Re: Unexpected result with inner product

[Dr . Jürgen Sauermann]

    Hi Elias,
      
      I suppose the reason is roughly this:
      
      Some interpreter, including IBM APL2 and GNU APL, sometimes
      allow 1-element vertors (lets call them quasi-scalars) in places
      where strictly speaking scalars would be required.
      
      Your partial results 0/x if some a=0 is always a vector while 1/x
      for some other a-1 is always 1 1-element vector which is subject
      to be being treated as a scalar instead.
      
      When the the inner product f.g and the outer product ∘.g gets
       a non-scalar result from g then it will enclose that result
      before
      the f/ and disclose it again after the f/.
      
      The final disclose will in your case see a mix of 0-element and
      1-element vectors and will scalar-entend the 1-element
      quasi-scalars to the common shape of all items which is,
      in your example empty).
      
      A different A reveals this:
      
            (1⌈A≠0) +.Q B
         6  6 
         6  6 
         6  6 
      
      Best Regards,
      Jürgen

On 1/26/22 5:25 AM, Elias Mårtenson wrote:

Consider the following code:

    A←3 4⍴1 3 2 0 2 1 0 1 4 0 0 2
    B←4 2⍴4 1 0 3 0 2 2 0
    Q←{⍺/⍵}
    (A≠0) +.Q B

My reading (and implementation) of the ISO spec suggests the output should be the following:

┏━━━┓
┃4 6┃
┃6 4┃
┃6 1┃
┗━━━┛

However, in GNU APL I get this:

┏→━━━━━━┓
↓┏⊖┓ ┏⊖┓┃
┃┃0┃ ┃0┃┃
┃┗━┛ ┗━┛┃
┃┏⊖┓ ┏⊖┓┃
┃┃0┃ ┃0┃┃
┃┗━┛ ┗━┛┃
┃┏⊖┓ ┏⊖┓┃
┃┃0┃ ┃0┃┃
┃┗━┛ ┗━┛┃
┗∊━━━━━━┛

Which one is correct?

Regards,

Elias