[Jprogramming] APL Question

[Raul Miller]

On 6/12/07, Will Slade <willhslade at gmail.com> wrote:
> I can return the first dimension of A like so: A[1;;]
>
> Is there a way to return the nth dimension of A without knowing what the
> dimensions of A are? I'd like the flexibility to arbitrarily return the nth
> dimension, but I don't know how to do this in APL, and the error messages
> are a little cryptic for someone with my inexperience.

As you posted this to the J programming forum, I'm presuming you're
interested in knowing how to do this from a J point of view as well.

Of course, J makes this rather simple (since you can ignore trailing
dimensions when you use its index function).  But some approaches
from J should also be valid in APL.

For example, let's say your array has dimensions of either 2 3 5 7
or 2 3 4, and you want the second item, and you want to limit yourself
to J operations which correspond to APL operations.  (I'll presume your
APL has nested arrays).

index=:4 :0
   shape=.$y
   inds=:i.&.>shape
   inds=. (<x),}.inds
   indices=. >+/&.>/inds*&.>|.*/\|.}.shape,1
   indices { ,y
)

Translation notes:
   $ is rho, =. is gets and }. is 1 drop
   i.&.> is iota each
   (<x),}.inds can instead use indexing or whatever you like
   >+/&.>/ is jot dot plus reduce, then removing the nesting
   indices {,y is (,y)[indices]

Anyways, I think this approach should work for most APLs,
but I haven't actually tried it.

-- 
Raul