Manipulate Array of objects

Question

I have an array of objects that appears as follows:

var dataArray = [{ "moment": "18 January 2019", "val": 53 },
                 { "moment": "18 January 2019", "val": 69 },
                 { "moment": "18 January 2019", "val": 52 },
                 {"moment": "21 January 2019", "val": 52 },
                 { "moment": "21 January 2019", "val": 52 },
                 { "moment": "21 January 2019", "val": 52 }]

What I would like to do is to put in a new array the value of the element moment and the average of val for that moment. eg:

result =[{moment: "18 January 2019", "val": 58},
{moment: "21 January 2019", "val": 52}].

For now I managed to put a single occurence of each date in the new array result but I am having hard time to put the average since I don't know how to make the difference between values for a date from values for another date. Any help would be appreciated.

Peace

break down the problem. #1 how to get the avg? Variable avg is gotten by for data in dataArray divided by dataArray.length. #2 do your other iteration(s) — Ronnie Smith
– Ronnie Smith, Commented Jan 22, 2019 at 4:54

Code Maniac · Accepted Answer · 2019-01-22 04:56:17Z

You can do it using reduce.

Here first we aggregate the values by moment and also count no. of similar moments. and than map through it and find the average using the aggregated value and count.

var dataArray = [{ "moment": "18 January 2019","val": 53 },{ "moment": "18 January 2019", "val": 69 },{ "moment": "18 January 2019", "val": 52 },{"moment": "21 January 2019", "val": 52 },                { "moment": "21 January 2019", "val": 52 },                { "moment": "21 January 2019", "val": 52 }]
                 
let output = dataArray.reduce((op,cur)=>{
  if( op[cur.moment] ){
    op[cur.moment].val += cur.val;
    op[cur.moment].count++;
  } else {
    op[cur.moment] = cur
    op[cur.moment].count = 1;
  }
  return op;
},{})

let final = Object.values(output).map(e => {
   return {
    moment: e.moment,
    val: e.val/e.count
   } 
})

console.log(final)

Our_Benefactors · Accepted Answer · 2019-01-22 04:54:57Z

0

var totalCount = 0;
var divBy = 0;
var momentToCheck = "21 January 2019"
for(int i= 0; i<dataArray.length; i++) {
    if (dataArray[i].moment === momentToCheck) {
        totalCount+= dataArray[i].val;
        divBy++;
    }
}
var newAvg = totalCount/divBy;
dataArray.push({moment:momentToCheck, val:newAvg});

edited Jan 22, 2019 at 4:54

answered Jan 22, 2019 at 4:52

Our_Benefactors

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6 Comments

Prashant Zombade Over a year ago

I think he wants averaged values for each date.

Prashant Zombade Over a year ago

How about "18 January 2019" and other dates that may come later?

Our_Benefactors Over a year ago

@PrashantZombade make a function that takes a moment as an argument, call that function in a loop.

Prashant Zombade Over a year ago

That will make unnecessary call to the functions to and fro. It might be good idea in this case to process the input in one go.

Our_Benefactors Over a year ago

@PrashantZombade as other posters have suggested, use reduce()

|

Matt Way · Accepted Answer · 2019-01-22 05:03:21Z

It might make sense to first perform a grouping, and then to perform the average of each group. For example:

const dataArray = [{ "moment": "18 January 2019", "val": 53 },
                   { "moment": "18 January 2019", "val": 69 },
                   { "moment": "18 January 2019", "val": 52 },
                   { "moment": "21 January 2019", "val": 52 },
                   { "moment": "21 January 2019", "val": 52 },
                   { "moment": "21 January 2019", "val": 52 }]
                   
const groups = dataArray.reduce((r, { moment, val }) => {
  (r[moment] = r[moment] || []).push(val)  
  return r
}, {})

const avg = arr => arr.reduce((r, x) => r + x, 0) / arr.length

const result = Object.entries(groups).map(([moment, arr]) => ({
  moment,
  avg: avg(arr)
}))

console.log(result)

Mario · Accepted Answer · 2019-01-22 05:03:59Z

You can achieve the expected result in the following way using reduce

var dataArray = [{
        "moment": "18 January 2019",
        "val": 53
    },
    {
        "moment": "18 January 2019",
        "val": 69
    },
    {
        "moment": "18 January 2019",
        "val": 52
    },
    {
        "moment": "21 January 2019",
        "val": 52
    },
    {
        "moment": "21 January 2019",
        "val": 52
    },
    {
        "moment": "21 January 2019",
        "val": 52
    }
];

const result = dataArray.reduce((accumulator, currentValue, index, array) => {
    if (!accumulator[currentValue.moment]) {
        accumulator[currentValue.moment] = array
            .filter(arr => arr.moment === currentValue.moment)
            .reduce((acc, val, idx, coll) => {
                acc += val.val;

                if (idx === coll.length - 1) {
                    return acc/coll.length;
                } else {
                    return acc;
                }
        }, 0);
    }

    return accumulator;
}, []);

console.log(result); // [ '18 January 2019': 58, '21 January 2019': 52 ]

brk · Accepted Answer · 2019-01-22 05:05:25Z

Use array reduce and inside the call back check if the arument array have moment with same value

var dataArray = [{
    "moment": "18 January 2019",
    "val": 53
  },
  {
    "moment": "18 January 2019",
    "val": 69
  },
  {
    "moment": "18 January 2019",
    "val": 52
  },
  {
    "moment": "21 January 2019",
    "val": 52
  },
  {
    "moment": "21 January 2019",
    "val": 52
  },
  {
    "moment": "21 January 2019",
    "val": 52
  }
]

let avgVal = dataArray.reduce(function(acc, curr) {
  let findIfHasDate = acc.findIndex((item) => {
    return item.moment === curr.moment

  });

  if (findIfHasDate === -1) {
    acc.push({
      moment: curr.moment,
      totalVal: curr.val,
      totalcount: 1

    })
  } else {
    acc[findIfHasDate].totalVal = (acc[findIfHasDate].totalVal + curr.val);
    acc[findIfHasDate].totalcount += 1;
  }

  return acc;
}, []).map((item) => {
  return {
    moment: item.moment,
    avg: (item.totalVal) / item.totalcount
  }
})

console.log(avgVal)

kornieff · Accepted Answer · 2019-01-23 01:51:48Z

0

const sumRow = (sum, count) => ({ sum, count })

const summarize = data => data.reduce((a, c) => (prev => (
  { ...a, [c.moment]: sumRow(prev.sum + c.val, prev.count + 1) }
))(a[c.moment] || sumRow(0, 0)), {})

const average = summary => Object.keys(summary).map(
  k => ({ moment: k, val: (summary[k].sum / summary[k].count) })
)
 
console.log(average(summarize([
  { "moment": "18 January 2019", "val": 53 },
  { "moment": "18 January 2019", "val": 69 },
  { "moment": "18 January 2019", "val": 52 },
  { "moment": "21 January 2019", "val": 52 },
  { "moment": "21 January 2019", "val": 52 },
  { "moment": "21 January 2019", "val": 52 }
])))

edited Jan 23, 2019 at 1:51

answered Jan 22, 2019 at 7:30

kornieff

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