Remove list entries depending on other list

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Hi all, I have two lists of classes as shown below - both classe have a common property - using Linq how can I remove items from the artist list that don't have any Albums ? In SQL I would say

C#

Delete from Artist where ArtistName not in(Select ArtistName from Albums)


class Artist
{
    String ArtistName {get;set;}
}

class Albums
{
    String ArtistName {get;set;}
    String AlbumName {get;set;}
}

Posted 3-Dec-15 4:58am

pkfox

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BillWoodruff 3-Dec-15 13:50pm

An artist with no albums stops being an artist ? An album can have only one artist ?

I think the design of your classes should reflect your estimation of the most frequently used type of queries users will want to do.

I also think that unless you are absolutely certain the design requirement will never change, it is best to implement general solutons that anticipate being extended, changed, in the future.

If you are really interested in this type of approach, let me know, and I'll post an example.

pkfox 3-Dec-15 14:29pm

I know what your saying Bill but the data is coming from a source that is not under my control, it's a music database and an artist can exist if they contribute to an album but are not necessarily the *album artist* so an entry is made as a *contributor* but not a corresponding album

BillWoodruff 4-Dec-15 2:18am

Hi PkFox, Understood. Is it the case that the data you receive, for each album, contains both a list of "primary" artists, and another list of "contributing artists" ? Is it the case that you are getting Artists information always combined with Album data ?

Even though you may have no control over the incoming JSON, that doesn't stop you transforming those "resurrected" class instances into a structure you design to meet the possible modal needs of your users ... does it ?

cheer, Bill

pkfox 4-Dec-15 3:18am

Food for thought thanks Bill - you mentioned you could send me something - please do.

BillWoodruff 4-Dec-15 3:33am

"you mentioned you could send me something - please do" I'll post an alternative "solution" here today, or tomorrow (GMT+0700).

But, please tell me exactly what Type objects you get when you deserialize the JSON: list of ? array of ? ... or ?

cheers, Bill

pkfox 4-Dec-15 4:41am

Hi Bill, I get a list of albums, tracks and artists - they are part of Logitech Media Server ( used to be Squeezecenter )

BillWoodruff 4-Dec-15 5:10am

Please be more specific: you get one list of albums, one list of artists, one list of tracks ... or ... ?

pkfox 4-Dec-15 8:31am

Sorry Bill I should be clearer - yes I get one list of each

BillWoodruff 4-Dec-15 23:23pm

Hi PkFox, I'm having trouble getting your scenario in "focus" here (perhaps it's my old eyes); assuming you have three lists, what in the structure of these lists links artist to album(s)/track(s).

If you could add a sample of the JSON you are dealing with to your original post, that might be helpful.

By the way have you seen this CP article:

http://www.codeproject.com/Articles/125126/Controlling-a-SqueezeBox-Server-using-C-VS-Par

pkfox 5-Dec-15 3:42am

Hi Bill I've read that article and it's very good in fact I use some of his methods to talk to the music server. The structure of the classes is very basic unfortunately I can't paste them here because of the formatting but I'll do my best to explain, the Artist class has a ArtistID member as a primary key, the Album also has ArtistID and AlbumID as the primary key, the Tracks class has AlbumID and TrackID as the primary key. Thanks for your interest.

BillWoodruff 5-Dec-15 4:52am

Are you at the point, now, where everything is working the way you want it to ? If not, then I think posting the structure of the Classes as part of your original post, where you can use formatting, would be a good idea. Can an Album or Track have more than one Artist ... didn't you mention "contributing" artist ?

pkfox 5-Dec-15 7:12am

Hi Bill, I have things working using Matts solution of removing artists that don't have a corresponding album which is not ideal but it does work

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Matt T Heffron · Accepted Answer · 2015-12-03T06:48:00

Since you stated in your reply to Original Griff that you cannot change the design, here's what I'd do using the classes as you described:

C#

class Artist
{
    String ArtistName {get;set;}
}
 
class Albums
{
    String ArtistName {get;set;}
    String AlbumName {get;set;}
}

Assuming the names of the variables of the two Lists are:
AllAlbums and AllArtists
Then, to delete the Artists for which there are no albums:

C#

HashSet<string> AlbumArtistSet = new HashSet<string>(AllAlbums.Select(al => al.ArtistName));
var trimmedArtists = AllArtists.Where(a => AlbumArtistSet.Contains(a.ArtistName)); // .ToList() if necessary

This makes only one pass through each list.

=================
Edit: MTH
Since the data is being reparsed from JSon, why not reparse it into more useful data structures along the line of what Original Griff proposed?
Or, if you aren't using a DB yourself, then why not setup something like:

C#

// Still assuming the same AllAlbums and AllArtists as above.
// (Shouldn't the Albums class be named Album since it represents ONE album?)
Dictionary<string, List<Albums>> AlbumsByArtistName = new Dictionary<string, List<Albums>>();

// Now populate the dictionary lists.
foreach (var al in AllAlbums)
{
  string name = al.ArtistName;
  List<Albums> albums;
  if (!AlbumsByArtistName.TryGetValue(name, out albums)
  {
    albums = new List<Albums>();
    AlbumsByArtistName.Add(name, albums);
  }
  albums.Add(al);
}
// Now to "trim" the AllArtists list you can:
var trimmedArtists = AllArtists.Where(a => AlbumsByArtistName.Contains(a.ArtistName)); // .ToList() if necessary

Still only one pass through each collection, but now you can access the subset collection of albums by artist name. You can setup similar dictionaries for other properties that you might want subset access. Just remember to add and delete everywhere appropriately.
At this point you've probably noticed that this is looking a bit like a DB. :-)
Using your own in-process (in-memory?) DB, independent from the data source, might be the best solution.

OriginalGriff · Accepted Answer · 2015-12-03T05:25:00

To be honest, I wouldn't.
Instead of keeping the ArtistName in two places, I'd change the design slightly:

C#

class Artist
    {
    public String ArtistName { get; set; }
    }

class Album
    {
    public Artist Artist { get; set; }
    public String AlbumName { get; set; }
    }

Which would be more like what you would do in a DB.
Then it's trivial:

C#

            List<Artist> artists = new List<Artist>();
            Artist joe = new Artist() { ArtistName = "Joe" };
            Artist mike = new Artist() { ArtistName = "Mike" };
            Artist sally = new Artist() { ArtistName = "Sally" };
            artists.Add(joe);
            artists.Add(mike);
            artists.Add(sally);
            List<Album> albums = new List<Album>();
            albums.Add(new Album() {Artist = mike, AlbumName = "A1"});
//
            var lessArtists = artists.Except(albums.Select(a => a.Artist).Distinct());

Remove list entries depending on other list

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