Re: New keyword name for linalg.lu

That would break a lot of code, I'm afraid. We can deprecate it possibly,
but I'd say that would take a lot of time.

The helper function is actually ->  np.eye(n)[P, :] that's probably why
mathematica doesn't even bother with the matrix.

On Mon, Apr 24, 2023 at 3:51 PM Evgeni Burovski <[email protected]>
wrote:

> FWIW, I'd suggest to reconsider adding the keyword at all.
> Maybe return a permutation vector (ideally, a LAPACK convention) and
> add a helper function to convert it to a permutation matrix?
>
>
> On Mon, Apr 24, 2023 at 12:41 PM Ilhan Polat <[email protected]> wrote:
> >
> > Dear all,
> >
> > In [1], I have proposed a new keyword for scipy.linalg.lu function,
> with prospective name "p_indices" as a bool switch.
> >
> > This will have the effect of having the permutation matrix P in A = P L
> U decomposition returned as a vector of indices such as [3, 1, 2, 0]
> meaning first and fourth row is swapped. With this you can still compute A
> through L[P, :] @ U without allocating a very costly 2D matrix of 1s and
> 0s. In tall arrays, say 400x10, the resulting 2D array P is 400x400 and in
> the vector form this reduces to 400x1.
> >
> > Hence, the switch to control this behavior is done by a new keyword and
> I don't know if "p_indices" is a nice one (doesn't feel like it). I
> considered, return_indices, perm_as_1d, p_as_1d, p_as_vec and bunch more
> but either too many underscores or too verbose.
> >
> > Thus, I'd like to ask for either validation or new ideas. We should
> maybe ping the NumPy audience for this too as this has no counterpart in
> NumPy.
> >
> > Thanks,
> > ilhan
> >
> > [1] : https://github.com/scipy/scipy/pull/18358
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