Calculate average of duplicates in a javascript array of objects

Question

I have an array of objects:

[
{"market": "Qacha's nek","commodity": 55,"price": "90","month": "04","year": "2017"}, 
{"market": "Mohales Hoek","commodity": 55,"price": "75","month": "04","year": "2017"}, 
{"market": "Mafeteng","commodity": 55,"price": "75","month": "04","year": "2017"}, 
{"market": "Maseru","commodity": 55,"price": "69","month": "04","year": "2017"}, 
{"market": "Butha-Buthe","commodity": 55,"price": "66","month": "04","year": "2017"}, 
{"market": "Leribe","commodity": 55,"price": "64","month": "04","year": "2017"}, 
{"market": "Butha-Buthe","commodity": 55,"price": "65","month": "04","year": "2017"}, 
{"market": "Thaba-Tseka","commodity": 55,"price": "82","month": "04","year": "2017"},
{"market": "Thaba-Tseka","commodity": 55,"price": "81","month": "04","year": "2017"},
{"market": "Maseru",    "commodity": 55,"price": "74,99","month": "04","year": "2017"}
]

I'm trying to aggregate duplicates by price average.

So the keys to identifying duplicated rows are all the properties except price, that must be aggregated by average.

In the data above, for example, line 5 and 7:

5) "market": "Butha-Buthe","commodity": 55,"price": "66","month": "04","year": "2017"
7) "market": "Butha-Buthe","commodity": 55,"price": "65","month": "04","year": "2017"

are duplicates and I want to merge them and make the average of their price value.

I was trying to use the reduce function, but I can't figure out how to identify duplicated values, especially if they are not sorted.

I post the code, but it's useless as I can't understand how to identify duplicates with reduce:

var data = [
{"market": "Qacha's nek","commodity": 55,"price": "90","month": "04","year": "2017"}, 
{"market": "Mohales Hoek","commodity": 55,"price": "75","month": "04","year": "2017"}, 
{"market": "Mafeteng","commodity": 55,"price": "75","month": "04","year": "2017"}, 
{"market": "Maseru","commodity": 55,"price": "69","month": "04","year": "2017"}, 
{"market": "Butha-Buthe","commodity": 55,"price": "66","month": "04","year": "2017"}, 
{"market": "Leribe","commodity": 55,"price": "64","month": "04","year": "2017"}, 
{"market": "Butha-Buthe","commodity": 55,"price": "65","month": "04","year": "2017"}, 
{"market": "Thaba-Tseka","commodity": 55,"price": "82","month": "04","year": "2017"},
{"market": "Thaba-Tseka","commodity": 55,"price": "81","month": "04","year": "2017"},
{"market": "Maseru","commodity": 55,"price": "74,99","month": "04","year": "2017"}
];

var avg = data.reduce(function(result, current) {
			console.log(result,current);
      if(!result){
      	result=current;
      }
      else {
      	if(result.market==current.market){
        	console.log(current.market);
        }
      }
});

Here a jsfiddle where I was trying to understand how the reduce function works: https://jsfiddle.net/brainsengineering/7tmdx0kg/7/

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– T.J. Crowder, Commented Mar 6, 2019 at 19:01
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– T.J. Crowder, Commented Mar 6, 2019 at 19:01
@JonasWilms unfortunately, the backend service returned this way... I know it shouldn't — Giox
– Giox, Commented Mar 6, 2019 at 19:09

Nina Scholz · Accepted Answer · 2019-03-06 19:20:43Z

2

You could take a combined key for the wanted properties and replace the price format to a numerical parsable format.

var data = [{ market: "Qacha's nek", commodity: 55, price: "90", month: "04", year: "2017" }, { market: "Mohales Hoek", commodity: 55, price: "75", month: "04", year: "2017" }, { market: "Mafeteng", commodity: 55, price: "75", month: "04", year: "2017" }, { market: "Maseru", commodity: 55, price: "69", month: "04", year: "2017" }, { market: "Butha-Buthe", commodity: 55, price: "66", month: "04", year: "2017" }, { market: "Leribe", commodity: 55, price: "64", month: "04", year: "2017" }, { market: "Butha-Buthe", commodity: 55, price: "65", month: "04", year: "2017" }, { market: "Thaba-Tseka", commodity: 55, price: "82", month: "04", year: "2017" }, { market: "Thaba-Tseka", commodity: 55, price: "81", month: "04", year: "2017" }, { market: "Maseru", commodity: 55, price: "74,99", month: "04", year: "2017" }],
    keys = ['market', 'commodity', 'month', 'year'],
    count = {},
    result = data.reduce(function (r, o) {
        var key = keys.map(function (k) { return o[k]; }).join('|');
        if (!count[key]) {
            count[key] = { sum: +o.price.replace(',', '.'), data: JSON.parse(JSON.stringify(o)) };
            count[key].data.count = 1;
            r.push(count[key].data);
        } else {
            count[key].sum += +o.price.replace(',', '.');
            count[key].data.price = (count[key].sum / ++count[key].data.count).toString();
        }
        return r;
    }, []);

console.log(result);

