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c#arraysalgorithmequality
izb
izb

Reputation: 51761

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

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?

Upvotes: 14

Views: 16418

Answers (6)

Adriaan Stander
Adriaan Stander

Reputation: 166386

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;

Upvotes: 1

Mohammad Mazaz
Mohammad Mazaz

Reputation: 316

Pseudocode :

A:array
B:array
C:hashtable

if A.length != B.length then return false;

foreach objA in A
{
H = objA;
if H is not found in C.Keys then
C.add(H as key,1 as initial value);
else
C.Val[H as key]++;
}

foreach objB in B
{
H = objB;
if H is not found in C.Keys then
return false;
else
C.Val[H as key]--;
}

if(C contains non-zero value)
return false;
else
return true;

Upvotes: 0

Ani
Ani

Reputation: 113402

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();

Upvotes: 19

Martin DeMello
Martin DeMello

Reputation: 12336

Convert both arrays to HashSets and use setequals

Upvotes: 2

usr-local-ΕΨΗΕΛΩΝ
usr-local-ΕΨΗΕΛΩΝ

Reputation: 26874

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)

Upvotes: 3

Albin Sunnanbo
Albin Sunnanbo

Reputation: 47038

I would use a HashSet, assuming there are no duplicates

string[] arr1 = new string[] { "cat", "dog", "mouse", "pangolin" };
string[] arr2 = new string[] { "dog", "pangolin", "cat", "mouse" };

bool result = true;
if (arr1.Length != arr2.Length)
{
    result = false;
}
else
{
    HashSet<string> hash1 = new HashSet<string>(arr1);
    foreach (var s in arr2)
    {
        if (!hash1.Contains(s))
            result = false;
    }
}

Edit:
If you just have four elements it might be faster to skip the HashSet and use arr1.Contains in the comparison. Measure and pick the fastest for your array size.

Upvotes: 0

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