CODEWITHSUNDEEP
javascript
Maik Lowrey
Maik Lowrey

Reputation: 17566

Grouping a javascript array over a loop

I have the following array:

[
    {'id': 1, 'rating': 5, 'rating_category': "a"},
    {'id': 1, 'rating': 1, 'rating_category': "a"},
    {'id': 1, 'rating': 4, 'rating_category': "b"},
    {'id': 1, 'rating': 5, 'rating_category': "b"},
]

And expect the following array:

[
   { category: "a", rating_average: 3, rating_count: 2},
   { category: "b", rating_average: 4.5, rating_count: 2},
]

My approach works only partially. I just don't get the array format I want. I get a [key : {}, key : {}] but I want to get [{},{}]. My problem is checking if the key already exists in the array.

const data = [
    {'id': 1, 'rating': 5, 'rating_category': "a"},
    {'id': 1, 'rating': 1, 'rating_category': "a"},
    {'id': 1, 'rating': 4, 'rating_category': "b"},
    {'id': 1, 'rating': 5, 'rating_category': "b"},
]

r = []
data.forEach(e => {
  if (! r[e.rating_category]) {
    r[e.rating_category] = {
      rating: e.rating, 
      count: 1      
    };
  } else {    
    r[e.rating_category]['rating'] += e.rating
    r[e.rating_category]['count']++
  }
  
})
console.log(r.a, r.b)

Upvotes: 0

Views: 57

Answers (3)

Nina Scholz
Nina Scholz

Reputation: 386604

You could take a function for grouping, like the (hopefully) upcoming new prototype Array#groupBy and take the average by using a sum function and return a new object for each group.

Array.prototype.groupBy ??= function (callbackfn, thisArg) {
    const O = Object(this);
    const len = O.length >>> 0;
    if (typeof callbackfn !== 'function') throw new TypeError(callbackfn + ' is not a function');

    let k = 0;
    const groups = {};

    while (k < len) {
        const Pk = Number(k).toString();
        const kValue = O[Pk];
        const propertyKey = callbackfn.call(thisArg, kValue, Number(k), O);
        (groups[propertyKey] ??= []).push(kValue);
        ++k;
    }
    return groups;
};

const
    sum = (array, key) => array.reduce((s, v) => s + (key ? v[key] : v), 0),
    data = [{ id: 1, rating: 5, rating_category: "a" }, { id: 1, rating: 1, rating_category: "a" }, { id: 1, rating: 4, rating_category: "b" }, { id: 1, rating: 5, rating_category: "b" }],
    result = Object
        .entries(data.groupBy(({ rating_category }) => rating_category))
        .map(([category, a]) => ({ category, rating_average: sum(a, 'rating') / a.length, rating_count: a.length }));

console.log(result);
.as-console-wrapper { max-height: 100% !important; top: 0; }

Upvotes: 0

mhodges
mhodges

Reputation: 11116

Here's a way you can do it fairly efficiently. Basically the idea is to group the ratings by category first, then calculate the length and average of the ratings for each category and then format your return value, like so:

let data = [
    {'id': 1, 'rating': 5, 'rating_category': "a"},
    {'id': 1, 'rating': 1, 'rating_category': "a"},
    {'id': 1, 'rating': 4, 'rating_category': "b"},
    {'id': 1, 'rating': 5, 'rating_category': "b"},
]
// group the ratings by category
let grouped = data.reduce((res, curr) => {
  if (!res[curr.rating_category]) {
    res[curr.rating_category] = [];
  }
  res[curr.rating_category].push(curr.rating);
  return res;
}, {});
// calculate length & average of each category and format return value
let formatted = Object.entries(grouped).map(([key, value]) => {
  let average = value.reduce((res, curr) => res + curr, 0) / value.length;
  return { category: key, rating_count: value.length, rating_average: average }
});

console.log(formatted)

Upvotes: 1

Barmar
Barmar

Reputation: 781038

Add the category as a property in the object. Then at the end you can use Object.values() to get all the properties as an array.

r should be an object, not an array.

const data = [
    {'id': 1, 'rating': 5, 'rating_category': "a"},
    {'id': 1, 'rating': 1, 'rating_category': "a"},
    {'id': 1, 'rating': 4, 'rating_category': "b"},
    {'id': 1, 'rating': 5, 'rating_category': "b"},
]

const r = {};
data.forEach(e => {
  if (! r[e.rating_category]) {
    r[e.rating_category] = {
      category: e.rating_category,
      rating: e.rating, 
      count: 1      
    };
  } else {    
    r[e.rating_category]['rating'] += e.rating
    r[e.rating_category]['count']++
  }
  
})
console.log(Object.values(r))

Upvotes: 1

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