DIP80: phobos additions

[Rikki Cattermole via Digitalmars-d]

On 12/06/2015 9:30 a.m., jmh530 wrote:
> On Tuesday, 9 June 2015 at 03:26:25 UTC, Ilya Yaroshenko wrote:
>
>> There are
>> https://github.com/9il/simple_matrix and
>> https://github.com/9il/cblas .
>> I will try to rework them for Phobos.
>>
>> Any ideas and suggestions?
>>
>
> A well-supported matrix math library would definitely lead to me using D
> more. I would definitely applaud any work being done on this subject,
> but I still feel there are some enhancements (most seemingly minor) that
> would really make a matrix math library easy/fun to use.
>
> Most of what I discuss below is just syntactical sugar for some stuff
> that could be accomplished with loops or std.algorithm, but having it
> built-in would make practical use of a matrix math library much easier.
> I think Armadillo implements some of these as member functions, whereas
> other languages like R and Matlab have them more built-in.
>
> Disclaimer: I don't consider myself a D expert, so I could be horribly
> wrong on some of this stuff.
>
> 1) There is no support for assignment to arrays based on the values of
> another array.
> int[] A = [-1, 1, 5];
> int[] B = [1, 2];
> int[] C = A[B];
>
> You would have to use int[] C = A[1..2];. In this simple example, it’s
> not really a big deal, but if I have a function that returns B, then I
> can’t just throw B in there. I would have to loop through B and assign
> it to C. So the type of assignment is possible, but if you’re frequently
> doing this type of array manipulation, then the number of loops you need
> starts increasing.
>
> 2) Along the same lines, there is no support for replacing the B above
> with an array of bools like
> bool[] B = [false, true, true];
> or
> auto B = A.map!(a => a < 0);
>
> Again, it is doable with a loop, but this form of logical indexing is a
> pretty common idiom for people who use Matlab or R quite a bit.
>
> 3) In addition to being able to index by a range of values or bools, you
> would want to be able to make assignments based on this. So something like
> A[B] = c;
>
> This is a very common operation in R or Matlab.
>
> 4) Along the lines of #2, as an alternative to map, there is no support
> for array comparison operators. Something like
> int[3] B;
> B[] = A[] + 5;
>
> works, but
>
> bool[3] B;
> B[] = A[] > 0;
>
> doesn’t (I’m also not sure why I can’t just write auto B[] = A[] + 5;,
> but that’s neither here nor there). Moreover, it seems like only the
> mathematical operators work in this way. Mathematical functions from
> std.math, like exp, don’t seem to work. You have to use map (or a loop)
> with exp to get the result. I don’t have an issue with map, per se, but
> it seems inconsistent when some things work but not others.
>
> 5) You can only assign scalars to slices of arrays. There doesn’t seem
> to be an ability to assign an array to a slice. For instance, in #1, I
> couldn’t write A[0..1] = B; or A[0, 1] = B; instead of what I had
> written for C.
>
> 6) std.range and std.algorithm seem to have much better support for one
> dimensional containers than if you want to treat a container as
> two-dimensional. If you have a two-dimensional array and want to use map
> on every element, then there’s no issue. However, if you want to apply a
> function to each column or row, then you’d have to use a for loop (not
> even foreach).
>
> This seems to be a more difficult problem to solve than the others. I’m
> not sure what the best approach is, but it makes sense to look at other
> languages/libraries. In R, you have apply, which can operate on any
> dimensional array.  Matlab has arrayfun. Numpy has apply_along_axis.
> Armadillo has .each_col and .each_row (one other thing about Armadillo
> is that you can switch between what underlying matrix math library is
> being used, like OpenBlas vs. Intel MKL).


Humm, work on getting gl3n into phobos or work on my ODBC driver 
manager. Tough choice.