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
