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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: [mg71350] Re: List difference using patterns and substitutions.
  • From: Bill Rowe <readnewsciv at sbcglobal.net>
  • Date: Wed, 15 Nov 2006 06:44:41 -0500 (EST)

On 11/14/06 at 5:06 AM, ncc1701zzz at gmail.com (Nacho) wrote:

>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?

While there may be a fast way to do this with pattern matching, 
there already is a built-in function to do this. Specifically ListConvolve

For example,

In[1]:=
ListConvolve[{1,-1},{a,b,c}]

Out[1]=
{b-a,c-b}

Alternatively

In[2]:=
ListCorrelate[{-1,1},{a,b,c}]

Out[2]=
{b-a,c-b}
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