Return objects which contain Lists, based on the those lists matching

Question

I have Cell objects which contain a List<int> called PossibleValues. I'm trying to find a way to get a list of cells in which all members have matching PossibleValues. I currently have:

foreach (var cell in group)
{
    var cellsWithMatchingPossibleValues = group.Where(c => c.PossibleValues == cell.PossibleValues);
}

Unfortunately this isn't working, I suspect my linq statement isn't comparing the contents of PossibleValues, but instead comparing a reference of some kind, so that even in the case where both lists are composed of 3 and nothing else, cellsWithMatchingPossibleValues ends up only containing one cell, although I'm not certain, or sure how to get around that.

To formalise the question: How can I return objects which contain Lists based on the those lists matching?

group in foreach and group inside foreach is different, isn't it? — Valentin, Mar 13 '16 at 14:17
Valentin: Same group, I'm looking to find how many Cells within that group have matching PossibleValues lists, to do that I planned to loop through each cell and match that list against the lists of the whole group. I'd expect to always get 1 match with this, seeing as the cell is in the group. If you have a more efficient idea, I'd love to hear it. — MGDavies, Mar 13 '16 at 14:20
CodeConstruct: The issue isn't solved by doing a .ToList(); The resulting list still only contains one object when we would expect two. — MGDavies, Mar 13 '16 at 14:22
Please let me know if I'm missing something, but wouldn't the GroupBy function be more what you're looking for here? Also, doesn't this code re-initialize cellsWithMatchingPossibleValues with every iteration, basically removing whatever value was there before? — Dan Orlovsky, Mar 13 '16 at 14:37
What is `PossibleValues`? Is it a list of strings, ints, or objects? Your answer to this question will make a **huge** difference in whether the answer is facile or complex. — code4life, Mar 13 '16 at 14:53
Second question: are you iterating a collection of `Cell` objects and distilling a list of `Cell` objects having a common set of `PossibleValue`s, or do you have a single `Cell` object and trying to see which `Cell`s in a list match to it? — code4life, Mar 13 '16 at 14:57
Can you clarify what you mean by "matching"? Same elements? Same elements in the same order? — Bobson, Mar 13 '16 at 15:16
Valid questions: PossibleValues is a list of ints. Group is a List. Each Cell has a PossibleValues List. I'll be updating the main question to improve it in light of some of the questions I've had. — MGDavies, Mar 13 '16 at 15:19

score 2 · Answer 1 · edited May 23 '17 at 10:28

You can implement IEqualityComparer interface and use GroupBy method. Here you can find good GetHashCode for List, and here how to compare a lists.

    public class PossibleValuesCellComparer : IEqualityComparer<Cell>
    {
        public bool Equals(Cell x, Cell y)
        {
            return Enumerable.SequenceEqual(x.PossibleValues.OrderBy(t => t), y.PossibleValues.OrderBy(t => t));
        }


        public int GetHashCode(Cell cell)
        {
            var list = cell.PossibleValues.OrderBy(t => t);
            unchecked
            {
                int hash = 19;
                foreach (var obj in list)
                {
                    hash = hash * 31 + obj.GetHashCode();
                }
                return hash;
            }
        }
    }

     ....

     var g2 = group.GroupBy(x => x, new PossibleValuesCellComparer());

René Vogt · Answer 2 · 2016-03-13T14:42:29.793

You can implement your own IEqualityComparer<Cell> for your Cell class that states equality when the PossibleValues are equal like this:

public class CellComparer : IEqualityComparer<Cell>
{
    public bool Equals(Cell x, Cell y)
    {
        if (ReferenceEquals(x, null)) return ReferenceEquals(y, null);
        if (ReferenceEquals(y, null)) return false;
        return x.PossibleValues.SequenceEqual(y.PossibleValues);
    }
    public int GetHashCode(Cell obj)
    {
        if (obj == null) return 0;
        unchecked
        {
            int hash = 1;
            foreach (int h in obj.PossibleValues.Select(v => v?.GetHashCode() ?? 0))
                hash = (hash * 397) ^ h;
            return hash;
        }
    }
}

Then you can use this for a simple LINQ grouping like this:

var cellsGroupedByEqualValues = group.GroupBy(c => c, new CellComparer());

This returns an IEnumerable<IGrouping<Cell,Cell>> and you can iterate through it and receive the number of matching cells:

foreach(var groupedCells in cellsGroupedByEqualValues)
    Console.WriteLine(groupedCells.Count());

But these contain duplicates since GroupBy generates a IGrouping for every Cell and adds all matching cells to that. (still trying to find a good way around that)

But for now you can tell for every Cell how many other Cells with the same list of values there are.

Return objects which contain Lists, based on the those lists matching

2 Answers2