A HashSet works via hashing (via IEqualityComparer.GetHashCode) the objects you insert and tosses the objects into buckets per the hash. The buckets themselves are stored in an array, hence the O(1) part. For example (this is not necessarily exactly how the C# implementation works, it just gives a flavor) it takes the first character of … Read more
I think you want HashSet.Except. That is, rather than use Lists, use HashSets, and then the operation is available. This is a better type if what you are representing is really a ‘set’ anyway. (If you already have a list, you can just create a ‘new HashSet’ out of it.)
hash_set is an extension that is not part of the C++ standard. Lookups should be O(1) rather than O(log n) for set, so it will be faster in most circumstances. Another difference will be seen when you iterate through the containers. set will deliver the contents in sorted order, while hash_set will be essentially random … Read more
You don’t want union — as you said, it will create a new HashSet. Instead you can use Extend::extend: use std::collections::HashSet; fn main() { let mut a: HashSet<u16> = [1, 2, 3].iter().copied().collect(); let b: HashSet<u16> = [1, 3, 7, 8, 9].iter().copied().collect(); a.extend(&b); println!(“{:?}”, a); // {8, 3, 2, 1, 7, 9} } (Playground) Extend::extend is … Read more
You should use the HashSet<T> class, which is specifically designed for what you’re doing.
From the Javadoc: Note that this implementation is not synchronized. If multiple threads access a hash set concurrently, and at least one of the threads modifies the set, it must be synchronized externally Reading doesn’t modify a set, therefore you’re fine.
PRESENT is just a dummy value — the set doesn’t really care what it is. What the set does care about is the map’s keys. So the logic goes like this: Set.add(a): map.put(a, PRESENT) // so far, this is just what you said the key “a” is in the map, so… keep the “a” key, … Read more
This is why I don’t like to rely on auto-boxing. Java Collections cannot store primitives (for that you will need a third party API like Trove). So, really, when you execute code like this: hashSet.add(2); hashSet.add(5); What is really happening is: hashSet.add(new Integer(2)); hashSet.add(new Integer(5)); Adding a null to the hash set is not the … Read more
In .Net, what you are probably looking for is KeyedCollection http://msdn.microsoft.com/en-us/library/ms132438.aspx You can get around the nastiness of re-implementing this abstract class each time with some “generic” cleverness. (See IKeyedObject`1.) Note: Any data transfer object which implements IKeyedObject`1 should have an overridden GetHashCode method simply returning this.Key.GetHashCode(); and same goes for equals… My Base Class … Read more
I just ran into a similar scenario (“I am interested in a fast Add and Contains and Remove”) and implemented this sucker: using System.Collections.Generic; using System.Threading; namespace BlahBlah.Utilities { public class ConcurrentHashSet<T> : IDisposable { private readonly ReaderWriterLockSlim _lock = new ReaderWriterLockSlim(LockRecursionPolicy.SupportsRecursion); private readonly HashSet<T> _hashSet = new HashSet<T>(); #region Implementation of ICollection<T> …ish public … Read more