Calculate the weighted average with JavaScript from a 2d array

Question

I am trying to calculate the weighted average from a 2d javascript array. This is the array:

 var timestamp = [
  [1524751200, 6000],
  [1556287200, 6000],
  [1587909600, 6000],
  [1619445600, 6000],
  [1650981600, 3000],
  [1682517600, 3000],
  [1714140000, 3000],
  [1745676000, 3000],
  [1777212000, 1500],
  [1808748000, 1500],
  [1840370400, 1500],
  [1871906400, 1500],
  [1903442400, 750],
  [1934978400, 750],
  [1966600800, 750],
  [1998136800, 750],
  [2029672800, 375],
  [2061208800, 375],
  [2092831200, 375],
  [2124367200, 375],
  [2155903200, 187.5],
  [2187439200, 187.5],
  [2219061600, 187.5],
  [2250597600, 187.5],
  [2282133600, 93.75],
  [2313669600, 93.75],
  [2345292000, 93.75],
  [2376828000, 93.75],
  [2408364000, 46.875],
  [2439900000, 46.875],
  [2471522400, 46.875],
  [2503058400, 46.875],
  [2534594400, 23.4375],
  [2566130400, 23.4375],
  [2597752800, 23.4375],
  [2629288800, 23.4375],
  [2660824800, 11.71875],
  [2692360800, 11.71875],
  [2723983200, 11.71875],
  [2755519200, 11.71875],
  [2787055200, 5.859375],
  [2818591200, 5.859375],
  [2850213600, 5.859375],
  [2881749600, 5.859375],
  [2913285600, 2.9296875],
  [2944821600, 2.9296875],
  [2976444000, 2.9296875],
  [3007980000, 2.9296875],
  [3039516000, 1.46484375],
  [3071052000, 1.46484375],
  [3102674400, 1.46484375],
  [3134210400, 1.46484375],
  [3165746400, 0.732421875],
  [3197282400, 0.732421875],
  [3228904800, 0.732421875],
  [3260440800, 0.732421875],
  [3291976800, 0.3662109375],
  [3323512800, 0.3662109375],
  [3355135200, 0.3662109375],
  [3386671200, 0.3662109375],
  [3418207200, 0.18310546875],
  [3449743200, 0.18310546875],
  [3481365600, 0.18310546875],
  [3512901600, 0.18310546875],
  [3544437600, 0.091552734375],
  [3575973600, 0.091552734375],
  [3607596000, 0.091552734375],
  [3639132000, 0.091552734375],
  [3670668000, 0.0457763671875],
  [3702204000, 0.0457763671875],
  [3733826400, 0.0457763671875],
  [3765362400, 0.0457763671875],
  [3796898400, 0.02288818359375],
  [3828434400, 0.02288818359375],
  [3860056800, 0.02288818359375],
  [3891592800, 0.02288818359375],
  [3923128800, 0.011444091796875],
  [3954664800, 0.011444091796875],
  [3986287200, 0.011444091796875],
  [4017823200, 0.011444091796875],
  [4049359200, 0.0057220458984375],
  [4080895200, 0.0057220458984375],
  [4112431200, 0.0057220458984375],
  [4143967200, 0.0057220458984375],
  [4175503200, 0.0028610229492188],
  [4207039200, 0.0028610229492188],
  [4238661600, 0.0028610229492188],
  [4270197600, 0.0028610229492188],
  [4301733600, 0.0014305114746094],
  [4333269600, 0.0014305114746094],
  [4364892000, 0.0014305114746094],
  [4396428000, 0.0014305114746094],
  [4427964000, 0.0007152557373047],
  [4459500000, 0.0007152557373047],
  [4491122400, 0.0007152557373047],
  [4522658400, 0.0007152557373047],
  [4554194400, 0.00035762786865235],
  [4585730400, 0.00035762786865235],
  [4617352800, 0.00035762786865235],
  [4648888800, 0.00035762786865235]
];

Column 1 represents the timestamp, each one is the date 26.04 of each year for the next 100 years, and column 2 is a value that basically halves every 4 years. The weighted average must be calculated between all the values from column 2, and the number of times each value is taken into the calculation is the weight of that value. The condition that I must apply is a date that I take from the user input, and I only have to calculate the values that correspond to all the timestamps that are smaller than the date given by the user input. I have no idea where to start.

