How to use Comparer for a HashSet

Question

As a result of another question I asked here I want to use a HashSet for my objects

I will create objects containing a string and a reference to its owner.

public class Synonym
{
   private string name;
   private Stock owner;
   public Stock(string NameSynonym, Stock stock)
   {
       name=NameSynonym;
       owner=stock
   }
   // [+ 'get' for 'name' and 'owner']
}

I understand I need a comparer , but never used it before. Should I create a separate class? like:

public class SynonymComparer : IComparer<Synonym>
{
   public int Compare(Synonym One, Synonym Two)
   { // Should I test if 'One == null'  or  'Two == null'  ???? 
       return String.Compare(One.Name, Two.Name, true); // Caseinsesitive
   }

}

I prefer to have a function (or nested class [maybe a singleton?] if required) being PART of class Synonym instead of another (independent) class. Is this possible?

About usage: As i never used this kind of thing before I suppose I must write a Find(string NameSynonym) function inside class Synonym, but how should I do that?

public class SynonymManager
{ 
    private HashSet<SynonymComparer<Synonym>> ListOfSynonyms;

    public SynonymManager()
    {
        ListOfSymnonyms = new HashSet<SynonymComparer<Synonym>>();
    }

    public void SomeFunction()
    { // Just a function to add 2 sysnonyms to 1 stock
        Stock stock = GetStock("General Motors");
        Synonym otherName = new Synonym("GM", stock);
        ListOfSynonyms.Add(otherName);
        Synonym otherName = new Synonym("Gen. Motors", stock);
        ListOfSynonyms.Add(otherName);
    }

    public Synonym Find(string NameSynomym)
    {
       return ListOfSynonyms.??????(NameSynonym);
    }
 }

In the code above I don't know how to implement the 'Find' method. How should i do that?

Any help will be appreciated (PS If my ideas about how it should be implemented are completely wrong let me know and tell me how to implement)

Answer 1

Wouldn't it be more reasonable to scrap the Synonym class entirely and have list of synonyms to be a Dictonary (or, if there is such a thing, HashDictionary) of strings?

(I'm not very familiar with C# types, but I hope this conveys general idea)

The answer I recommend (edited, now respects the case):

    IDictionary<string, Stock>>  ListOfSynonyms = new Dictionary<string,Stock>>(); 
    IDictionary<string, string>> ListOfSynForms = new Dictionary<string,string>>(); 
    class Stock 
    {   
        ...
        Stock addSynonym(String syn) 
        {
            ListOfSynForms[syn.ToUpper()] = syn;
            return ListOfSynonyms[syn.ToUpper()] = this;
        }
        Array findSynonyms()
        {
            return ListOfSynonyms.findKeysFromValue(this).map(x => ListOfSynForms[x]);
        }
    }

    ...
    GetStock("General Motors").addSynonym('GM').addSynonym('Gen. Motors');
    ...
    try  
    {
        ... ListOfSynonyms[synonym].name ...
    }  
    catch (OutOfBounds e) 
    {
        ...
    } 
    ...
    // output everything that is synonymous to GM. This is mix of C# and Python
    ... GetStock('General Motors').findSynonyms()
    // test if there is a synonym
    if (input in ListOfSynonyms) 
    {
        ...
    }

Answer 2

You can always use LINQ to do the lookup:

public Synonym Find(string NameSynomym)
{
   return ListOfSynonyms.SingleOrDefault(x => x.Name == NameSynomym);
}

But, have you considered using a Dictionary instead, I believe it is better suited for extracting single members, and you can still guarantee that there are no duplicates based on the key you choose.

I am not sure that lookup time is of SingleOrDefault, but I am pretty sure it is linear (O(n)), so if lookup time is important to you, a Dictionary will provide you with O(1) lookup time.

Answer 3

A HashSet doesn't need a IComparer<T> - it needs an IEqualityComparer<T>, e.g.

public class SynonymComparer : IEqualityComparer<Synonym>
{
   public bool Equals(Synonym one, Synonym two)
   {
        // Adjust according to requirements
        return StringComparer.InvariantCultureIgnoreCase
                             .Compare(one.Name, two.Name);

   }

   public int GetHashCode(Synonym item)
   {
        return StringComparer.InvariantCultureIgnoreCase
                             .GetHashCode(item.Name);

   }
}

However, your current code only compiles because you're creating a set of comparers rather than a set of synonyms.

Furthermore, I don't think you really want a set at all. It seems to me that you want a dictionary or a lookup so that you can find the synonyms for a given name:

public class SynonymManager
{ 
    private readonly IDictionary<string, Synonym>> synonyms = new
        Dictionary<string, Synonym>>();

    private void Add(Synonym synonym)
    {
        // This will overwrite any existing synonym with the same name
        synonyms[synonym.Name] = synonym;
    }

    public void SomeFunction()
    { // Just a function to add 2 sysnonyms to 1 stock
        Stock stock = GetStock("General Motors");
        Synonym otherName = new Synonym("GM", stock);
        Add(otherName);
        ListOfSynonyms.Add(otherName);
        otherName = new Synonym("Gen. Motors", stock);
        Add(otherName);
    }

    public Synonym Find(string nameSynonym)
    {
       // This will throw an exception if you don't have
       // a synonym of the right name. Do you want that?
       return synonyms[nameSynonym];
    }
}

Note that there are some questions in the code above, about how you want it to behave in various cases. You need to work out exactly what you want it to do.

EDIT: If you want to be able to store multiple stocks for a single synonym, you effectively want a Lookup<string, Stock> - but that's immutable. You're probably best storing a Dictionary<string, List<Stock>>; a list of stocks for each string.

In terms of not throwing an error from Find, you should look at Dictionary.TryGetValue which doesn't throw an exception if the key isn't found (and also returns whether or not the key was found); the mapped value is "returned" in an out parameter.

Answer 4

Currently in my StockManager class I have this:

 private Dictionary<string, Stock> ListOfSynonyms; // List of all synonyms for stocks 

 public Stock GetStock(string NameStock)
 {
  Stock stock = null;
  if (NameStock != null)
  {
   string NameSynonym = NameStock.Trim().ToUpper();
   try
   { // Take care of NameSynonym not being present 
    stock = ListOfSynonyms[NameSynonym];
   }
   catch
   { // KeyNotFoundException will be thrown if NameSynonym is not present
   }
  }
  return stock;
 }

 public bool AddSynonym(string NameSynonym, Stock stock)
 {
  bool bAdded = false;
  if ((NameSynonym != null) && (stock !=  null))
  {
   string Synonym = NameSynonym.Trim().ToUpper();
   if ((Synonym != "") && (GetStock(Synonym) == null))
   {
    ListOfSynonyms[Synonym] = stock;
    bAdded = true;
   }
  }
  return bAdded;
 }

I think now I only need to be able to retrieve all synonyms for a particular stock (and delete them in case the stock will be removed from the ones I want to store). As it can be that 2 stocks will be merged ( because just a few sites provide the combination ISIN-StockName so the user [or program] has to be able to merge himself afterwards) I also need to change the stock a particular synonym is referring to afterwards.

(PS I have to go now, but [of course] will look at it later. Thanks for all answers to all)