Motivation
Recently, I've run into the problem of extracting unique values in a vector (of any integer, real or complex type or possibly even character). Consider for instance the following vector x = [1, 2, 3, 3, 4]. What I'd need is a function taking x as input and returning the vector y = [1, 2, 3, 4] as output. The interface for a real-valued vector could be as simple as
pure function unique(x, sorted) result(y)
real(dp), intent(in) :: x(:)
!! Array whose unique values need to be extracted.
logical(lk), optional, intent(in) :: sorted
!! Whether the output vector needs to be sorted or not (default .false. ?)
real(dp), allocatable :: y(:)
!! Vector containing only the unique values from x.
end functionThe output vector could be sorted or not, depending on the user's choice. I know that there are no Fortran intrinsic functions for that purpose, but I ain't sure something like that is already available in stdlib. If I'm wrong, could anyone point me to the correct function?
Prior Art
- In Matlab, there is the
unique function whose description is available here.
- Python has the
set function taking as input a list and returning only the unique elements of this list.
- Numpy has
np.unique whose description is available here.
- @jacobwilliams provides an integer-based implementation on his blog (here).
Additional Information
Both Matlab and Numpy's implementations cover a relatively large set of cases (1D-array, multidimensional arrays, different types, etc) and return values (the unique elements, the corresponding indices, indices to the reconstruct the original array from this unique set, etc).
I don't know if absolutely all these cases need to be covered (at least as a starting point). I would probably recommend to start with the simplest ones (i.e. only input vectors and output vector with the unique elements) as these are probably the most common situations where a unique function might be needed. That would include integer, real, complex and character 1D-arrays.
Motivation
Recently, I've run into the problem of extracting unique values in a vector (of any integer, real or complex type or possibly even character). Consider for instance the following vector
x = [1, 2, 3, 3, 4]. What I'd need is a function takingxas input and returning the vectory = [1, 2, 3, 4]as output. The interface for a real-valued vector could be as simple asThe output vector could be sorted or not, depending on the user's choice. I know that there are no Fortran intrinsic functions for that purpose, but I ain't sure something like that is already available in
stdlib. If I'm wrong, could anyone point me to the correct function?Prior Art
uniquefunction whose description is available here.setfunction taking as input alistand returning only the unique elements of this list.np.uniquewhose description is available here.Additional Information
Both Matlab and Numpy's implementations cover a relatively large set of cases (1D-array, multidimensional arrays, different types, etc) and return values (the unique elements, the corresponding indices, indices to the reconstruct the original array from this unique set, etc).
I don't know if absolutely all these cases need to be covered (at least as a starting point). I would probably recommend to start with the simplest ones (i.e. only input vectors and output vector with the unique elements) as these are probably the most common situations where a
uniquefunction might be needed. That would includeinteger,real,complexandcharacter1D-arrays.