Combine array of values with array of dates while averaging values for duplicate dates

Question

I can't work out the logic on this one, most of the other questions I've found have different data structures such as arrays of objects with duplicate keys.

I have two arrays of equal length - one is a list of values, the other is a corresponding date. For example:

values = [100,100,95,80,100,100,87,88,90,40];
dates = ['03 Dec', '03 Dec', '04 Dec', '04 Dec', '04 Dec', '04 Dec', '05 Dec', '05 Dec', '06 Dec', '06 Dec'];

The real arrays are much longer, circa 1,000 values with corresponding dates spanning 10 days. I am trying to get the average value for each date to make it much easier to display using apexcharts.

What I am trying to get to is something like this:

values = [100,93.75,87.5,65];
dates = ['03 Dec', '04 Dec', '05 Dec', '06 Dec'];

or indeed:

{03 Dec: '100', 04 Dec: '93.75', 05 Dec: '87.5', 06 Dec: '65'}

I've tried to combine with dates.reduce but I'm stuck with how to average the values for each day.

Any pointers would be gratefully received. Thank you.

Nikkorian · Accepted Answer · 2022-12-14 01:08:04Z

1

Try the following code. It results in an object with the dates as properties and the averages as corresponding values:

var values = [100,100,95,80,100,100,87,88,90,40];
var dates = ['03 Dec', '03 Dec', '04 Dec', '04 Dec', '04 Dec', '04 Dec', '05 Dec', '05 Dec', '06 Dec', '06 Dec'];
var averages={};
var lastdate="",total,count;
dates.forEach( (currentValue, index, )=>{
    if (currentValue !== lastdate) {
        if (lastdate !== "") {
            averages[lastdate] = total/count;
        }
        lastdate=currentValue;
        total=0;
        count=0;
    }
    total += values[index];
    count++;
})
averages[lastdate] = total/count;
console.log(averages);

answered Dec 14, 2022 at 1:08

Nikkorian

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4 Comments

Jonathan Nathanson Over a year ago

Thank you, this code achieves exactly what I needed. Can you explain the format for lastdate - I've not seen a variable declaration with comma seperated list of values before? Thank you again.

Nikkorian Over a year ago

Lastdate is simply a local variable used to store each of the values in the 'dates' variable (lastdate=currentValue;) as it's processed so we can tell if the next date processed is the same (so add to count and total) or a new date (so calculate the previous average then clear the variable count and total). Lastdate will be a string, same as the values in 'dates'.

Nikkorian Over a year ago

@Jonathan Nathanson, If my answer worked for you, please accept it.

Nikkorian Over a year ago

I just re-read your question. var lastdate="",total,count; is a short way of declaring three vars (lastdate, total, and count) in one line. I also gave lastdate an initial value of "".

Cooleronie · Accepted Answer · 2022-12-14 01:25:15Z

1

This should work. See comments in code for explanation, ended up making more sense that way.

const values = [100,100,95,80,100,100,87,88,90,40];
const dates = ['03 Dec', '03 Dec', '04 Dec', '04 Dec', '04 Dec', '04 Dec', '05 Dec', '05 Dec', '06 Dec', '06 Dec'];

const result = {};

for (let i = 0; i < dates.length; i++) {
  const date = dates[i];

  if (!result[date]) {
    // create a property in the object with the current date and set it to the sum of the values that match the current date
    result[date] = values.filter((value, index) => dates[index] === date).reduce((total, value) => total + value, 0);
  }
}

for (const date in result) {
  // Divide the sum of values for the current date by the number of values that match the current date
  result[date] /= values.filter((value, index) => dates[index] === date).length;
}

console.log(result);

answered Dec 14, 2022 at 1:25

Cooleronie

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1 Comment

Jonathan Nathanson Over a year ago

Thank you this works exactly as I need. I think this is potentially an easier to read/understand method than the other one provided. Much appreciated.

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