Introducing HashSet [Kim Hamilton]
HashSet<T> is in our latest CTP, and you can find it in the System.Collections.Generic namespace. The naming discussion over the last month has motivated me to recap some naming highlights for HashSet, so hang in til the end if you’re interested.
HashSet is an unordered collection containing unique elements. It has the standard collection operations Add, Remove, Contains, but since it uses a hash-based implementation, these operation are O(1). (As opposed to List<T> for example, which is O(n) for Contains and Remove.) HashSet also provides standard set operations such as union, intersection, and symmetric difference.
HashSet<int> theSet1 = new HashSet<int>(); theSet1.Add(1); theSet1.Add(2); theSet1.Add(2); // theSet1 contains 1,2 HashSet<int> theSet2 = new HashSet<int>(); theSet2.Add(1); theSet2.Add(3); theSet2.Add(4); // theSet2 contains 1,3,4 theSet1.UnionWith(theSet2); // theSet1 contains 1,2,3,4
HashSet’s default Add operation returns a bool letting you know whether the item was added, so in the code sample above, you could check the return type to check whether the item was already in the set.
bool added = theSet1.Add(2); // added is true added = theSet1.Add(2); // added is false
If you’re familiar with our ICollection<T> interface, notice that this means ICollection<T>.Add (returning void) has an explicit implementation, allowing HashSet<T> to introduce its own Add.
A note on uniqueness: HashSet determines equality according to the EqualityComparer you specify, or the default EqualityComparer for the type (if you didn’t specify). In the above example we didn’t specify an EqualityComparer so it will use the default for Int32. In the next example, we’ll use an OddEvenComparer, which considers items equal if they are both even or both odd.
class OddEvenComparer : IEqualityComparer<int> { public OddEvenComparer() {} public bool Equals(int x, int y) { return (x & 1) == (y & 1); } public int GetHashCode(int x) { return (x & 1); } } ... // Now use the comparer HashSet<int> oddEvenSet = new HashSet<int>(new OddEvenComparer()); oddEvenSet.Add(1); oddEvenSet.Add(3); oddEvenSet.Add(4); // oddEventSet contains 1,4; it considered 1 and 3 equal.
Notice the name UnionWith in the first example. UnionWith, as with the other set operations, modifies the set it’s called on and doesn’t create a new set. This distinction is important because the Linq operations Union, Intersect, etc on IEnumerable create a new set. So HashSet’s methods aren’t duplicating Linq; they’re provided in case you want to avoid creating a new set, and they’re distinguished by the With suffix.
Now for some naming fun, which will demonstrate some other framework guidelines. We would have liked to name this feature Set. This is because it’s preferred to use a common name rather than one that reveals details about the implementation. To borrow an example from Krzysztof Cwalina and Brad Abram’s book Framework Design Guidelines, a type used to submit print jobs to a print queue should be named Printer, and not PrintQueue. Applying this guideline to this class – HashSet, while more technically precise, isn’t as recognizable at Set. You can see this guideline in other class names in the System.Collections.Generic namespace: List<T> instead of ArrayList<T>, Dictionary<T> instead of Hashtable<T>.
This brings up the question of whether naming it Set would have been a bad idea if we add other sets in the future, such as an OrderedSet. However, a hash-based unordered set can reasonably be considered the “go-to” set because of its good performance, so distinguishing it with the name Set would still be acceptable.
Any guesses as to why we didn’t go with the name Set?
