This should work for non-scalar x?
rotate=: <@([ (]|(+ i.))&.> #@[{.$@]) { ]
Thanks,
--
Raul
On Fri, May 30, 2014 at 7:13 PM, Dan Bron <[email protected]> wrote:
> Thanks. Appreciate the improvements too.
>
> For non-scalar x, I think we'd have to mess with transposes, which is
> doable, though overkill for my current purposes.
>
> -Dan
>
> > On May 30, 2014, at 7:04 PM, Raul Miller <[email protected]> wrote:
> >
> > That looks right.
> >
> > Note that (((] | + + i.@]) #) { ]) can be replaced with (((] | [ + i.@])
> #)
> > { ]) or, even ((] | (+ i.)) #) { ]
> >
> > Note also that behavior will be different from |. when x is not a scalar.
> >
> > Thanks,
> >
> > --
> > Raul
> >
> >
> >
> >> On Fri, May 30, 2014 at 6:42 PM, Dan Bron <[email protected]> wrote:
> >>
> >> While trying to implement the Nautical Bell task on RosettaCode [1], I
> >> needed an arithmetic way to express a rotation. The short story is
> clocks
> >> on ships are origin-1, whereas modern clocks are origin zero (midnight
> is
> >> 00:00:00.000).
> >>
> >> Now, if I had a list of all timestamps, the index change is simple: _1
> |.
> >> list_of_timestamps . The problem is, I'm given timestamps as scalars
> >> (i.e. individual points in time), so I don't have a list. All I have is
> a
> >> timestamp and a maximum (which corresponds to the length of the list).
> >>
> >> Anyway, you can see the solution I cobbled together on the page, but
> what
> >> I'm interested in is a general way to express rotations arithmetically
> >> (i.e. as operations on indices). Is it true that after a rotation of R
> >> places, the indices i of a list of length L will be given by L | (L+R)
> + i
> >> ?
> >>
> >> Or, modelled in J:
> >>
> >> rotate =: ((] | + + i.@]) #) { ]
> >>
> >> _1 rotate i. 10
> >> 9 0 1 2 3 4 5 6 7 8
> >>
> >> _1 rotate 'hello world!'
> >> !hello world
> >>
> >> 2 rotate 'abcdefg'
> >> cdefgab
> >>
> >> (rotate"0 _~ i.@#) 'abcdefg'
> >> abcdefg
> >> bcdefga
> >> cdefgab
> >> defgabc
> >> efgabcd
> >> fgabcde
> >> gabcdef
> >>
> >> Can I rely on this identity for future work? Is there any scalar x and
> list
> >> y where x rotate y will differ from x |. y ?
> >>
> >> -Dan
> >>
> >> [1] Nautical Bell task on RosettaCode
> >> http://rosettacode.org/wiki/Nautical_bell#J
> >> ----------------------------------------------------------------------
> >> 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
