How to get Sum of “last” items of N hot Observable instances?

Question

EDIT: On 09/15/2013 - I am describing my scenario further broken into steps to help everybody understand my situation better. Added the source for whole application for down

Answer 1

The Rxx library has an overload of CombineLatest() which takes an IObservable<IObservable<T>>. If you use this overload, then the solution is easy:

var runningSum = allCqis
    .Select(cqi => cqi.StartWith(0)) // start each inner sequence off with 0
    .CombineLatest() // produces an IObservable<IList<decimal>>
    .Select(cqis => cqis.Sum()); // LINQ operator Sum(IEnumerable<decimal>)

Looking at the source code for Rxx.CombineLatest might be useful to see how the problem is solved "under the hood"

Answer 2

Lots of questions! Maybe time to brush up on your Rx skillz? Pretty much all of your recent questions are covered in my web site IntroToRx.com. Having a deep understanding of Rx, will allow you to answer these fairly simple questions much more quickly than asking on a forum. You should be able to read the book in less than 3 days.

Anyway....

1 Do you want a running sum or just a single sum at the end?

2 Then, do you want a sum for all values for all streams, or the sums of each stream?

To get a single sum value for a sequence you use the .Sum() operator. http://introtorx.com/Content/v1.0.10621.0/07_Aggregation.html#MaxAndMin

To get a running total, use the Scan operator. http://introtorx.com/Content/v1.0.10621.0/07_Aggregation.html#Scan

So the answer is probably something like this (untested):

sources.Select(source=>source.Scan(0m, (acc, value)=>acc+=value)).Merge();