Application.Match
 

Then how come this works:

arr3 = ColumnVector(arr2,1)
ResizeArray arr3, 5000, 0
For i = 1 to UBound(arr1,1)
lb = Application.Match(arr1(i,1),arr3,0)
ub = Application.match(arr1(i,1),arr3)
..
..
..

but w/o the ResizeArray arr3, 5000, 0 it doesn't.