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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: [mg71361] Re: [mg71306] List difference using patterns and substitutions.
  • From: Bob Hanlon <hanlonr at cox.net>
  • Date: Wed, 15 Nov 2006 06:45:47 -0500 (EST)
  • Reply-to: hanlonr at cox.net

a={1,2,3,5,5};

Rest[a]-Most[a]

{1,1,2,0}

Most is available in version 5 and later.

Rest[a]-Drop[a,-1]

{1,1,2,0}

ListConvolve[{1,-1},a]

{1,1,2,0}

Partition[a,2,1]/.{x_,y_}:>y-x

{1,1,2,0}

Thread[{Rest[a],Most[a]}]/.{x_,y_}:>x-y

{1,1,2,0}


Bob Hanlon

---- Nacho <ncc1701zzz at gmail.com> 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.
> 


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