Retrieve an object from hashset in C#

Question

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Why can't I retrieve an item from a HashSet without enumeration?

I need to add a lot of objects to a Set.and I should retrieve them very fast. The only way that I know is using hash. But the HashSet class in C# doesn't contain any 'Get' method. The dictionary class isn't useful because Finding an object is very time-consuming in a dictionary.

This might help in future: http://msdn.microsoft.com/en-us/library/ms229335(v=vs.90).aspx — immutabl, Jan 06 '11 at 10:25
@Masoud: Can you publish your code? Might be easier to reconstruct :-) — Kees C. Bakker, Jan 06 '11 at 10:31
The hash returned by `GetHashCode` isn't unique, so what you're trying to do doesn't make much sense. — CodesInChaos, Jan 06 '11 at 10:39
My question is simple. I just want to add an object to a set and retrieve it very fast. — Masoud, Jan 06 '11 at 10:48
Why are you using a hashset? Why not use a dictionary? Dictionary is fast. Or you can use Hashtable. — Devendra D. Chavan, Jan 06 '11 at 11:05
I used dictionary but it is the most time consuming retrieving I ever seen. A real hash should be able to retrieve data in O(1) — Masoud, Jan 06 '11 at 11:11
Possible answer at: http://stackoverflow.com/questions/1869452/a-faster-replacement-to-the-dictionarytkey-tvalue — Devendra D. Chavan, Jan 06 '11 at 11:42
@Masoud, how did you reach the conclusion that the Dictionary is time consuming and not retrieving data in O(1)? — Devendra D. Chavan, Jan 06 '11 at 11:44

score 14 · Answer 1 · answered Jan 06 '11 at 10:37

14

HashSets are basically used to test if an object is included in the collection. It is un-orderd and un-sorted, doesnt have an index that could be used to retrive an object.

If you need to store and retrieve objects, use other collection types like List, HashTable etc.

answered Jan 06 '11 at 10:37

Danish Khan

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I am sorry, I dont understand. Here is how you can use a HashTable. http://support.microsoft.com/kb/309357 – Danish Khan Jan 06 '11 at 11:05
There is a Hashtable class in System.Collections.Hashtable. http://msdn.microsoft.com/es-es/library/system.collections.hashtable.aspx – Devendra D. Chavan Jan 06 '11 at 11:05
The (generic) hashtable class is called `Dictionary`. – CodesInChaos Jan 06 '11 at 11:07
1

hashtable hsa no get method same as hashset. dictionary is too slow. I need a real hash that can retrieve added datas very fast. – Masoud Jan 06 '11 at 11:10
@Masoud I would recommend that you share the exact scenario that you want to implement. That way we would be able to help you better. – Danish Khan Jan 06 '11 at 11:12
You can store your data in the keys of a Dictionary. The values can be anything (don't care). For example booleans which are all 'true'. Finding your data will then be quick - O(1). – oferei Dec 07 '16 at 10:38
@oferei that does not work as there is still no way to access the existing Dictionary key using your 'equivalent' key. You need to use a Dictionary where the key and value are the same. – Andrew Hanlon Jan 25 '17 at 18:55

score 1 · Answer 2 · answered Jan 06 '11 at 10:37

The HashSet represents a number of values. You can look if a certain value is in the set by using Contains(). A Dictionary is for storing objects and retrieving it by its hash (key).

using System;
using System.Collections.Generic;

namespace ConsoleApplication1
{
    class Program
    {
        public enum EnumA
        {
            One,
            Two,
            Three,
            Four
        }

        static void Main(string[] args)
        {
            HashSet<EnumA> test = new HashSet<EnumA>();

            test.Add(EnumA.Four);

            Console.WriteLine("Contains one:");
            Console.WriteLine(test.Contains(EnumA.One));
            Console.WriteLine("Contains four:");
            Console.WriteLine(test.Contains(EnumA.Four));
            Console.WriteLine();
            Console.ReadLine();

            return;
        }
    }
}

Retrieve an object from hashset in C#

2 Answers2