Re: Potential problem in diadic iota (⍳) or in Pick (⊃)

[Kacper Gutowski]

On Thu, Aug 06, 2020 at 08:50:27PM -0400, Christian Robert wrote:
>      ∇z←(F fasteach) A;shp;vec;nub;res;idx
> shp←⍴A               ⍝ Shape and ...
> vec←,A               ⍝ ... ravel of arg.
> nub←∪ vec            ⍝ Vector of unique elements.
> nub←∪ vec            ⍝ Vector of unique elements.
> res←F¨nub            ⍝ Result for unique elts.
> idx←nub ⍳ vec        ⍝ Indices of arg in nub ...
> z←shp ⍴ idx ⊃¨⊂ res  ⍝ ... distribute result.
> ∇

The problem is indeed in iota and it reminds me a similar problem we had 
with an unique before.  For sufficiently big arrays, a different 
algorithm is used where the left argument is sorted beforehand to speed 
up searching; and something goes wrong there.

Smaller reproducer:

      A←⌽⍳64
      B←⍳6
      B≡A[A⍳B]
0

Should be 1, of crouse.

-k