Question

Let say we have two arrays:

DateTime[] wDates = new DateTime[20000];
double[] wValues = new double[20000];

Those two arrays are both sequentially ordered, that is given an int i, wValues[i] is for the date wDates[i].

Let us say we need to get the average value of wValues where the month of the date is January

using a standard loop this would be:

double wAvg = 0.0;
int wDataCount = 0;
for (int i=0; i < 20000; i++)
  if (wDates[i].Month == 1)
  {
    wAvg += wValues[i];
    wDataCount++;
  }

if (wDataCount > 0)
  wAvg /= wDataCount;

I am wondering how to do this in LINQ?
I could create a struct/class DateDouble that contains both values and then do something like:

List<DateDouble> wListData = new List<DateDouble>();
Add the items...
double wAvg = (from d in wListData
               where d.Date.Month == 1
               select d.Value).Average();

but creating thousands of DateDouble object would be a big memory overhead when done dozens of millions time a day. The same would happen with temporary objects and trying to use ‘index’ and joining the index on the arrays would create awful performances.

Is there a better way to achieve this in LINQ?

Thanks,
MM

Answer 1

There is an overloaded version of IEnumerable.Where() extension method which also considers the index in the predicate.

double average = wValues.Where((d, i) => wDates[i].Month == 1).Average();