce, he wont be getting the count of the repeated items, right? — vsnikhilvs, Aug 12 '21 at 16:36
This one has been answered here: https://stackoverflow.com/questions/5667888/counting-the-occurrences-frequency-of-array-elements — AllWorkNoPlay, Aug 12 '21 at 16:52

haim brandesdorfer · Answer 1 · 2021-08-12T17:43:01.830

It can be solved using Set and filter

let arr = [25, 25, 25, 20, 15, 10, 10, 5];

const newArr = [...new Set(arr)].map((x) => {
    const count = arr.filter(y => y == x).length
    return count > 1 ? x + " * " + count: x;
})

console.log(newArr) //  ["25 * 3", 20, 15, "10 * 2", 5]

or if you want the numeric value you can do that

let arr = [25, 25, 25, 20, 15, 10, 10, 5];
    const newArr = [...new Set(arr)].map((x) => arr.filter(y => y == x).length * x)
    console.log(newArr) // [75, 20, 15, 20, 5]

score 0 · Answer 2 · answered Aug 12 '21 at 16:55

You could use reduce and check if the current element in the iteration is equal to the previous element and then also what is the type of last element added to the accumulator.

const arr = [25, 25, 25, 20, 15, 10, 10, 5];
const result = arr.reduce((r, e, i, arr) => {
  if (i && e === arr[i - 1]) {
    if (typeof r[r.length - 1] === 'number') {
      r[r.length - 1] = `${e} * 2`;
    } else {
      const [key, value] = r[r.length - 1].split(' * ')
      r[r.length - 1] = `${key} * ${+value + 1}`
    }
  } else {
    r.push(e)
  }

  return r;
}, [])

console.log(result)

lejlun · Accepted Answer · 2021-08-12T17:05:29.797

You can use a Array#forEach loop to iterate through each item of the array, keeping track of how many times each item has been seen before.

Demo:

let arr = [25, 25, 25, 20, 15, 10, 10, 5];

let result = [], seenBefore = [];

arr.forEach((item) => {
  let seen = seenBefore.indexOf(item);
  if (seen !== -1) return result[seen].count++;
  result.push({ name: item, count: 1 });
  seenBefore.push(item);
});

result = result.map(({ name, count }) =>
  count > 1 ? `${name} * ${count}` : name
);

console.log(result);

The same technique but smaller using Array#reduce:

let arr = [25, 25, 25, 20, 15, 10, 10, 5];

let result = arr
  .reduce(
    (acc, item) => {
      acc[1].indexOf(item) !== -1 ? acc[0][acc[1].indexOf(item)].count++ : (acc[0].push({ name: item, count: 1 }), acc[1].push(item));
      return acc;
    },
    [[], []]
  )[0]
  .map(({ name, count }) => (count > 1 ? `${name} * ${count}` : name));

console.log(result);

Find duplicates of an array and replace it with its number

3 Answers3