Re: Any easy/build-in construct to extract submatrices from larger matrix?

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Fri, 6 Jul 2012 18:47:22 -0500, Randy Brukardt wrote:

> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
> news:3u95tkqb1l8t$.t7lzt049tvq8.dlg@40tude.net...
>> On Wed, 4 Jul 2012 20:33:32 -0500, Randy Brukardt wrote:
> ...
>>> This is distributed overhead, which programs would  have to pay whether or
>>> not they used any 2-d slices. It would make all array parameters more
>>> expensive.
>>
>> By-value passed slices should impose no overhead.
> 
> Sure. but they're not a general solution.

I suppose it is for a great majority of people asking for this feature.
E.g. those doing linear algebra stuff.

> That is, they don't work for slices of arrays of by-reference types,

Yes, but nD arrays of those are rare. One important exception could be
arrays of controlled types used to implement fat pointers, e.g. handles.

>>>> Slices should be first class, that would remove the burden of
>>>> implementation from compiler writers. The rest is just same problematic as
>>>> with referencing single array/container element. There is no big
>>>> difference for one element or a set of elements.
>>>
>>> That's not really possible so long as slices are what C calls l-values. The
>>> problem is passing slices as parameters and allowing assignments into 
>>> them.
>>
>> Why does this work for array elements then?
> 
> (There is a "not" missing in what I wrote above.)
> 
> It works for array elements only because they are contiguous and passing a 
> single address is sufficient to represent the slice, as it is for any array 
> objects. (Recall that we're talking about *constrained* array parameters 
> here.)

That is why you could pass it copy-in/copy-out as you would with scalar
array elements. Yes, it would be much work and likely many compiler bugs,
but not impossible.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de