How can I determine the quantity of pairs in an array using JavaScript?

Question

How can I determine the quantity of pairs in an array using JavaScript?

My current approach is not providing me with the desired result. I am facing an issue where pairs of 1 are being counted multiple times, which is incorrect. What I actually need is to count only the "perfect" pairs, such as the pair between 5 and 2 in this case. As of now, my solution is returning 4 when it should be 2.

I am struggling to find a way to achieve this. Any suggestions would be greatly appreciated.

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) )

javascript arrays

Answer 1

Answer №1

To find the number of pairs in a list, you can filter the object values by 2 and then count them

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

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

  arr.forEach((num) => {
    obj[num] = obj[num] ? obj[num] + 1 : 1;
  });
  
  return Object.values(obj).filter(e => e == 2).length;
};

console.log(countPairs(numbers))

Answer 2

To find the number of pairs in a list, you can filter the object values by 2 and then count them

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

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

  arr.forEach((num) => {
    obj[num] = obj[num] ? obj[num] + 1 : 1;
  });
  
  return Object.values(obj).filter(e => e == 2).length;
};

console.log(countPairs(numbers))

Answer 3

Answer №2

Here's a concise way to count pairs in an array using the Map method:

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

let exampleArray = [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(exampleArray));

Answer 4

Here's a concise way to count pairs in an array using the Map method:

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

let exampleArray = [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(exampleArray));

Answer 5

Answer №3

or that

const 
  ar1 = [12, 5, 5, 2, 1, 1, 1, 1, 2]
, countPerfectPairs = arr => arr.reduce((r,val,i,{[i+1]:next})=>
    {
    if(!r.counts[val])
      { 
      r.counts[val] = arr.filter(x=>x===val).length
      if (r.counts[val]===2) r.pairs++
      }
    return  next ? r : r.pairs
    },{counts:{},pairs:0}) 


console.log( countPerfectPairs(ar1) )

If you prefer Details:

const 
  ar1 = [12, 5, 5, 2, 1, 1, 1, 1, 2]
, countPerfectPairs = arr => arr.reduce((r,val)=>
    {
    if(!r.counts[val])
      { 
      r.counts[val] = arr.filter(x=>x===val).length
      if (r.counts[val]===2) r.pairs++
      }
    return r
    },{counts:{},pairs:0}) 


console.log( countPerfectPairs(ar1) )

Answer 6

or that

const 
  ar1 = [12, 5, 5, 2, 1, 1, 1, 1, 2]
, countPerfectPairs = arr => arr.reduce((r,val,i,{[i+1]:next})=>
    {
    if(!r.counts[val])
      { 
      r.counts[val] = arr.filter(x=>x===val).length
      if (r.counts[val]===2) r.pairs++
      }
    return  next ? r : r.pairs
    },{counts:{},pairs:0}) 


console.log( countPerfectPairs(ar1) )

If you prefer Details:

const 
  ar1 = [12, 5, 5, 2, 1, 1, 1, 1, 2]
, countPerfectPairs = arr => arr.reduce((r,val)=>
    {
    if(!r.counts[val])
      { 
      r.counts[val] = arr.filter(x=>x===val).length
      if (r.counts[val]===2) r.pairs++
      }
    return r
    },{counts:{},pairs:0}) 


console.log( countPerfectPairs(ar1) )

How can I determine the quantity of pairs in an array using JavaScript?

Answer №1

Answer №2

Answer №3

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