Creating a Multidimensional array from a list of objects

Question

I have a list of "Entry" objects (that each have an ID), and want to create a 2D array based by splitting them up by ID:

class Entry
{
    public int ID { get; set; }
    public int value { get; set; }

    public Entry(int id, int value)
    {
        this.ID = id;
        this.value = value;
    }
}

Population:

        List<Entry> entries = new List<Entry>();
        entries.Add(new Entry(0, 20));
        entries.Add(new Entry(1, 20));
        entries.Add(new Entry(1, 25));
        entries.Add(new Entry(2, 21));
        entries.Add(new Entry(2, 23));

I need to build a Multidimensional list of entries split up by ID. The following code builds the array:

        List<Entry> entriesZero = new List<Entry>();
        entriesZero.Add(new Entry(0, 20));

        List<Entry> entriesOne = new List<Entry>();
        entriesOne.Add(new Entry(1, 20));
        entriesOne.Add(new Entry(1, 25));

        List<Entry> entriesTwo = new List<Entry>();
        entriesTwo.Add(new Entry(2, 21));
        entriesTwo.Add(new Entry(2, 23));

        List<List<Entry>> entriesByID = new List<List<Entry>>();
        entriesByID.Add(entriesZero);
        entriesByID.Add(entriesOne);
        entriesByID.Add(entriesTwo);

What would be the best way to accomplish something like this? I could do it with multiple foreach loops but I'm wondering if there's a better way in LINQ.

Erik Philips · Accepted Answer · 2013-03-21 22:40:43Z

8

Do you definitely need it to be a list grouped by ID? A Lookup will do this for you really easily:

ILookup<int, Entry> entriesById = entries.ToLookup(e => e.Id);

You can still iterate over that, but you can also look up all the entries for any ID.

If you really, really need a List<List<Entry>> you could do this:

var listOfLists = entriesById.GroupBy(e => e.Id)
                             .Select(g => g.ToList())
                             .ToList();

... but I'd go with the lookup.

edited Mar 21, 2013 at 22:40

Erik Philips

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answered Mar 21, 2013 at 22:39

Jon Skeet

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1 Comment

David Over a year ago

I didn't know about lookups, that's great! Though I still need to have a list of lists, because it's being set to as a DataContext for a chart.

Tim Schmelter · Accepted Answer · 2013-03-21 22:43:13Z

1

You need just GroupBy + ToList(one one the group itself an one on the whole thing):

List<List<Entry>> entriesByID = entries.GroupBy(entry => entry.ID)
            .Select(g => g.ToList())
            .ToList();

answered Mar 21, 2013 at 22:43

Tim Schmelter

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Comments

Nefarion · Accepted Answer · 2013-03-21 22:47:52Z

0

You could also do this:(Non-LINQ)

Dictionary<int , List<Entry>> dictionary = new ...

foreach (var v in entries)
    dictionary[v.ID].Add(v);

edited Mar 21, 2013 at 22:47

answered Mar 21, 2013 at 22:41

Nefarion

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