In a word: performance.
a=. 1e6?@$0
b=. <:+:1e6?@$0
c=. 1e6?@$2
10 ts 'r=. c}b,:a'
0.0066557413 8390912
10 ts 'c}b,:a'
0.021307987 41945088
On Thu, Dec 22, 2011 at 11:00 AM, Johann Hibschman <jhibsch...@gmail.com>wrote:
> Thanks, everyone. I never understood item-amend until now. That's
> exactly what I was looking for. For context, I was reading through the
> APL book and hit the "/a,u,b/" notation, and wondered how to do that
> in J.
>
> One question though: why is the particular form (result=:c}b,:a)
> singled out, rather than the general (c}b,:a)?
>
> On Wed, Dec 21, 2011 at 10:54 AM, Henry Rich <henryhr...@nc.rr.com> wrote:
> > Note that
> >
> > c}b,:a
> >
> > does indeed compute b,:a. But if it's used in the precise form
> >
> > result =: c}b,:a
> >
> > the computation is avoided.
> >
> > b =: 1e6$4
> > a =: 1e6$4
> > c=: ? 1e6$ 2
> > ts 'c}b,:a'
> > 0.0401299 2.09725e7
> > ts 'q =: c}b,:a'
> > 0.0112949 4.19558e6
> >
> > There are other special cases; see the Special Code page.
> >
> > Henry Rich
> >
> > On 12/21/2011 11:43 AM, Marshall Lochbaum wrote:
> >> A more obvious way is to use (c { b,.a) . The key is to realize that
> saying
> >> (if c then a else b) is just selection: if c is 0, we pick b, and if c
> is
> >> 1, we pick a. So we rephrase as (c { b,a), and J's implicit rank allows
> us
> >> to do (c { b,.a) for vector c, a, and b.
> >>
> >> That said, (c} b,:a) is faster, and uses less memory by not actually
> >> computing b,:a . So it's a better choice.
> >>
> >> Marshall
> >>
> >> On Wed, Dec 21, 2011 at 10:26 AM, Brian Schott<schott.br...@gmail.com
> >wrote:
> >>
> >>> One way is the following.
> >>>
> >>> c}b,:a
> >>> 10 2 30
> >>>
> >>>
> >>> --
> >>> (B=)
> >>> ----------------------------------------------------------------------
> >>> For information about J forums see http://www.jsoftware.com/forums.htm
> >>>
> >> ----------------------------------------------------------------------
> >> For information about J forums see http://www.jsoftware.com/forums.htm
> >>
> > ----------------------------------------------------------------------
> > For information about J forums see http://www.jsoftware.com/forums.htm
> ----------------------------------------------------------------------
> For information about J forums see http://www.jsoftware.com/forums.htm
>
--
Devon McCormick, CFA
^me^ at acm.
org is my
preferred e-mail
----------------------------------------------------------------------
For information about J forums see http://www.jsoftware.com/forums.htm
