Count non null elements in an array of lists

Question

I couldn't find this anywhere else. I've an array of lists:

public List<xmldata>[] XMLArrayList = new List<xmldata>[9999];

To initialize and insert a list into each position, i do the following:

for(int m=0; m< XList.XMLArrayList.Count(); m++)
{
    XList.XMLArrayList[m] = new List<xmldata>();
}

But i would like to count how many elements there aren't null. EX: Positions 0 to 5 have a List on them. But other positions not.

Tried a linq approach:

int count = XList.XMLArrayList.Count(x => x != null);

But it returns me the array size (9999). How can i count the non null elements on an array of lists ? Ps: Already tried Dictionary and And List of List - this approach works best for achieving what i need.

Thanks.

If you initialize all the array positions to new list then none of them are null and you should get the length of the array. — juharr, Feb 25 '16 at 19:41
Yeah - i do initialize all of 'em. But how can i find on all initialized positions those who contains a xmldata object ? — Pablo Costa, Feb 25 '16 at 19:42
The Alexander's solution is the right one, there is a big difference between an empty list and a null list. — Gusman, Feb 25 '16 at 19:43

score 5 · Accepted Answer · edited Feb 25 '16 at 19:43

5

Try this:

int count = XList.XMLArrayList.Count(x => x.Count()>0);

edited Feb 25 '16 at 19:43

Blorgbeard

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answered Feb 25 '16 at 19:41

Alexandr Kudryashov

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Exactly ! WOrked as expected. Thanks. – Pablo Costa Feb 25 '16 at 19:44
1

Since these are lists you can use the `Count` property instead of the `Count()` extension method. – juharr Feb 25 '16 at 19:46
2

This is slightly cleaner: `XList.XMLArrayList.Count(c => c.Any());` – S. Brentson Feb 25 '16 at 19:49
3

Checking for non-empty collections is more ideomatically done using `enumeration.Any()` rather than `enumeration.Count() > 0`. it is also faster (O(1) vs O(n)). if Count() is evaluated on a heavy sql query it'll be very expensive. `Any()` will just check if enumerating yields an element and then return, regardless of the size of the enumeration. – sara Feb 25 '16 at 19:52
@kai, Since these are lists the `Count()` extenstion method will just end up returning the `Count` property value which is O(1). – juharr Feb 25 '16 at 19:57
@juharr that's true I guess, I'd still argue it's more correct, both from a semantic and from a maintainability point of view to use `Any` when checking for emptyness, even if the practical difference is negligible in this case (good habits and all that) – sara Feb 25 '16 at 19:59

Viru · Answer 2 · 2016-02-25T19:56:38.617

0

you can also do this

XList.XMLArrayList.Where(x => x.Any()).Count();

edited Feb 25 '16 at 19:56

answered Feb 25 '16 at 19:50

Viru

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1

this won't compile, you're using XMLArrayList as a method. – sara Feb 25 '16 at 19:53

Count non null elements in an array of lists

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