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Implement a `unique` function returning only the unique values in a vector.

Aperta
  • Qualcosa non va?
#940 6 commenti 3 reazioni 0 assegnatari Vedi su GitHub

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Valutazione

Difficoltà
5/5
Tempo stimato
Più di una settimana
Idoneità per principianti
25/100
Tipo di issue
Funzionalità
Chiarezza
Abbastanza chiara
Stato di attività
Ferma
Stack tecnologico
fortran
Ambito
data

Direzione di ricerca

Start by checking whether stdlib already provides a suitable function, as the issue asks. The proposed starting scope is a unique function for one dimensional integer, real, complex, and character vectors, with optional sorting; the issue does not identify files or tests. Define and agree on the supported cases and behavior before implementation, then verify the result covers the agreed scope.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

idea topic: algorithms topic: container
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 function

The 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.

Lingua principale
Fortran
Stelle
1.4k
Fork
233
Merge medio
3g 6h
PR unite (30g)
5

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