It seems that "such that" is indistinguishable (to me) with select. While I do
not use a "select" utility adverb (I write out a # expression each time),
making these utility adverbs is a great step in learning J, and making the
language fit the concepts you already understand in your mind. With that said,
my ideal definition of select is:
select =: 1 : 'u # ]'
The big difference with yours is that u can be a dyadic verb.
1000 (< *:) select i.50
________________________________
From: Jon Hough <jgho...@outlook.com>
To: "programm...@jsoftware.com" <programm...@jsoftware.com>
Sent: Friday, December 26, 2014 12:07 PM
Subject: Re: [Jprogramming] "Such that" syntax in J
Another example
suchthat =: adverb define
(#~u) y
)
Find all positive int x less than 100 such that x^2 > 1000
((1000&<)@:*:) suchthat i. 100
This seems to be what I wanted.
Come to think of it, since the expression is
{x | x < 100 AND x^2 > 1000}
the i. 100 part should belong on the left side since it is part of the "such
that" condition.
> Date: Fri, 26 Dec 2014 09:00:15 -0800
> From: rogerhui.can...@gmail.com
> To: programm...@jsoftware.com
> Subject: Re: [Jprogramming] "Such that" syntax in J
>
> Using -: in that context requires extra parens. Besides, ←→ is prettier.
>
> On Fri, Dec 26, 2014 at 8:52 AM, R.E. Boss <r.e.b...@outlook.com> wrote:
> >
> > Why not use the primitive '-:' ?
> >
> > (+/(1+i._)^-s) -: */%1-(p:i._)^-s
> >
> >
> > R.E. Boss
> >
> >
> > > -----Original Message-----
> > > From: programming-boun...@forums.jsoftware.com [mailto:programming-
> > > boun...@forums.jsoftware.com] On Behalf Of Roger Hui
> > > Sent: vrijdag 26 december 2014 17:28
> > > To: Programming forum
> > > Subject: Re: [Jprogramming] "Such that" syntax in J
> > >
> > > ←→ are metasymbols, commonly used in mathematics to denote
> > > equivalence.
> > >
> > >
> > >
> > > On Fri, Dec 26, 2014 at 8:20 AM, Jon Hough <jgho...@outlook.com> wrote:
> > > >
> > > > Thanks for replying.
> > > >
> > > > I'm confused about two things.
> > > >
> > > > (1) I'm worried I haven't explained myself well.
> > > > I mean I want to find all x given g(x) is true for some function g.
> > > >
> > > > i.e. the set { x | g(x) is true }
> > > > (You may have answered this, I need to read your example more
> > carefully,
> > > > but this leads me to problem 2...)
> > > >
> > > > (2) I don't understand what
> > > > ←→
> > > > means in terms of J. These are not ASCII characters or J primitives.
> > > >
> > > > Thanks.
> > > >
> > > > > Date: Fri, 26 Dec 2014 08:13:40 -0800
> > > > > From: rogerhui.can...@gmail.com
> > > > > To: programm...@jsoftware.com
> > > > > Subject: Re: [Jprogramming] "Such that" syntax in J
> > > > >
> > > > > f x
> > > > >
> > > > > where x is an array of all values of interest. For (countably)
> > infinite
> > > > > sets you have to express x in terms of i._ . For example, to
> > express the
> > > > > Euler product formula for the Riemann zeta function,
> > > > >
> > > > > +/(1+i._)^-s ←→ */%1-(p:i._)^-s
> > > > >
> > > > >
> > > > >
> > > > > On Fri, Dec 26, 2014 at 8:04 AM, Jon Hough <jgho...@outlook.com>
> > > wrote:
> > > > > >
> > > > > > In Haskell (and other languages I'm sure) one can express the idea
> > of
> > > > > > "such that" to denote one expression depending on another. In
> > Haskell
> > > > we
> > > > > > can use the syntax:
> > > > > >
> > > > > > [ f x | x <- xs ]
> > > > > >
> > > > > > which means
> > > > > > "the list of all f x
> > > > > > such that x is drawn from xs."
> > > > > >
> > > > > > So | is syntactically similar to the English "such that". (and very
> > > > much
> > > > > > like | in mathematical sets)
> > > > > >
> > > > > > I would like to know if J has a construct to express the above
> > Haskell
> > > > > > code.
> > > > > >
> > > > > > Thanks,
> > > > > > Jon.
> > > > > >
> > > > > >
> > ----------------------------------------------------------------------
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> > > > > >
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