[erlang-questions] Matrix rotator

Johnny Billquist <>
Mon Jun 15 02:32:05 CEST 2009


Tony Arcieri wrote:
> On Sun, Jun 14, 2009 at 2:08 PM, Johnny Billquist < 
> <mailto:>> wrote:
> 
>     The problem is rather trivial in itself. Are you looking for some
>     particularly efficient algorithm, or what?
> 
> 
> Whatever you'd like, but creative or clever solutions are always 
> interesting.

Ok. A really trivial solution then:

-----------------
rotate([]) ->
     [];
rotate(L = [H|_]) ->
     rotate(length(H),L,[]).

rotate(0,_,R) ->
     lists:reverse(R);
rotate(N,L,R) ->
     rotate(N-1,L,[extract(N,L,[])|R]).

extract(_,[],R) ->
     lists:reverse(R);
extract(N,[H|T],R) ->
     extract(N,T,[lists:nth(N,H)|R]).
------------------


And an example run:

17> test:rotate([[a,b,c],[d,e,f]]).
[[c,f],[b,e],[a,d]]
18>

	Johnny


-- 
Johnny Billquist                  || "I'm on a bus
                                   ||  on a psychedelic trip
email:              ||  Reading murder books
pdp is alive!                     ||  tryin' to stay hip" - B. Idol



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