Comments
Anonymous
November 09, 2006
FXCop rule prohibits naming a type after a language keyword. "set" is also used in C#, but also in VB6, though it is either gone or optional in VB.NET. It would be nice to support operator overloading for sets.Anonymous
November 09, 2006
Because "set" is a reserved contextual keyword http://msdn2.microsoft.com/en-gb/library/x53a06bb.aspxAnonymous
November 09, 2006
I would strongly prefer the name Set instead of HashSet. As you state, HashSet refers too strongly to the implementation. Users of the class don't really care about the implementation, we just care about the functionality and performance characteristics.Anonymous
November 09, 2006
Even though you no longer have to use Set when assigning references in VB, it is still a keyword. It is used in declaring property accessors: Property Text() As String Get Return _text End Set(ByVal value As String) _text = value End Set End Property I'm not sure if Set is a true keyword or a contextual keyword, but either way there's potential for confusion.Anonymous
November 09, 2006
PowerCollections uses Set<T>... I can't say I've ever had a problem with keyword collision with it.Anonymous
November 09, 2006
You've been kicked (a good thing) - Trackback from DotNetKicks.comAnonymous
November 09, 2006
HashSet is a new generic collection that has been added to the System.Collections.Generic namespace.Anonymous
November 09, 2006
> Any guesses as to why we didn’t > go with the name Set? I assume you mean Set<T>. That makes the VB argument mostly irrelevant. I am really glad that you did not call it Set<T>. I eagerly await the day that ISet<T> is introduced along with other set types, TreeSet<T>, OrderSet<T> etc. However, I think it is a mistake to introduce HashSet<T> now without an ISet<T>. A lot of code will need to be rewritten later (such as updating signatures) because of this choice. I hope the type is unsealed. Lets say I want to design a TreeSet<T>. It really does not make sense for me to extend HashSet<T> to do this. Yet without an ISet<T>, there really aren't any other options. I could ignore HashSet<T> entirely and write everything from scratch. However, that does not fit well into the framework. Please reconsider this issue.Anonymous
November 09, 2006
A Set type is great to have. I've gotten by with Dictionary<KeyType, object> so far (leaving the value as null), but this is even better. Another type that I use often is Dictionary<KeyType, List<ValueType>> - a key maps to a set of zero or more values. I would love to see something like this in the BCL.Anonymous
November 09, 2006
Hello Kim, When and where this collection will be available?Anonymous
November 09, 2006
The BCL Team work in a new type collection called (temporary) HashSet that is a collection containingAnonymous
November 09, 2006
I think this is a great idea, as it is really a missing feature currently. Thus why don't you go a step beyond by creating an HashList which would be o(1) (or did I miss it)Anonymous
November 10, 2006
Hi Isreal, It's in our latest Orcas CTP, which you can download here: http://www.microsoft.com/downloads/details.aspx?FamilyId=C09B5A2D-EB6A-44B6-8BBD-3764A2FDA9CE&displaylang=en It's in System.Core.dll, in the System.Collections.Generic namespace. Thanks, KimAnonymous
November 12, 2006
The comment has been removedAnonymous
November 12, 2006
Hi Dono, >> A lot of code will need to be rewritten later (such as updating signatures) because of this choice. We had a lot of discussion on this point during the design of HashSet (which Justin summarized); we'll most likely follow up soon with some more details. Thanks, KimAnonymous
November 17, 2006
Kim, By HashList I was referring to an IList<T> implementation which would be O(1) for Add, Remove and Contains. Isn't it one of the main matters of HashSet ?Anonymous
November 22, 2006
The comment has been removedAnonymous
November 22, 2006
Hi Diegov, Earlier versions of our design used Add/TryAdd, but we removed it to be consistent with the set interpretation of add. Even though a set is maintained such that it doesn't contain duplicate items, attempting to add a duplicate isn't exceptional. We provided the Add that returns bool for cases where the user may want different control flow based on whether the item was already present in the set, but this shouldn't be achieved through exceptions (with their associated perf cost, etc). So in fact, this isn't a change of semantics of Add. You can think of it this way -- if the items are equal according to the equality comparer, then it's an implementation choice whether the original item is replaced by the new duplicate item. After Add is finished, the item is present in the set. I should also mention that we came to this decision only after much debate. Using Add/TryAdd was a common suggestion, but more than 2/3rds of people reviewing the API thought that having Add throw on duplicates would be surprising for a set. Some readers may be wondering why IDictionary throws on duplicate adds. That's because it takes both a key and a value, and although the key may be the same, the values may differ. Thanks, KimAnonymous
November 22, 2006
Hi Sebastien, >> Isn't it one of the main matters of HashSet ? Not exactly; HashSet is unordered and doesn't contain duplicates, but an IList is ordered and may contain duplicates. Thanks, KimAnonymous
November 22, 2006
The comment has been removedAnonymous
February 07, 2007
System.Collections.Generic.HashSetAnonymous
February 15, 2007
Незабаром в нас буде нова версія .Net Framework 3.5, яка схоже, як і 3.0, буде спиратися на базовий .NetAnonymous
June 12, 2007
Since it's coming up on 2 years since I complained that the BCL didn't have Set<T> I figured IAnonymous
August 08, 2007
While installing VisualStudio 2008 Beta2 I was surprised that the NET framework 2.0 installation gotAnonymous
September 11, 2007
Generic function to removeDuplicates from Generic ListAnonymous
September 13, 2007
Visual Studio (VS) 2008 and .Net 3.5, code named “Orcas”, will be released in the near future. This releaseAnonymous
October 22, 2007
Mein TechTalk ist nun zu Ende. Meine letzte Station heute in Berlin war Lustig und Amüsant, ich hoffeAnonymous
November 06, 2007
The comment has been removedAnonymous
November 06, 2007
The comment has been removedAnonymous
November 19, 2007
.NET Framework 3.5 and Visual Studio 2008 have officially shipped! Soma has the announcement on his blogAnonymous
April 22, 2008
In mathematics, a Set is typically thought of as a collection of distinct objects that is usually defined