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MathGroup Archive 2006

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Re: Something in the spirit of letrec

  • To: mathgroup at smc.vnet.net
  • Subject: [mg68460] Re: Something in the spirit of letrec
  • From: Peter Pein <petsie at dordos.net>
  • Date: Sun, 6 Aug 2006 02:56:31 -0400 (EDT)
  • References: <eauvlv$172$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Mariano Suárez-Alvarez schrieb:
> Hi all,
> 
> I'm trying to somehow come up with a function WithSelf such that
> evaluating
> 
>   WithSelf[{a,self[1]+b,self[2]+2}]
> 
> will result in {a,a+b,a+b+c}. The idea is that the each
> item in the argument may depend on the previous ones;
> so for example "self[1]+b" means "what's in the first position
> in this list, plus b". There will be no "forward references"
> in the argument.
> 
> I've tried with
> 
>   SetAttributes[WithSelf, HoldFirst];
>   WithSelf[l_, self_] := Module[{ll, r = {}},
>       ll = Function[self, #] & /@ l;
>       Scan[AppendTo[r, #[r]] &, ll];
>       r
>       ]
> 
> This gives the correct correct results, but emits warnings:
> for example,
> 
>   WithSelf[{1, self[[1]] + 2, self[[2]] + 3, self[[1]]}, self]
> 
> results in {1, 3, 6, 1}, which is OK, but emits a few of
> 
>   Part::partd: Part specification self[[1]] is longer than depth of
> object.
> 
> I guess there is some evaluation going on which I am
> not seeing...
> 
> I'm quite sure what I am trying to do is possible... but how?
> 
> Ideas?
> 
> -- m
> 

Hi Mariano,

how about

In[1]:= WithSelf[l_, s_:self] := l //. s -> (l[[#1]] & )
In[2]:= WithSelf[{1, self[1] + 2, self[2] + 3, self[1]}]
Out[2]= {1, 3, 6, 1}

?

Peter


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