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MathGroup Archive 2005

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Re: letrec/named let

  • To: mathgroup at smc.vnet.net
  • Subject: [mg56767] Re: [mg56707] letrec/named let
  • From: Daniel Roy <droy at mit.edu>
  • Date: Thu, 5 May 2005 06:03:03 -0400 (EDT)
  • Organization: MIT
  • References: <200505040433.AAA06220@smc.vnet.net> <opsp9mdot7iz9bcq@monster.ma.dl.cox.net>
  • Sender: owner-wri-mathgroup at wolfram.com

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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