What's the asymptotic complexity of GroupBy operation?

Question

I am interested in the asymptotic complexity (big O) of the GroupBy operation on unindexed datasets. What\'s the complexity of the best known algorithm and what\'s the compl

Answer 1

About Linq, I guess you want to know about the Linq-to-object group by complexity (Enumerable.GroupBy).

Checking the implementation with ILSpy, it appears to me it is O(n). (.Net Framework 4 series.)

It enumerates the source collection once. For each element, it computes its grouping key. Then it checks if it has already the key in a hashtable mapping to elements lists, adding the key to the hashtable if it is missing. Then it adds the element to the corresponding entry list in the hashtable.