Services & Resources / Wolfram Forums
-----
 /
MathGroup Archive
2001
*January
*February
*March
*April
*May
*June
*July
*August
*September
*October
*November
*December
*Archive Index
*Ask about this page
*Print this page
*Give us feedback
*Sign up for the Wolfram Insider

MathGroup Archive 2001

[Date Index] [Thread Index] [Author Index]

Search the Archive

Swiftest code to turn list of ordered pairs into Markov matrix

  • To: mathgroup at smc.vnet.net
  • Subject: [mg29683] Swiftest code to turn list of ordered pairs into Markov matrix
  • From: "Seth Chandler" <SChandler at Central.UH.Edu>
  • Date: Tue, 3 Jul 2001 04:40:37 -0400 (EDT)
  • Organization: University of Houston
  • Sender: owner-wri-mathgroup at wolfram.com

Suppose one has a list of ordered pairs oplist such as
{{1,5},{1,6},{2,3},{2,3} ... }. All the values in the list are positive
integers. The highest value in oplist is known to be some value z.

Now one wishes to form a matrix such that the value at element i,j is equal
to
Count[oplist,{i,j}]. One terribly slow way to do this is

Table[Count[oplist,{i,j}],{i,1,z},{j,1,z}]

A faster way to do this is as follows:
Fold[ReplacePart[#,Extract[#,#2]+1,{#2}]&,Table[0,{z},{z}],oplist]

Does anyone have a considerably faster way? A speed up of 100% or more would
be very helpful.

For what it's worth Count[oplist,{something_,_}] is the same for all values
of something.

P.S. The problem arises in converting a representation of a directed graph
into a Markov transition matrix.

Thanks,

Seth J. Chandler
Associate Professor of Law
University of Houston Law Center





  • Prev by Date: Re: Surface Plots in Mathematica
  • Next by Date: AW: Making a LIst Reconsidered
  • Previous by thread: Re: Re: Equations
  • Next by thread: Re: Swiftest code to turn list of ordered pairs into Markov matrix