Filtering a List using lambda

Question

I have an object which has properties ID, brandID, brandName, NumPages, and Type.

i need to show the top 5 brands by numPage size, a brand may have multiple IDs, so I need to group by brand

 listing.OrderByDescending(o => o.numPage).GroupBy(o=> o.brandName).Take(5).ToList();

is alone the lines of what im looking for but this is not valid code.

Why it is not being valid? Are you getting an error message or just not getting the wanted results. — Xtian Macedo
– Xtian Macedo, Commented Jan 14, 2012 at 0:18

JaredPar · Accepted Answer · 2012-01-14 00:18:56Z

2

It sounds like a given brand name may have several ID's and that you want the top 5 brand's sorted by numPage. Is that correct

If so try the following

var query = listing
  .GroupBy(x => x.brandName)
  .OrderByDescending(brands => brands.Sum(x => x.numPage))
  .Select(x => x.Key)
  .Take(5);

Note: After the GroupBy operation you're now passing around a collection of the brand objects instead of single ones. Hence to order by the numPage we need to sum it for all of the brand objects in the group. The .Select(x => x.Key) will select back out the original brandName on which the group is based

answered Jan 14, 2012 at 0:18

JaredPar

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15 Comments

user743382 Over a year ago

An IGrouping doesn't have a numPage member, so that shouldn't compile.

JaredPar Over a year ago

@hvd I verified in Visual Studio that this compiles. One of my earlier versions had a typo but the current one should work

user743382 Over a year ago

Yes, my comment was based on the .OrderByDescending(x => x.numPage.Sum()) which you've indeed fixed.

JaredPar Over a year ago

@hvd i think i get bitten by that mistake every time I hand role a groupby :(

tap Over a year ago

i get an Arguement null exection and it's not valid code, I should mention instead of a var query, im doing this in a viewModel being called from return View(new viewModel(listing.GroupBy... but that shouldn't make a difference

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Giorgio Minardi · Accepted Answer · 2012-01-14 00:36:18Z

just tried and it works:

public class Listing
{
    public int ID { get; set; }
    public int BrandID { get; set; }
    public string BrandName { get; set; }
    public int NumPages { get; set; }
    public Type Type { get; set; }    
}

Here the filtering

Listing listing1 = new Listing() { NumPages = 2, BrandName = "xx" };
Listing listing2 = new Listing() { NumPages = 2, BrandName = "xx" };
Listing listing3 = new Listing() { NumPages = 2, BrandName = "xx" };
Listing listing4 = new Listing() { NumPages = 3, BrandName = "xxxxx" };

List<Listing> allListings = new List<Listing>() { listing1, listing2, listing3, listing4 };

var result = allListings.OrderByDescending(x => x.NumPages).GroupBy(x => x.BrandName).Take(5);

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