CODEWITHSUNDEEP
javascriptarraysalgorithmaverage
user2217057
user2217057

Reputation: 237

JavaScript arrays - efficiently calculate average in given intervals

I have two arrays, one of them represents data, and the other one - intervals. Both are sorted and their start and end values match. I go through nested for loops to calculate the average of data points in a given interval. As a result, I end up with one data value for each interval. For smaller size arrays, < 100-500 length, these linear loops do the work, however, this approach becomes an issue with several thousand data points. Any recommendation will be appreciated.

Please see a simplified code below with a link to JSfiddle at the end

var TimelineArray = [0, 10, 20, 30, 40, 40, 60, 70, 80, 90, 100],
	DataArray = [0, 2, 4, 5, 8, 11, 19, 22, 24, 25, 30, 31, 38, 39, 51, 56, 57, 58, 59, 64, 74, 76, 89, 91, 92, 94, 98, 100],
	DataArrayA = [];

for (i = 0; i < TimelineArray.length-1; i++) {
	var dataPointsInGivenTimeInterval = [];
	for (j = 0; j < DataArray.length; j++) {
		if (DataArray[j] > TimelineArray[i] && DataArray[j] <= TimelineArray[i+1]) {
			dataPointsInGivenTimeInterval.push(DataArray[j]);
		}
	};
	if (dataPointsInGivenTimeInterval.length == 0) {
		DataArrayA.push(null);
	}
	else {
		var sumOfdataPoints = null;
		for (k = 0; k < dataPointsInGivenTimeInterval.length; k++) {
			sumOfdataPoints += dataPointsInGivenTimeInterval[k];
		}
		var avg = sumOfdataPoints / dataPointsInGivenTimeInterval.length;
		DataArrayA.push(avg);
	}
} // end for

console.log(TimelineArray);
console.log(DataArrayA);
.as-console-wrapper {
  max-height: 100% !important;
  top: 0;
}

The console output is

[0, 10, 20, 30, 40, 40, 60, 70, 80, 90, 100]
[4.75, 15, 25.25, 36, null, 56.2, 64, 75, 89, 95]

Here is the code at JSfiddle - calculating average values for given intervals

Upvotes: 1

Views: 632

Answers (3)

John Boker
John Boker

Reputation: 83709

Not sure if it'll be any faster, but here's a crack at it in a different way:

var TimelineArray = [0, 10, 20, 30, 40, 40, 60, 70, 80, 90, 100],
DataArray = [0, 2, 4, 5, 8, 11, 19, 22, 24, 25, 30, 31, 38, 39, 51, 56, 57, 58, 59, 64, 74, 76, 89, 91, 92, 94, 98, 100],
DataArrayA = [];
  
function avg(arr){
  if(arr!= null && arr.length > 0)
    return arr.reduce(function(a, b){ return a+b;}, 0) / arr.length;
  return null;
}
for(var i = 0; i < TimelineArray.length-1; i++){
  var interval = [TimelineArray[i], TimelineArray[i+1]];
  var data = DataArray.filter(function(a){ return a > interval[0] && a <= interval[1]});
  DataArrayA.push(avg(data));
}

console.log(DataArrayA);

edit 1: removed a loop.

Upvotes: 0

Oriol
Oriol

Reputation: 288120

Since the arrays are sorted, you can do it linearly with respect to the size of the timeline and data:

var timeline = [0, 10, 20, 30, 40, 40, 60, 70, 80, 90, 100],
    data = [0, 2, 4, 5, 8, 11, 19, 22, 24, 25, 30, 31, 38, 39, 51, 56, 57, 58, 59, 64, 74, 76, 89, 91, 92, 94, 98, 100];
var averages = new Array(timeline.length - 1);
for (var i = 0, j = 0; i < timeline.length; i++) {
  var sum = 0,
      items = 0;
  for (; data[j] <= timeline[i]; j++) {
    sum += data[j];
    ++items;
  }
  if(i) averages[i-1] = sum / items;
}
console.log(averages);
.as-console-wrapper {
  max-height: 100% !important;
  top: 0;
}

Upvotes: 2

Arnauld
Arnauld

Reputation: 6110

You don't need to re-scan DataArray from the beginning on each iteration.

var TimelineArray = [0, 10, 20, 30, 40, 40, 60, 70, 80, 90, 100];
var DataArray = [0, 2, 4, 5, 8, 11, 19, 22, 24, 25, 30, 31, 38, 39, 51, 56, 57, 58, 59, 64, 74, 76, 89, 91, 92, 94, 98, 100];

var res = [], pos = 0;

TimelineArray.forEach(function(v, i) {
  for(var sum = 0, n = 0; DataArray[pos] <= v; n++) {
    sum += DataArray[pos++];
  }
  i && res.push(n ? sum / n : null);
});

console.log(res);

Upvotes: 1

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