Select a group of n objects into a list of a list of objects

Question

I have Object A in which I have lengths. I would like to order by length descending, then I would like to group them by threes and return that list of a list of objects.

I can get the grouping to work, but all i get is the key of the grouping and not the items.

public class a { public string Id { get; set; } public int Length { get; set; } } List<a> c = Instantiate a list c.OrderByDescending(x => x.Length) .Select((e, i) => new { Item = e, Grouping = (i / 3) }) .GroupBy(x => x.Grouping) .Select(x => x.Key) .ToList()

I think it has something to do with my group by but I cant seem to get it to work. What I would like is a List<List<a>> that have at most three items.

as I understood correctly, just turn .Select(x => x.Key) into .Select(x => x.ToList()) — Victor
– Victor, Commented Apr 16, 2015 at 23:30
also, i / 3 will break your collection into 3 pieces, to partition by length you may look at answer stackoverflow.com/questions/5215469/… — Victor
– Victor, Commented Apr 16, 2015 at 23:36
@Victor, that is what I'd like. At least for now I'd like to just simply break into groups of three when it is sorted by length. — Herrozerro
– Herrozerro, Commented Apr 17, 2015 at 12:10

david.s · Accepted Answer · 2015-04-16 23:30:38Z

2

Use this .Select(grp => grp.ToList()) instead of .Select(x => x.Key). This will return the group as a List<a>.

answered Apr 16, 2015 at 23:30

david.s

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1 Comment

Joe Farrell Over a year ago

I would add to this that he probably wants .GroupBy(x => x.Grouping, x => x.Item). This will produce a list of lists of as, whereas what he's got now will produce a list of lists of objects of that anonymous type created in the first Select().

PiotrWolkowski · Accepted Answer · 2015-04-17 00:02:11Z

Following query will generate a list of lists where the inner list contains three items:

var listOfTriplets = c.OrderByDescending(x => x.Length) .Select((x, i) => new { Index = i, Value = x }) .GroupBy(x => x.Index / 3) .Select(x => x.Select(v => v.Value).ToList()) .ToList();

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