I guess a pattern of "filter" is
(#~ filter) y
where filter is a monad verb that return 0 or 1
For your case, it would (untested)
(#~ odd) i.9
for Chris' example the filter is 1: = #...@q:
there are other variations such ({~ filter) y where filter return index to
be taken.
Втр, 24 Авг 2010, Alex Gian писал(а):
> Apologies for the lame noob query again, but I've already been through a
> few pages of scribblings and unsuccessful attempts.
> I've also read and followed Chris Burke's "Elegant Program - #~1:=...@q:"
> which deals with a similar theme, and searched the site without much to
> show for it. Also tried using the "13 :" cheat, no joy there, either.
>
> Basically I want a pretty bog standard filter function, textbook h.o.p.
> example as given in the Lisp family, but obviously in J tacit style, not
> recursive. Say the left argument will be a boolean function and the
> right arg the string to filter.
> So, e.g.
>
> odd =: 2&|
>
> NB. This is OK
> (odd i.9) # i.9
> 1 3 5 7
>
> NB. but if I try to make "filter" a function, somehow along the lines
> NB. This definitely does NOT work
> filter =: ([]) # ]
> odd filter i.9
> 1 0 0 1 1 1 0 0 0 0 1 1 1 1 1 0 0 0 0 0 0 1 1 1 1 1 1 0 0 0 0 0 0 0 0
>
> The above is one of the less bizarre of my attempts. Seems to have
> inverted left and right, gawdknowswhy.
> What am I doing wrong?
>
> I have to say, that as an interested and eager newcomer, the hurdles for
> such basic, almost calculator-like functionality - take for granted in
> any modern language - are the biggest turn-off.
>
> Perhaps when I've got my head around it, I could write up a little "lame
> noob's" guide.
> Yes, dot product and matrix multiplication will also feature in it!
> Perhaps a simple file (loadable by default) implementing such functions
> would be useful to other newcomers. Just my 2¢
>
> At the moment J feels not so much like an array language, but like an
> assembler for an array system. :D
>
>
>
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regards,
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