Re: List difference using patterns and substitutions.
- To: mathgroup at smc.vnet.net
- Subject: [mg71322] Re: List difference using patterns and substitutions.
- From: Jean-Marc Gulliet <jeanmarc.gulliet at gmail.com>
- Date: Wed, 15 Nov 2006 06:43:12 -0500 (EST)
- Organization: The Open University, Milton Keynes, UK
- References: <ejc53g$6qf$1@smc.vnet.net>
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.
>
One approach could be
lst = {1, 2, 3, 5, 5};
Most[RotateLeft[lst] - lst]
--> {1, 1, 2, 0}
Another one would be
Partition[list, 2, 1] /. {a_, b_} -> b - a
--> {1, 1, 2, 0}
Regards,
Jean-Marc