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MathGroup Archive 2001

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Re: Swiftest code to turn list of ordered pairs into Markov matrix

  • To: mathgroup at smc.vnet.net
  • Subject: [mg29714] Re: Swiftest code to turn list of ordered pairs into Markov matrix
  • From: "Allan Hayes" <hay at haystack.demon.co.uk>
  • Date: Wed, 4 Jul 2001 03:08:39 -0400 (EDT)
  • References: <9hs1a0$bs9$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Seth,

z= 50;

oplist= Table[Random[Integer,{1,z}],{100000},{2}];

a=Fold[ReplacePart[#,Extract[#,#2]+1,#2]&,Table[0,{z},{z}],oplist];//Timing

        {44.6 Second,Null}

(*I replaced {#2} with #2 -- the former gave trouble that I have not yet
tracked down*)

(b=Table[0,{z},{z}];
      MapThread[(b[[Sequence@@#1]]=#2)&,
            {First/@#,Length/@#
              }]&[
        Split[Sort[oplist]]]);//Timing

        {4.84 Second,Null}

a===b

        True
--
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

"Seth Chandler" <SChandler at Central.UH.Edu> wrote in message
news:9hs1a0$bs9$1 at smc.vnet.net...
> 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
>
>
>
>




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