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

edited Mar 6, 2019 at 19:20

answered Mar 6, 2019 at 19:13

Nina Scholz

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7 Comments

Jonas Wilms Over a year ago

Thought of using a hashtable for counts, but I couldn't find a way without transforming the object at least two times, this combined object/array is just very elegant :)

Giox Over a year ago

@nina always the best on map and reduce! This solution is really the best as I can define also the keys. And would it possible to add a new property to the object for keeping track of the count (how many elements have been aggregated)?

Nina Scholz Over a year ago

@Giox, now with count in the result set.

mkhanoyan Over a year ago

This code certainly does the job but I wouldn't want to maintain it for sure. Not sure OP will even understand how it actually works...

mkhanoyan Over a year ago

There is a trick that @NinaScholz is using there. After pushing the data to the result array (in the r.push(count... line), she relies on the fact that javascript is a "pass by reference" language. You need to understand that when she updates the price in count object (last line of the else block) she is also updating the result array. I think it would benefit you to start from scratch and try to solve the problem without using reduce first.

|

p.s.w.g · Accepted Answer · 2019-03-06 19:26:58Z

Group prices together for each item by adding them to an array in your reduce call. You can keep track of which items are duplicated in the same function. Then use loop over the duplicate items to compute the averages.

Note I had to change your price 74,99 to 74.99 to parse more easily. You'll probably want some sort of localization/globalization library if this is critical in your use case.

var data = [
{"market": "Qacha's nek","commodity": 55,"price": "90","month": "04","year": "2017"}, 
{"market": "Mohales Hoek","commodity": 55,"price": "75","month": "04","year": "2017"}, 
{"market": "Mafeteng","commodity": 55,"price": "75","month": "04","year": "2017"}, 
{"market": "Maseru","commodity": 55,"price": "69","month": "04","year": "2017"}, 
{"market": "Butha-Buthe","commodity": 55,"price": "66","month": "04","year": "2017"}, 
{"market": "Leribe","commodity": 55,"price": "64","month": "04","year": "2017"}, 
{"market": "Butha-Buthe","commodity": 55,"price": "65","month": "04","year": "2017"}, 
{"market": "Thaba-Tseka","commodity": 55,"price": "82","month": "04","year": "2017"},
{"market": "Thaba-Tseka","commodity": 55,"price": "81","month": "04","year": "2017"},
{"market": "Maseru","commodity": 55,"price": "74.99","month": "04","year": "2017"}
];

function parsePrice(str) {
  // TODO: localization
  return +str;
}

function formatPrice(num) {
  return num.toFixed(2);
}

function getHashKey(item) {
  return JSON.stringify([item.market, item.commodity, item.month, item.year]);
}

var duplicatedItems = {};
var prices = data.reduce(function(result, current) {
  var key = getHashKey(current);
  if (key in result) {
    result[key].push(parsePrice(current.price));
    duplicatedItems[key] = current;
  } else {
    result[key] = [parsePrice(current.price)];
  }
  return result;
}, {});
var avg = Object.keys(duplicatedItems).map(function(key) {
  var item = duplicatedItems[key];
  var avgPrice = prices[key].reduce(function(acc, price) { return acc + price; }, 0) / prices[key].length;
  return {
    market: item.market,
    commodity: item.commodity,
    price: formatPrice(avgPrice),
    month: item.month,
    year: item.year
  };
});

console.log(avg);

Jonas Wilms · Accepted Answer · 2019-03-06 19:05:57Z

0

You could insert the values into a new array and merge if it already exists:

 const result = [];

 outer: for(const {  market, commodity,  price, month, year } of input) {
    for(const other of result) {
       if(market === other.market && commodity === other.commodity && month === other.month && year === other.year) {
         other.prices.push(+price);

         continue outer;
       }
    }
    result.push({ market, commodity, prices: [+price], month, year });
 }

 for(const group of result)
   group.price = group.prices.reduce((a, b) => a + b, 0) / group.prices.length;

answered Mar 6, 2019 at 19:05

Jonas Wilms

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2 Comments

Giox Over a year ago

is it compatible with IE11?

Jonas Wilms Over a year ago

@giox no, it isn't. Transpile it if needed.

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