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MathGroup Archive 2006

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Re: List difference using patterns and substitutions.

  • To: mathgroup at smc.vnet.net
  • Subject: [mg71332] Re: [mg71306] List difference using patterns and substitutions.
  • From: János <janos.lobb at yale.edu>
  • Date: Wed, 15 Nov 2006 06:43:34 -0500 (EST)
  • References: <200611141006.FAA06710@smc.vnet.net>

On Nov 14, 2006, at 5:06 AM, Nacho wrote:

> Hello.
>
> I'm trying to figure how can I build a difference list from another
> using only patterns and rule substitutions.
>
> The idea is to get from a list, another, one element shorter, where
> each value is the substraction of two consecutive elements in the
> original list, that is, from {1,2,3,5,5} get {1,1,2,0}.
>
> I've been thinking about it for a while, and I know several methods
> using a more traditional programming style (with For, for example),  
> but
> I have no idea if it is possible to make it simple and fast with rule
> substitutions.
>
> Any idea?
>
> Thanks in advance.

I am a mule when it comes to rules, but here is a newbie approach  
without For.

In[1]:=
lst = {1, 2, 3, 5, 5};

In[2]:=
(#1[[2]] - #1[[1]] & ) /@
   Partition[lst, 2, 1]
Out[2]=
{1, 1, 2, 0}

János


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