c# How to get count of value in a List based on value in another List using LINQ

Question

I understand how you can get the count of a List where each value might have some number. For example if I wanted the number of times a value was equal to 1 in "ListA" I would do the following:

int countOf1=ListA.Count(x => x ==1);

If I have several Lists, and I want to get the count of ListB where ListA is equal to some value, how can I modify the above command to do so?

To illustrate the question, I will show a picture in a spreadsheet of what this would look like. In the pic, you can see ListA and ListB will always have the same number of values, as I want information on each row to be an 'observation' of sorts.

I could do this with a loop, but I would like my script to be a bit simpler. I also realize for this example, I could just get the count of ListA where the value is equal to 1. But I will have some other things I want to do in the future like compute averages, percentiles or other aggregations on ListB, where ListA is equal to some value.

Is this possible to do?

Picture of Lists

To clarify... for every ListA[x] == 1, you want to get ListB[x]? — Cory Nelson
– Cory Nelson, Commented Mar 24, 2016 at 4:42
Yes, the best way to describe would be if I wanted to get the average of all the values in ListB WHERE ListA==1. — pelt
– pelt, Commented Mar 24, 2016 at 15:27

Jason Boyd · Accepted Answer · 2016-03-24 04:38:04Z

2

So it looks like what you want to do is join list A onto list B. You don't specify in your question how the two lists should be joined but based on your image I am guessing you want to do it by the index - i.e. A[0] to B[0], A[1] to B[1], etc. If that is the case you can use Zip. Then if you wanted to take the average of B where A equals 1, for example, you could do the following:

ListA
.Zip(
    ListB,
    (a, b) => new { A = a, B = b })
.Where(x => x.A == 1)
.Average(x => x.B);

answered Mar 24, 2016 at 4:38

Jason Boyd

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