Why doesn't Distinct (Linq) work in a List of int[]?

Question

I wrote this method. It's supposed to return all triplets [a, b, c] from an array of integers for which a ^ 2 + b ^ 2 = c ^ 2.

public static List<int[]> AllTripletsThatFulfilla2b2c2Equality(int[] n)
    {
        var combinations = from i in n
                           from j in n
                           from p in n
                           select new int[][] {new int[]{ i, j, p }, new int[] {p, j, i}, new int[] {i, p, j}};
        
        return combinations.Where(x => x.Length == x.Distinct().Count() && Math.Pow(x[0], 2) + Math.Pow(x[1], 2) == Math.Pow(x[2], 2)).Distinct().ToList();
    }

Distinct doesn't work.

I've tried several other ways to write this, including

public static List<int[]> AllTripletsThatFulfilla2b2c2Equality(int[] n)
    {
        var combinations = from i in n
                           from j in n
                           from p in n
                           select new[] { i, j, p };
        combinations = combinations.ToList();
        Func<int[], List<int[]>> combo = x =>
        {
            List<int[]> list = new List<int[]>();
            list.Add(x);
            if (!combinations.Contains(new [] { x[0], x[2], x[1] }))
            {
                list.Add(new[] { x[0], x[2], x[1] });
            }


            if (!combinations.Contains(new[] { x[2], x[1], x[0] }))
            {
                list.Add(new[] { x[2], x[1], x[0] });
            }
            list.Add(new [] { x[0], x[2], x[1] });
            list.Add(new [] { x[2], x[1], x[0] });
            return list;
        };

        return combinations.SelectMany(combo).Distinct().Where(x => x.Length == x.Distinct().Count() && Math.Pow(x[0], 2) + Math.Pow(x[1], 2) == Math.Pow(x[2], 2)).ToList();
    }

and even added a comparer for Distinct:

        public class TripletComparer : IEqualityComparer<int[]>
    {
        public bool Equals(int[] x, int[] y)
        {
            return x[0] == y[0] && x[1] == y[1] && x[2] == y[2];
        }

        public int GetHashCode(int[] obj)
        {
            return obj.GetHashCode();
        }
    }

Same result when tested, this one:

Assert.Equal() Failure
Expected: List<Int32[]> [[3, 4, 5], [4, 3, 5]]
Actual:   List<Int32[]> [[3, 4, 5], [3, 4, 5], [4, 3, 5], [4, 3, 5], [4, 3, 5], ...]

Somehow, it doesn't see that there are items that repeat themselves. Does anyone have any idea why, and how to fix it?

because there is no overloaded comparsion and default check only if it's the same object (same point in the memory) — Selvin, Jun 30 '20 at 10:10
I mean you need to use `Distinct(IEnumerable, IEqualityComparer)` and pass `IEqualityComparer` to point what do you mean when you are saying that `int[]` is equal to another `int[]` — Selvin, Jun 30 '20 at 10:12
Look into the duplicate, in particular this answer, if you can´t (or dont want to) change the class: https://stackoverflow.com/a/1365761/2528063 — MakePeaceGreatAgain, Jun 30 '20 at 10:16
and additional resource https://stackoverflow.com/questions/3232744/easiest-way-to-compare-arrays-in-c-sharp — Selvin, Jun 30 '20 at 10:17
@Selvin How does this link help? OP needs to provide some means for equality anyway, no matter if he´s using `Distinct` or `SequenceEquals`. — MakePeaceGreatAgain, Jun 30 '20 at 10:19
I can´t see where you provide an instance of the equality-comparer to `Distinct`. — MakePeaceGreatAgain, Jun 30 '20 at 10:21
@HimBromBeere with *provide some means for equality* for `int[]` — Selvin, Jun 30 '20 at 10:21
your `GetHashCode`-implementation seems odd. Usually you compute a hashcode based on the exact same members you use within the actual `Equals`-check. — MakePeaceGreatAgain, Jun 30 '20 at 10:23
@HimBromBeere fixed GetHashCode, it worked, thank you so much — , Jun 30 '20 at 10:30

score 0 · Accepted Answer · answered Jun 30 '20 at 10:34

Your GetHashCode-implementation seems odd. Usually you compute a hashcode based on the exact same members you use within the actual Equals-check. Otherwise Distinct will probably not even call into your Equals-method.

public bool Equals(int[] x, int[] y)
{
    return x[0] == y[0] && x[1] == y[1] && x[2] == y[2];
}

public int GetHashCode(int[] obj)
{
    int hash = 17;
    // Suitable nullity checks etc, of course :)
    hash = hash * 23 + obj[0].GetHashCode();
    hash = hash * 23 + obj[1].GetHashCode();
    hash = hash * 23 + obj[2].GetHashCode();
    return hash;
}

Disclaimer: the hashcode-code is adapted from https://stackoverflow.com/a/263416/2528063.

Why doesn't Distinct (Linq) work in a List of int[]?

1 Answers1