Select an element by index from a .NET HashSet

问题

At the moment I am using a custom class derived from HashSet. There's a point in the code when I select items under certain condition:

var c = clusters.Where(x => x.Label != null && x.Label.Equals(someLabel));

It works fine and I get those elements. But is there a way that I could receive an index of that element within the collection to use with ElementAt method, instead of whole objects?

It would look more or less like this:

var c = select element index in collection under certain condition;
int index = c.ElementAt(0); //get first index
clusters.ElementAt(index).RunObjectMthod();

Is manually iterating over the whole collection a better way? I need to add that it's in a bigger loop, so this Where clause is performed multiple times for different someLabel strings.

Edit

What I need this for? clusters is a set of clusters of some documents collection. Documents are grouped into clusters by topics similarity. So one of the last step of the algorithm is to discover label for each cluster. But algorithm is not perfect and sometimes it makes two or more clusters with the same label. What I want to do is simply merge those cluster into big one.

回答1:

Sets don't generally have indexes. If position is important to you, you should be using a List<T> instead of (or possibly as well as) a set.

Now SortedSet<T> in .NET 4 is slightly different, in that it maintains a sorted value order. However, it still doesn't implement IList<T>, so access by index with ElementAt is going to be slow.

If you could give more details about why you want this functionality, it would help. Your use case isn't really clear at the moment.

回答2:

In the case where you hold elements in HashSet and sometimes you need to get elements by index, consider using extension method ToList() in such situations. So you use features of HashSet and then you take advantage of indexes.

HashSet<T> hashset = new HashSet<T>();

//the special situation where we need index way of getting elements
List<T> list = hashset.ToList();

//doing our special job, for example mapping the elements to EF entities collection (that was my case)

//we can still operate on hashset for example when we still want to keep uniqueness through the elements

回答3:

There's no such thing as an index with a hash set. One of the ways that hash sets gain efficincy in some cases is by not having to maintain them.

I also don't see what the advantage is here. If you were to obtain the index, and then use it this would be less efficient than just obtaining the element (obtaining the index would be equally efficient, and then you've an extra operation).

If you want to do several operations on the same object, just hold onto that object.

If you want to do something on several objects, do so on the basis of iterating through them (normal foreach or doing foreach on the results of a Where() etc.). If you want to do something on several objects, and then do something else on those several same objects, and you have to do it in such batches, rather than doing all the operations in the same foreach then store the results of the Where() in a List<T>.

来源：https://stackoverflow.com/questions/3828973/select-an-element-by-index-from-a-net-hashset

标签

.net

hashset