Re: Pattern Matching in Lists
- To: mathgroup at smc.vnet.net
- Subject: [mg35586] Re: Pattern Matching in Lists
- From: "Allan Hayes" <hay at haystack.demon.co.uk>
- Date: Sun, 21 Jul 2002 01:01:03 -0400 (EDT)
- References: <ah5qce$59o$1@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
[second posting in view of reported technical problem]
Anthony,
Take
w = Table[Random[Integer], {200000}
My first thought was, and several posts used this,
Count[Partition[w, 2,1],{1,0}]//Timing
{3.24 Second,49851}
Later it occured to me to use arithmetic, which turned out to be twice as
fast:
Count[ Drop[w,-1] - Drop[w,1],1]//Timing
{1.49 Second,49851}
This is close to Selwyn Hollis's code
Count[Drop[w+2RotateRight[w],1],2]//Timing
{1.6 Second,49851}
--
Allan
---------------------
Allan Hayes
Mathematica Training and Consulting
Leicester UK
www.haystack.demon.co.uk
hay at haystack.demon.co.uk
Voice: +44 (0)116 271 4198
Fax: +44 (0)870 164 0565
"Anthony Mendes" <amendes at zeno.ucsd.edu> wrote in message
news:ah5qce$59o$1 at smc.vnet.net...
> Hello,
>
> Suppose w={1,1,1,0,0,1,0,1,0,0,1,0,0}.
>
> How can I count the number of occurrences of a 1 in w immediately
> followed by a 0 in w?
>
> I have tried every incarnation of Count[] I can think of; for example,
>
> Count[w,{___,1,0,___}]
>
> does not seem to work. In general, how can I count the number of
> occurrences of a 1 followed by a 0 in a list of 1's and 0's? Thank you!
>
>
> --
> Tony
> _____________________
> amendes at math.ucsd.edu
>
>