What should the output be, and what would be the reasoning behind it? — Jack Bashford, Feb 24 '19 at 10:43
Welcome to SO. Please read [What topics can I ask about](http://stackoverflow.com/help/on-topic) and [How to ask a good question](http://stackoverflow.com/help/how-to-ask) and [The perfect question](http://codeblog.jonskeet.uk/2010/08/29/writing-the-perfect-question/). SO is **not the place to ask for a tutorial, nor the place to find free programmers**. Your question should _show_ that you've put some efford into solving your own problem and should be answerable without writing half a book. — icecub, Feb 24 '19 at 10:45
The output must be just an integer that represents the weighted average. I'm trying to calculate the compound interest of an investment with interest rate that changes every 4 years. The interest rate is proportional to the value from column 2 that halves every 4 years. — GeorgeS, Feb 24 '19 at 10:53
The output will have to be the weighted average as I explained in the question. I apologize, not really understanding what you're asking, I'm very new to this. — GeorgeS, Feb 24 '19 at 11:11

adiga · Accepted Answer · 2019-02-24T11:04:49.230

You could filter the dates which are smaller than the input date and then calculate the average of the values at index = 1 using reduce

var timestamp=[[1524751200,6000],[1556287200,6000],[1587909600,6000],[1619445600,6000],[1650981600,3000],[1682517600,3000],[1714140000,3000],[1745676000,3000],[1777212000,1500],[1808748000,1500],[1840370400,1500],[1871906400,1500],[1903442400,750],[1934978400,750],[1966600800,750],[1998136800,750],[2029672800,375],[2061208800,375],[2092831200,375],[2124367200,375],[2155903200,187.5],[2187439200,187.5],[2219061600,187.5],[2250597600,187.5],[2282133600,93.75],[2313669600,93.75],[2345292000,93.75],[2376828000,93.75],[2408364000,46.875],[2439900000,46.875],[2471522400,46.875],[2503058400,46.875],[2534594400,23.4375],[2566130400,23.4375],[2597752800,23.4375],[2629288800,23.4375],[2660824800,11.71875],[2692360800,11.71875],[2723983200,11.71875],[2755519200,11.71875],[2787055200,5.859375],[2818591200,5.859375],[2850213600,5.859375],[2881749600,5.859375],[2913285600,2.9296875],[2944821600,2.9296875],[2976444000,2.9296875],[3007980000,2.9296875],[3039516000,1.46484375],[3071052000,1.46484375],[3102674400,1.46484375],[3134210400,1.46484375],[3165746400,0.732421875],[3197282400,0.732421875],[3228904800,0.732421875],[3260440800,0.732421875],[3291976800,0.3662109375],[3323512800,0.3662109375],[3355135200,0.3662109375],[3386671200,0.3662109375],[3418207200,0.18310546875],[3449743200,0.18310546875],[3481365600,0.18310546875],[3512901600,0.18310546875],[3544437600,0.091552734375],[3575973600,0.091552734375],[3607596000,0.091552734375],[3639132000,0.091552734375],[3670668000,0.0457763671875],[3702204000,0.0457763671875],[3733826400,0.0457763671875],[3765362400,0.0457763671875],[3796898400,0.02288818359375],[3828434400,0.02288818359375],[3860056800,0.02288818359375],[3891592800,0.02288818359375],[3923128800,0.011444091796875],[3954664800,0.011444091796875],[3986287200,0.011444091796875],[4017823200,0.011444091796875],[4049359200,0.0057220458984375],[4080895200,0.0057220458984375],[4112431200,0.0057220458984375],[4143967200,0.0057220458984375],[4175503200,0.0028610229492188],[4207039200,0.0028610229492188],[4238661600,0.0028610229492188],[4270197600,0.0028610229492188],[4301733600,0.0014305114746094],[4333269600,0.0014305114746094],[4364892000,0.0014305114746094],[4396428000,0.0014305114746094],[4427964000,0.0007152557373047],[4459500000,0.0007152557373047],[4491122400,0.0007152557373047],[4522658400,0.0007152557373047],[4554194400,0.00035762786865235],[4585730400,0.00035762786865235],[4617352800,0.00035762786865235],[4648888800,0.00035762786865235]];

const calculateAverage = (date) => {
  const filter = timestamp.filter(a => a[0] * 1000 < new Date(date));
  const average = filter.reduce((r,a) => r + a[1], 0) / filter.length;
  return average;
}

console.log(calculateAverage("10/10/2023"))

There is a multiplication by 1000 in filter because you mentioned these were future dates. So, I'm assuming the first column is in seconds and not milliseconds

Yes, the timstamp is in seconds. This is a good start! I just need to calculate the weighted average, from what I see you are calculating the mean between all values. Many thanks!!! — GeorgeS, Feb 24 '19 at 11:07
@GeorgeS for the date `10/10/2023`, what will be output? Is it not `5000`? `(6000 + 6000 + 6000 + 6000 + 3000 + 3000) / 6` is same as `(4* 6000 + 2* 3000)/ ( 4 + 2)` — adiga, Feb 24 '19 at 11:08
Yes, I misinterpreted your code. Yes, this is correct :) Thak you! — GeorgeS, Feb 24 '19 at 11:14

Calculate the weighted average with JavaScript from a 2d array

1 Answers1