CODEWITHSUNDEEP
javascriptarraysgroup-bymean
Cameron Farquhar
Cameron Farquhar

Reputation: 35

How do I group by one element and find the average of another in Javascript?

I am working on a project comparing the age of wine to it's score and want to make a line chart with age on the X axis and average score for each age on the Y. I have an array of objects each containing two elements, age and score and want to group by each age and find the average score for that age.

I have a CSV file created by doing a request from Global Wine Score that has vintage, review date, score etc. In python, I created a column for age by subtracting the review year from the vintage. In Javascript I created a new array doing this:

    var newArr = countryFilter.map(function(elem) {
            return {
                age: elem.wine_age,
                score: elem.score
            };
        });

here is what my array looks like:

0: Object { age: "116", score: "98.51" }

​​​​
age: "116"

​​​​
score: "98.51"

​​​
​​​
1: Object { age: "113", score: "84.46" }

​​​​
age: "113"

​​​​
score: "84.46"

length: 23258

I tried this, but any object with more than 1 unique score yields NAN.

 const reduced = newArr.reduce(function(m, d){
        if(!m[d.age]){
          m[d.age] = {...d, count: 1};
          return m;
        }
        m[d.age].score += d.score;
        m[d.age].count += 1;
        return m;
     },{});

     const result = Object.keys(reduced).map(function(k){
         const item  = reduced[k];
         return {
             wine_age: item.age,
             score: item.score/item.count,
         }
     })

     console.log(result);

How would I go about finding the average wine score per wine age?

Upvotes: 1

Views: 126

Answers (2)

Kinglish
Kinglish

Reputation: 23654

Here's one way to get it. You'll end up with an object of age:average pairs:

let grps={} 
data.forEach(g => grps.hasOwnProperty(g.age)? // first we set up the age -> array of scores object
     grps[g.age].push(+g.score) : grps[g.age] = [+g.score]),
 avgs = Object.assign(...Object.entries(grps) // then we take that object and calculate the avg based on the total score tally / number of scores
     .map(g=>({[g[0]]:
     (g[1].reduce((b,a)=> b+a)/grps[g[0]].length).toFixed(2)}))) // reduce is a perfect vehicle for tallying up arrays of numbers.


let data = [
{ age: "113", score: "84.46" },
  { age: "113", score: "88.23" },
  { age: "112", score: "84.46" },
  { age: "113", score: "10.58" },
]
let grps={}, s = data.forEach(g=>grps.hasOwnProperty(g.age)?grps[g.age].push(+g.score) : grps[g.age] = [+g.score]), avgs = Object.assign(...Object.entries(grps).map(g=>({[g[0]]:(g[1].reduce((b,a)=> b+a)/grps[g[0]].length).toFixed(2)})))
console.log(avgs)

Upvotes: 0

Guerric P
Guerric P

Reputation: 31815

You could group the objects in sub-arrays by age, then reduce the sub-arrays into single objects with the average score:

const input = [
  { age: "113", score: "84.46" },
  { age: "113", score: "88.23" },
  { age: "112", score: "84.46" },
  { age: "113", score: "10.58" },
];

const result = input
  .sort((a, b) => a.age - b.age)
  .reduce(
    (acc, cur, i, { [i - 1]: last }) =>
      cur.age === last?.age
        ? Object.assign([...acc], {
            [acc.length - 1]: [...acc[acc.length - 1], cur],
          })
        : [...acc, [cur]],
    []
  )
  .map((x) => ({
    age: x[0].age,
    score: x.reduce((a, b) => a + Number(b.score), 0) / x.length,
  }));

console.log(result);

Upvotes: 1

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