Question

I want to create class representing n-dimensional array, but where is a commutative access to its elements. e.g: a[new[] {4, 7, 55}] == a[new[] {55, 4, 7}]

I write this code, where I implement interface IEqualityComparer in order to compare keys (which are arrays) by their real content, but not refs.

using System;
using System.Collections.Generic;
using System.Linq;

class NArray
{
    public int this[int[] x]
    {
        get
        {
            Array.Sort(x);
            return array[x];
        }
        set
        {
            Array.Sort(x);
            array[x] = value;
        }
    }

    public void Remove(int[] x)
    {
        Array.Sort(x);
        array.Remove(x);
    }

    Dictionary<int[], int> array = new Dictionary<int[], int>(new ArrCmpr());
}

class ArrCmpr : IEqualityComparer<int[]>
{
    public bool Equals(int[] a, int[] b)
    {
        return a.Length == b.Length && Enumerable.Range(0, a.Length).All(i => a[i] == b[i]);
    }

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

But when I start to use this class I encounter an exception: “System.Collections.Generic.KeyNotFoundException: The given key was not present in the dictionary.” This exception occurs in next both cases when I try to output the element to console:

NArray a = new NArray();
a[new[] { 1, 3, 2 }] = 4;

Console.WriteLine(a[new[] { 3, 2, 1 }]); //error

 

NArray b = new NArray();
b[new[] { 1, 2, 3 }] = 4;
Console.WriteLine(b[new[] { 1, 2, 3 }]);    //error

So what is the cause of that problem and how could I fix it?

Answer 1

That’s because your implementation of GetHashCode is incorrect: two different arrays with the same items in the same order usually won’t have the same hashcode (because the values are not taken into account), so Equals is never called.

You need an implementation of GetHashCode that takes the values in the array into account:

class ArrCmpr : IEqualityComparer<int[]>
{
    public bool Equals(int[] a, int[] b)
    {
        return a.SequenceEqual(b);
    }

    public int GetHashCode(int[] a)
    {
        return a.Aggregate(0, (acc, i) => unchecked(acc * 457 + i * 389));
    }
}