HashSet vs. List performance

Question

It\'s clear that a search performance of the generic HashSet class is higher than of the generic List class. Just compare the has

Answer 1

Whether to use a HashSet<> or List<> comes down to how you need to access your collection. If you need to guarantee the order of items, use a List. If you don't, use a HashSet. Let Microsoft worry about the implementation of their hashing algorithms and objects.

A HashSet will access items without having to enumerate the collection (complexity of O(1) or near it), and because a List guarantees order, unlike a HashSet, some items will have to be enumerated (complexity of O(n)).

Answer 2

It depends. If the exact answer really matters, do some profiling and find out. If you're sure you'll never have more than a certain number of elements in the set, go with a List. If the number is unbounded, use a HashSet.

Answer 3

Depends on a lot of factors... List implementation, CPU architecture, JVM, loop semantics, complexity of equals method, etc... By the time the list gets big enough to effectively benchmark (1000+ elements), Hash-based binary lookups beat linear searches hands-down, and the difference only scales up from there.

Hope this helps!

Answer 4

You can use a HybridDictionary which automaticly detects the breaking point, and accepts null-values, making it essentialy the same as a HashSet.

Answer 5

The breakeven will depend on the cost of computing the hash. Hash computations can be trivial, or not... :-) There is always the System.Collections.Specialized.HybridDictionary class to help you not have to worry about the breakeven point.

Answer 6

The answer, as always, is "It depends". I assume from the tags you're talking about C#.

Your best bet is to determine

1. A Set of data
2. Usage requirements

and write some test cases.

It also depends on how you sort the list (if it's sorted at all), what kind of comparisons need to be made, how long the "Compare" operation takes for the particular object in the list, or even how you intend to use the collection.

Generally, the best one to choose isn't so much based on the size of data you're working with, but rather how you intend to access it. Do you have each piece of data associated with a particular string, or other data? A hash based collection would probably be best. Is the order of the data you're storing important, or are you going to need to access all of the data at the same time? A regular list may be better then.

Additional:

Of course, my above comments assume 'performance' means data access. Something else to consider: what are you looking for when you say "performance"? Is performance individual value look up? Is it management of large (10000, 100000 or more) value sets? Is it the performance of filling the data structure with data? Removing data? Accessing individual bits of data? Replacing values? Iterating over the values? Memory usage? Data copying speed? For example, If you access data by a string value, but your main performance requirement is minimal memory usage, you might have conflicting design issues.