Answering myself, I suppose I. fulfills most of the criteria for "such that"
e.g. find all primes less than 100.
We want {x | x< 100, x e.PRIMES}
so
( I. @: (1&p:) { ])
does what I want (assuming input is i. 100). But it is difficult to find a
definitive "such that" expression in the above J code.
Perhaps
(I.@: u) { ]
?
> From: [email protected]
> To: [email protected]
> Date: Fri, 26 Dec 2014 16:20:24 +0000
> Subject: Re: [Jprogramming] "Such that" syntax in J
>
> 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: [email protected]
> > To: [email protected]
> > 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 <[email protected]> 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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