Intersect two arrays in VBA (no excel) - arrays

Question

I have two string arrays with quite a few items. The items are unique in each array: I sort the array alphabetically and remove duplicates as I do so.
I want to know how many of the items in array a are also present in array b. As soon as there is even one of such items that occurs in both arrays, I "know enough".
And even though both arrays are sorted alphabetically first, doing a double loop as below is very heavy on the processor and terribly slow.
So I'm looking for an alternative, or even an other approach from the start...
Dim aryA(1 To 10000) As String
Dim aryB(1 To 10000) As String
Dim x As Long
Dim y As Long
Dim Counter as long
Counter = 0
'fill arrays with many values
For x = 1 to 10000
For y = 1 to 10000
If aryA(x) = aryB(y) Then
counter = counter + 1
Exit For
End If
Next y
Next x
If Flag = True Then
'run amazing code here
End If
End Sub```
Thanks!

Answer 1

You should try using Dictionaries instead of arrays.
I did a test with 1'000, 10'000 and 100'000 words and compared all elements (so not breaking when a duplicate was found) on a very regular computer. Using 1'000 elements was ready instantly, 10'000 took not much more that the blink of the eye, 100'000 2-3 seconds.
I don't know how your original data is from or how you will your arrays. The following code just copies the arrays into dictionaries and then loop over the keys of the first dictionary and looks if it is present in the second. No need to have the data sorted, and duplicates are eliminated automatically when the dictionary is build.
If your not familiar with Dictionaries: You need to add a reference to the "Microsoft Scripting Runtime" (or change code to late binding).
Sub arrayTest()
' (omitting my test code to fill up the arrays)
' Create Dictionaries
Dim d1 As New Dictionary
Dim d2 As New Dictionary
For x = 1 To UBound(aryA)
If Not d1.Exists(aryA(x)) Then d1.Add aryA(x), vbNullString
Next x
For y = 1 To UBound(aryB)
If Not d2.Exists(aryB(y)) Then d2.Add aryB(y), vbNullString
Next
' Compare the dictionaries
Counter = 0
Dim k
For Each k In d1.Keys
If d2.Exists(k) Then Counter = Counter + 1
Next
Debug.Print "Done: " & Counter
End Sub
If you just need this check once and don't do anything with the data afterwards, of course it is not necessary to create the second dictionary:
For x = 1 To UBound(aryA)
If Not d1.Exists(aryA(x)) Then d1.Add aryA(x), vbNullString
Next x
For y = 1 To UBound(aryB)
If d1.Exists(aryB(y)) Then Counter = Counter + 1
Next
Debug.Print "Done: " & Counter

Answer 2

If both array are sorted you could just go through them considering their ordering in linear time:
Pseudo code:
i=0;
j=0;
while i < len(aryA) and j < len(aryB) do
if aryA[i] == aryB[j] then
counter++;
i++;
j++;
elseif aryA[i] > aryB[j] then
j++
else
i++;
fi
done