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MathGroup Archive 2005

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Re: letrec/named let

  • To: mathgroup at smc.vnet.net
  • Subject: [mg56770] Re: [mg56707] letrec/named let
  • From: DrBob <drbob at bigfoot.com>
  • Date: Thu, 5 May 2005 06:03:30 -0400 (EDT)
  • References: <200505040433.AAA06220@smc.vnet.net> <opsp9mdot7iz9bcq@monster.ma.dl.cox.net> <1115235138.31794.120.camel@30-5-214.wireless.csail.mit.edu>
  • Reply-to: drbob at bigfoot.com
  • Sender: owner-wri-mathgroup at wolfram.com

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
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>>
>>
>>
>
>
>
>



-- 
DrBob at bigfoot.com


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