Re: scipy.linalg.lu considerations

Oh and just to add, if permute_l=True is given "P" would return empty but
still 3 output arguments.

On Thu, Apr 6, 2023 at 2:17 PM Ilhan Polat <[email protected]> wrote:

> Dear all,
>
> As I recently figured out (again) that SciPy has some custom Fortran code
> for generating LU decomposition and determinant [1], I started
> decommissioning those parts as we are slowly enabling nDarray support and
> some performance tweaks in the meantime [2].
>
> I am about to post a PR for scipy.linalg.lu and it occured to me that we
> still have the legacy behavior that returns different number of outputs
> depending on the parameters (from the matlab inspired times). We painfully
> learned in the past not to do this anymore but we still have this behavior
> in parts of the API. I can definitely keep this behavior with no problem.
> Hence it is not a matter of possibility. But;
>
> As everybody knows, lu performs the decomposition A = P L U with P
> permutation matrix, lower and upper trapezoidal arrays. Now, forming a P as
> a complete and very sparsely populated array is a waste of memory if you
> are not interested and hence other software offers to return it as a 1D
> permutation array (SciPy doesn't but it is trivial to add this ala matlab
> [3] )
>
> So proposition 1: Adding the output_form (or some other kwarg name) If the
> input is ndarray with shape (..., n, m) P becomes (..., min(m, n)) in the
> output_form='vector' case.
>
> Proposition 2: Going through the very annoying process of deprecating the
> mutating output arg number and return always p, l, u. Note that this is a
> central function in the number crunching community as there is no
> substitute in NumPy.
>
> I want to get some feedback from the folks about this to assess the weight
> of these propositions. Proposition 1 is basically just informing, I am +1
> with the second one but definitely not looking forward to it :)
>
> Let me know,
>
> Best,
> ilhan
>
>
>
> [1]: https://github.com/scipy/scipy/blob/main/scipy/linalg/src/lu.f
> [2]: https://github.com/scipy/scipy/pull/18225
> [3]: https://www.mathworks.com/help/matlab/ref/lu.html#d124e909668
>

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