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From: cs450a03@uc780.umd.edu
Newsgroups: comp.lang.apl
Subject: RE: J and nested vectors
Message-ID: <22MAR91.00311616@uc780.umd.edu>
Date: 22 Mar 91 00:31:16 GMT
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Since no responses have reached my site yet, I guess I'll field this one... 

Ken Block writes:
>I am under the impression that J does not have nested vectors.
>Is this true?

Not really

>How could I do something like the following apl: 
>x function foreach ((1 2 3) (5 6 7) (1 2 5) (2 3 4))

The function part might be written as   f &. >   if you wanted to
deal with it as a nested array.  Note that you could also convert that
thing to a matrix, and use   f " 1   with no loss in information or
functionality.

&.   by the way means "use the function on the right as a
pre-processor (and it's inverse as a post-processor) for the function
on the left."  The granularity is determined by the granularity of the
"function on the right".  So (f) would get evaluated once for the
array 1 2 3, another time for the array 5 6 7, and so on.

"   means simply, apply the function (on the left) using on arrays of
rank n (the number on the right).  So you get a foreach type operation
that's useful on regular arrays.

To construct the nested array, you could write:
   1 2 3; 5 6 7; 1 2 5; 2 3 4

Grab a copy of J and try it out.  Personally, I prefer answering
questions of people who are stuck than I do people who've never even
gotten that far.

Raul Rockwell