Re: [eigen] On the implementation of STL iterator for Eigen::Matrix

[Christoph Hertzberg <chtz@xxxxxxxxxxxxxxxxxxxxxxxx>]

Hi, thanks for the interest!

On 2018-10-02 15:14, Brook Milligan wrote:
> > On Oct 2, 2018, at 6:38 PM, Gael Guennebaud <gael.guennebaud@xxxxxxxxx> wrote:
> >
> > we're eventually implementing STL iterators to iterator over the coefficients of a vector/matrix as well as over the columns or rows of a matrix, and your inputs might be welcome to help converging to a stable API, see below.
> >
> > You can watch the WIP in PR 519 [1], and find some background in bug 231 [2]. Basically, to avoid ambiguities, we decided that iterators over coefficients would be defined for 1D expression only, meaning that to iterate over the elements of a 2D Matrix you'll have to explicitly reshape it as a 1D vector first.
>
> Have you considered a solution such as used by Boost GIL (Generic Image Library)?  That has the concept of "pixel locators" which allow both 1-D and 2-D (i.e., up/down, right/left) movement through the pixels of an image.
>
> In fact, I would be surprised if you cannot adapt a matrix to the GIL "image" concept and get a whole lot for free.
>
> In any case, I think it would be worth investigating before you settle on the arbitrary restriction of a 1-D-only API.

The main idea here is to have something which is compatible with the std 
algorithms. And ranged-for-loops.
Glancing over the GIL documentation on PixelLocators they seem to 
intentionally distinguish them from Iterators (which they implement, as 
well).

I would not rule out that Eigen could also provide something like 2D 
locators (not sure if there is a direct use case for that, though). But 
that would be independent of the current iterator implementation (of 
course, some code could be shared).


Christoph



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