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From: sam@kalessin.jpl.nasa.gov (Sam Sirlin)
Newsgroups: comp.lang.apl
Subject: Re: Questions about J.
Message-ID: <1991Jan9.171347.19535@jato.jpl.nasa.gov>
Date: 9 Jan 91 17:13:47 GMT
References: <15160009@hpdmd48.boi.hp.com>
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Reply-To: sam@kalessin.jpl.nasa.gov (Sam Sirlin)
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In article <15160009@hpdmd48.boi.hp.com>, rrr@hpdmd48.boi.hp.com (Rudi Rynders) writes:
|> OK, how DOES one catenate two m-rowed arrays in the "horizontal" direction in J?
|> A pretty basic question , I admit. But why is it so hard to find?
|> 
|> Anyone care to explain in detail the "OVER" and "BY" constructs?.
|> 
|> Rudi Rynders  (rrr@ hpdmd48.boi.hp.com)

To catenate two arrays on their last axis, use under:

  a ,&.|: b

This takes the transpose to start, catenates, then transposes the result (I 
think). It does seem to work at least for matrices. I suspect that with 
dyadic transpose one could catenate about arbitrary axes, albeit rather 
more cryptically than in traditional APL. And people used to say that APL was
a write only language! As to over and by, my list (2.7) doesn't have those 
names, so I would have to consult other documentation. Did you send away to
J for this? At least it gives mathematical details, which means you can 
eventually (using tests) figure them out. Consider it a puzzle.



I've not been able to find a way to do the equivalent of reassignment:

  A[I] .is X

in J. Is this really impossible? Doesn't this make it hard to do some things
that FORTRASH even does easily (such as setting up similar arrays)? Or 
maybee I should compare to Matlab, the other ascii APL derivative.


-- 
Sam Sirlin
Jet Propulsion Laboratory         sam@kalessin.jpl.nasa.gov