C# List of objects binary search with a comparer

Question

I have an object named Artist.

public class Artist
{
    public string artistID { get; set; }
    public string artistName { get; set; }

    public Artist(string artistID, string artistName)
    {
        this.artistID = artistID;
        this.artistName = artistName;

    }

    public Artist()
    {
    }
    public Artist(string artistID)
    {
        this.artistID = artistID;
    }

Which has a custom Comparer like this:

public class CompareArtists : IComparer<Artist>
{

  public int Compare(Artist x, Artist y)
  {
      if (x.artistID == null) return -1;
      if (y.artistID == null) return 1;

    return String.Compare(x.artistID, y.artistID);
   }

}

I am trying to see whether an artist is contained in my list of artists in the main program. And I am doing that like this:

Artist tempArtist = new Artist(user[1]);
Artist result = new Artist();

position = artists.BinarySearch(tempArtist, dc);
result = artists.ElementAt(position);
                       users.ElementAt(counter).favoriteArtists.Add(result,Int32.Parse(user[2]));

The result of the binarysearch, is a negative number which from what I read on MSDN, is the complement of the next largest index. And it is shown when no results were found. However, I am 100% certain that the item I am searching for in my artists list does actually exist there. When I use a linear method, e.g. artists.Single or artists.Find, I get a result.

Do you see anything wrong with my comparer or the way I use binary search?

ps: dc is my comparer: CompareArtists dc = new CompareArtists(); and user[] is an array of strings that were split from a line I read with stream reader. it cotains the artist ID at [1] thanks in advance

Is your list sorted? Because you can use BinarySearch on only a sorted list. — L.B
– L.B, Commented Feb 22, 2014 at 23:09
Also note that your comparer does the wrong thing if it's presented with two artists both of whom have a null ID. It should return 0 in that case. — Jon Skeet
– Jon Skeet, Commented Feb 22, 2014 at 23:10
The list was generated sequentially by reading a file where the IDs are actually ordered. So I would say yes. If that is what you mean — A.A.
– A.A., Commented Feb 22, 2014 at 23:11

Selman Genç · Accepted Answer · 2014-02-22 23:16:27Z

4

You need to sort your list before BinarySearch, see documentation

Searches the entire sorted List for an element using the specified comparer and returns the zero-based index of the element.

artists.Sort(new CompareArtists());
position = artists.BinarySearch(tempArtist, dc);

Also you should use same comparer in Sort and BinarySearch to make it work

edited Feb 22, 2014 at 23:16

answered Feb 22, 2014 at 23:11

Selman Genç

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2 Comments

A.A. Over a year ago

You were all right, I imagined that because the list was generated sequentially, it would actually be ordered. But upon sorting the artists list, my code works. I am confused, but it works.

A.A. Over a year ago

Quick followup. I was actually comparing them as string, in my sequential generation they were ordered as Integers. As strings, the ordering is different. That is why it did not work.

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