What's the fastest way to compare two arrays for equality?

Question

I have two arrays of objects which are likely to have the same values, but in a different order, e.g.

{ "cat", "dog", "mouse", "pangolin" }

{ "dog", "pangolin", "cat", "mouse" }

I wish to treat these two arrays as equal. What's the fastest way to test this?

Answer 1

I can't guarantee that this is the fastest, but it's certainly quite efficient:

bool areEquivalent = array1.Length == array2.Length 
                     && new HashSet<string>(array1).SetEquals(array2);

EDIT: SaeedAlg and Sandris raise valid points about different frequencies of duplicates causing problems with this approach. I can see two workarounds if this is important (haven't given much thought to their respective efficiencies):

1.Sort the arrays and then compare them sequentially. This approach, in theory, should have quadratic complexity in the worst case. E.g.:

return array1.Length == array2.Length
       && array1.OrderBy(s => s).SequenceEqual(array2.OrderBy(s => s));

2.Build up a frequency-table of strings in each array and then compare them. E.g.:

if(array1.Length != array2.Length)
   return false;

var f1 = array1.GroupBy(s => s)
               .Select(group => new {group.Key, Count = group.Count() });

var f2 = array2.GroupBy(s => s)
               .Select(group => new {group.Key, Count = group.Count() });

return !f1.Except(f2).Any();

Answer 2

I think the only reasonable way is to sort them and then compare.

Sorting requires O(n logn) and comparing O(n), so that's still a total of O(n logn)

Answer 3

Have you tried something like

string[] arr1 = {"cat", "dog", "mouse", "pangolin"};

string[] arr2 = {"dog", "pangolin", "cat", "mouse"};

bool equal = arr1.Except(arr2).Count() == 0 && arr2.Except(arr1).Count() == 0;