How to group array of dates by year and subtract last value - first value?

Question

I have the array of dates and I want to group dates by year and subtract the last value - the first value. for example, I have the following array:

const data = [
  { x: "2021-10-17T14:38:45.540Z", y: 2 },
  { x: "2021-10-17T13:38:45.540Z", y: 2 },
  { x: "2021-09-16T14:36:46.540Z", y: 1 },
  { x: "2021-01-04T14:35:46.540Z", y: 2 },
  { x: "2021-01-01T14:30:46.540Z", y: 1 },
  { x: "2020-02-01T06:28:47.520Z", y: 12 },
  { x: "2020-02-01T07:28:47.520Z", y: 12 },
  { x: "2019-04-13T10:19:20.034Z", y: 20 },
  { x: "2018-01-01T09:09:19.134Z", y: 4 },
  { x: "2017-01-01T12:09:19.034Z", y: 11 },
  { x: "2016-01-02T12:10:20.034Z", y: 24 },
  { x: "2016-01-02T11:10:20.034Z", y: 14 }
];

This is what I tried Group array of object by year and subtract the last value - the first value

I got this result which sum the values :

 [
  {
    "value": 8,
    "label": "2021"
  },
  {
    "value": 24,
    "label": "2020"
  },
  {
    "value": 20,
    "label": "2019"
  },
  {
    "value": 4,
    "label": "2018"
  },
  {
    "value": 11,
    "label": "2017"
  },
  {
    "value": 38,
    "label": "2016"
  }
]

Expected output for year (value = last value - first value):

 [
  {
    "value": 1,
    "label": "2021"
  },
  {
    "value": 0,
    "label": "2020"
  },
  {
    "value": 20,
    "label": "2019"
  },
  {
    "value": 4,
    "label": "2018"
  },
  {
    "value": 11,
    "label": "2017"
  },
  {
    "value": 10,
    "label": "2016"
  }
]

This is the group by year function :

function group_by_year(arr) {
  return Object.values(
    arr.reduce((a, { x: date_string, y: value }) => {
      const { year } = get_date_parts(date_string);
      if (a[year] === undefined) {
        a[year] = { value: 0, label: year };
      }
      a[year].value += value; // the problem (lastValue - firstValue)

      return a;
    }, {})
  );
}

The problem is how to change this line a[year].value += value; to a[year].value = lastValue - firstValue;

score 1 · Accepted Answer · answered Oct 26 '21 at 12:07

Inside the accumulator, you are just adding the values from array. This will generate total value against an year.

I have used the below logic.

Kept the firstValue, lastValue, label and occurance against each year.
firstValue holds the first occurance value of that year.
lastValue holds the last occurance value of that year.
label year value
occurance count of each year in the array.

From the Accumulated result obtained from Array.reduce, run Array.map to generate the output of your desire. It will be lastValue - firstValue. Use Math.abs if you are interested in positive difference only without sign.

Working Fiddle

const data = [{ x: "2021-10-17T14:38:45.540Z", y: 2 },{ x: "2021-10-17T13:38:45.540Z", y: 2 },{ x: "2021-09-16T14:36:46.540Z", y: 1 },{ x: "2021-01-04T14:35:46.540Z", y: 2 },{ x: "2021-01-01T14:30:46.540Z", y: 1 },{ x: "2020-02-01T06:28:47.520Z", y: 12 },{ x: "2020-02-01T07:28:47.520Z", y: 12 },{ x: "2019-04-13T10:19:20.034Z", y: 20 },{ x: "2018-01-01T09:09:19.134Z", y: 4 },{ x: "2017-01-01T12:09:19.034Z", y: 11 },{ x: "2016-01-02T12:10:20.034Z", y: 24 },{ x: "2016-01-02T11:10:20.034Z", y: 14 }];

function get_date_parts(iso_string) {
  const [year, month, day, hr, min, sec] = iso_string.split(/\D/g);
  return { year, month, day, hr, min, sec };
}

function group_by_year(arr) {
  const grouppedYear = Object.values(
    arr.reduce((a, { x: date_string, y: value }) => {
      const { year } = get_date_parts(date_string);
      if (a[year] === undefined) {
        a[year] = {
          firstValue: value,
          lastValue: value,
          label: year,
          occurance: 1,
        };
      } else {
        a[year].lastValue = value;
        ++a[year].occurance;
      }
      return a;
    }, {})
  );
  const returnValue = grouppedYear.map(({firstValue, lastValue, label, occurance}) => ({value: occurance > 1 ? lastValue - firstValue : firstValue, label}))
  return returnValue;
}
const output = group_by_year(data);
console.log(output)

how to add the [{x,y}] data list of the groupedYear to the returnValue? I tried `a[year].data = !a[year].data ? [].concat([{ x: date_string, y: value }]) : a[year].data.concat([{ x: date_string, y: value }]);` but it did not add the first value. always missing a value. — Dev M, Oct 27 '21 at 11:26
mb, I was missing this line. `if (a[year] === undefined) { a[year] = { data: [{ x: date_string, y: value }] //...}}` — Dev M, Oct 27 '21 at 11:50

score 0 · Answer 2 · answered Oct 26 '21 at 11:40

0

using moment library you can extract date string into year moment("2021-10-17T14:38:45.540Z").format('YYYY') after preparing object of array you can simply sort the array and find the answer

here you have to make separate group of array of same year data and then first and last index can simply find in for loop

answered Oct 26 '21 at 11:40

Harsh Mendapara

33
1
11

Considering the moment [project's status](https://momentjs.com/docs/#/-project-status/) you may want to reconsider suggesting its use. – Heretic Monkey Oct 26 '21 at 11:50
@Harsh Mendapara I tried that with momentjs the problem is some years is not displayed. https://stackoverflow.com/questions/69615993/how-to-group-array-of-dates-and-sum-value-by-day-1hour-6hours-8hours-week-m/69616505?noredirect=1#comment123212543_69616505 can I use array.reduce to get the first value and the last value? I alrady grouped the dates in the group_by_year method. the problem is how to change this line `a[year].value += value;` to `a[year].value = lastValue - firstValue;` – Dev M Oct 26 '21 at 11:51

How to group array of dates by year and subtract last value - first value?

2 Answers2