Re: [Bug-apl] Supporting negative ranks for ⍤ operator

[Alexey Veretennikov]

Hi,

Here is the definition
http://help.dyalog.com/14.1/Content/Language/Primitive%20Operators/Rank.htm

Juergen Sauermann <address@hidden> writes:

> Hi Louis,
>
> just for curiosity, where are negative ranks defined?
> My version of "Mastering Dyalog APL" (ISBN : 978-0-9564638-0-7) does
> not even mention the rank operator.
>
> /// Jürgen
>
> On 04/28/2016 12:31 AM, Louis de Forcrand wrote:
>
>
>  Whoops. Looks like I got here too late. 
>  Well done!
>
>  Louis
>
>  On 28 Apr 2016, at 00:29, Louis de Forcrand <address@hidden> wrote:
>
>  The three-item form is used if the associated function is ambivalent 
> (applied to the P-cells of ⍵
>  if monadic, applied to corresponding Q-cells of ⍺ and R-cells of ⍵ if 
> dyadic). I don't believe it is
>  possible to define ambivalent functions in ISO APL however, so it is kind of 
> redundant. It is
>  probably left over from Sharp APL. 
>
>  As to the negative rank, I believe that it is an obvious flaw in the 
> standard. Being able to apply
>  a function to the items of the argument is incredibly useful. Of course this 
> is achievable like so:
>  {⍺ (f ⍤ (¯1+≢⍴⍺⍵)) ⍵} ⍝ without ⍺ for the monadic form
>  Now that I've checked, I'm pretty sure that GNU APL does support rank 
> (negative or not), but
>  not with all primitives. Try it out with ⊖ or +⌿ on a rank-3 or above array, 
> and then with
>  ]BOXING and ⊂ on the same array… looks like a bug to me.
>
>  The rank operator isn't easy to grasp, and it's surely harder to implement.
>
>  Good luck,
>  Louis
>
>  On 27 Apr 2016, at 13:14, Jay Foad <address@hidden> wrote:
>
>  Incidentally, it works like this in Dyalog and NARS2000 too, though
>  the Dyalog documentation doesn't mention the 3-item form.
>
>  Jay.
>
>  On 27 April 2016 at 09:02, Jay Foad <address@hidden> wrote:
>
>
>  Given g ← f⍤P Q R:
>  P is the monadic rank
>  Q is the left rank
>  R is the right rank
>
>  So:
>  g Y applies g to the P-cells of Y
>  X g Y applies g to the Q-cells of X and the R-cells of Y
>
>  The ⌽3⍴⌽y1 stuff is just a too-cute way of saying that you can specify
>  fewer than 3 values in the right operand, and:
>  R is shorthand for R R R
>  Q R is shorthand for R Q R
>
>
>

-- 
Br,
/Alexey