how to get the number of pair values in array? Javascript

November 11, 2021

I am trying this solution but it is not giving me the result I want.
I don’t want to count the number of pairs of 1 cause it is not really a pair as it appears 4 times.
I need to count the "perfect" pairs, like in this case: 5 and 2. Right now this is giving me 4 as result and it should be 2.

How could I achieve that? I am stuck.

let ar1 = [12, 5, 5, 2, 1, 1, 1, 1, 2];

const countPairs = (ar) => {
  let obj = {};

  ar.forEach((item) => {
    obj[item] = obj[item] ? obj[item] + 1 : 1;
  });

  return Object.values(obj).reduce((acc, curr) => {
    acc += Math.floor(curr / 2);
    return acc;
  }, 0);
};

console.log( countPairs(ar1) )

>Solution :

This can be one-liner using Map as:

const countPairs(arr)  => [...arr.reduce((dict, n) => dict.set(n, (dict.get(n) ?? 0) + 1), new Map()).values(),].filter((n) => n === 2).length;

let ar1 = [12, 5, 5, 2, 1, 1, 1, 1, 2];

const countPairs = (arr) =>
  [
    ...arr
      .reduce((dict, n) => dict.set(n, (dict.get(n) ?? 0) + 1), new Map())
      .values(),
  ].filter((n) => n === 2).length;

console.log(countPairs(ar1));