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MathGroup Archive 2005

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Re: letrec/named let

  • To: mathgroup at smc.vnet.net
  • Subject: [mg56774] Re: [mg56707] letrec/named let
  • From: Daniel Roy <droy at mit.edu>
  • Date: Thu, 5 May 2005 06:04:20 -0400 (EDT)
  • Organization: MIT
  • References: <200505040433.AAA06220@smc.vnet.net> <opsp9mdot7iz9bcq@monster.ma.dl.cox.net> <1115235138.31794.120.camel@30-5-214.wireless.csail.mit.edu> <opsp9n4xswiz9bcq@monster.ma.dl.cox.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Ahah.. This works.  Very good solution!  (much faster than my solution
whose performance is dependent on the absolute size of the values !
ugly)

dan

On Wed, 2005-05-04 at 14:51 -0500, DrBob wrote:
> Clear@CompressNumericalSequence
> CompressNumericalSequence[s_List] := Module[{u = Union@s},
>      s /. Thread[u -> Range@Length@u]
>      ]
> CompressNumericalSequence@{5, 1, 1, 2, 3, 4}
> 
> {5,1,1,2,3,4}
> 
> Bobby
> 
> On Wed, 04 May 2005 15:32:18 -0400, Daniel Roy <droy at MIT.EDU> wrote:
> 
> > I just figured out why the suggestions I've been given fail on my test
> > cases.  Its because the sets that I am passing can have repeated digits.
> >
> > CompressNumericalSequence@{10,2,2,4,7,8}
> > {5,1,1,2,3,4}
> >
> > Otherwise, your solution of Ordering@Ordering is MUCH faster.
> > Unfortuantely, it gives the wrong answer on the above test.
> >
> > -dan
> >
> >
> > On Wed, 2005-05-04 at 14:14 -0500, DrBob wrote:
> >> I have no idea what "named let" does -- not a hell of a lot, I suspect -- but here's a simple replacement for the other routine:
> >>
> >> CompressNumericalSequence[s_List]:=Ordering@Ordering@s
> >> CompressNumericalSequence@{10,2,4,7,8}
> >>
> >> {5,1,2,3,4}
> >>
> >> Bobby
> >>
> >> On Wed, 4 May 2005 00:33:53 -0400 (EDT), Daniel Roy <droy at mit.edu> wrote:
> >>
> >> > hi.  i'm a lisper/schemer and i'm working with mathematica.  i
> >> > appreciate the lisp-like nature of mathematica but i can't seem to
> >> > easily replicate some of the functionality i like which is forcing me to
> >> > write ugly side-effect code.
> >> >
> >> > for instance, how do you do the equivalent of a "named let" in
> >> > mathematica (NOTE! I know i can take the max of a list, this is just a
> >> > simple example of a named let)
> >> >
> >> > (define (max-of-list lst)
> >> >   (let loop ((lst (cdr lst))
> >> >              (best (car lst)))
> >> >     (if (null? lst)
> >> >         best
> >> >         (loop (cdr lst)
> >> >               (if (> (car lst) best)
> >> >                   (car lst)
> >> >                   best)))))
> >> >(max-of-list '(1 2 3 4 5 2))
> >> >> 5
> >> >
> >> > Here is a mathematica function to compress a sequence numerically.
> >> > here is one attempt using functions where i pass the function to
> >> > itself... there has to be a better way
> >> >
> >> > CompressNumericalSequence[S_] := Module[
> >> >       {C = Function[{C, R, i},
> >> >             If[i < Max[R],
> >> >               If[Length[Position[R, i]] == 0,
> >> >                 C[C, (If[# > i, # - 1, #]) & /@ R, i],
> >> >                 C[C, R, i + 1]],
> >> >               R]]},
> >> >       C[C, S, 1]];
> >> >
> >> > CompressNumericalSequence[{10, 2, 4, 7, 8}]
> >> > {5, 1, 2, 3, 4}
> >> >
> >> > Also, is it possible to do letrec in mathematica?  (essentially, i know
> >> > i can do recursive function declarations at the top level... my question
> >> > is whether i can do them at lower levels?)...
> >> >
> >> > thanks, dan
> >> >
> >> >
> >> >
> >> >
> >> >
> >> >
> >> >
> >>
> >>
> >>
> >
> >
> >
> >
> 
> 
> 


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