       Re: letrec/named let

• To: mathgroup at smc.vnet.net
• Subject: [mg56768] Re: [mg56707] letrec/named let
• From: DrBob <drbob at bigfoot.com>
• Date: Thu, 5 May 2005 06:03:11 -0400 (EDT)
• References: <200505040433.AAA06220@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```Here are four ways to compute the maximum. Possibly some of them are in the same spirit as your "named let" solution.

Clear@max
max[s:{first_,others___}]:=max[first,{others}]
max[best_,{}]:=best
max[best_,rest:{next_,others___}]/;best>next:=max[best,{others}]
max[best_,rest:{next_,others___}]:=max[next,{others}]

max@{1,2,3,4,5,2}

5

max=Fold[Max,First@#,Rest@#]&;
max@{1,2,3,4,5,2}

5

max=Fold[Max,-Infinity,#]&;
max@{1,2,3,4,5,2}

5

The built-in Max can be replaced by If[#1>#2,#1,#2]& in the last two solutions.

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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