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MathGroup Archive 2004

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Re: newbie is looking for a customDistribution function

  • To: mathgroup at smc.vnet.net
  • Subject: [mg50411] Re: [mg50383] newbie is looking for a customDistribution function
  • From: Tomas Garza <tgarza01 at prodigy.net.mx>
  • Date: Thu, 2 Sep 2004 04:34:45 -0400 (EDT)
  • References: <200409010549.BAA29848@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

From a formal point of view, a distribution function is defined on the
reals. You can't, therefore, define it on lists of two, three, or more
symbols. A frequency function, on the other hand, may be defined on any set
of events. But, then, it is rather trivial for your problem: once you have
your list distpro, just divide each value in the list by the sum of all the
values, and that's it. Or, maybe, I misunderstood your question?

Tomas Garza
Mexico City
----- Original Message ----- 
From: "János" <janos.lobb at yale.edu>
To: mathgroup at smc.vnet.net
Subject: [mg50411] [mg50383] newbie is looking for a customDistribution function


> Hi,
>
> I looked for it in the archives, but found none.  I am looking for ways
> to create a custom distribution, which I can call as a function.  Here
> is an example for illustration.  Let's say I have a list created from a
> 4 elements alphabet  {a,b,c,d}:
>
> In[1]:=
> lst={a,a,b,c,a,d,a,c,c,a}
>
> Out[1]=
> {a,a,b,c,a,d,a,c,c,a}
>
> Distribute gives me - thanks David Park - all the two element
> combinations of {a,b,c,d}
>
> In[11]:=
> twocombs=Distribute[Table[{a,b,c,d},{2}],List]
>
> Out[11]=
> {{a,a},{a,b},{a,c},{a,d},{b,a},{b,b},{b,c},{b,d},{c,a},{c,b},{c,c},{c,d}
> ,{
>    d,a},{d,b},{d,c},{d,d}}
>
> I can count the occurrence of an element of twocombs in lst with the
> following function:
>
> occuranceCount[x_List] := Count[Partition[lst, 2, 1], x]
>
> Mapping this function over twocombs gives me the number of occurances
> of elements of twocombs in lst:
>
> In[12]:=
> distro=Map[occuranceCount,twocombs]
>
> Out[12]=
> {1,1,1,1,0,0,1,0,2,0,1,0,1,0,0,0}
>
> It shows that for example {c,a} occurs twice, {d,a} occurs once and
> {d,c} or {d,d} never occur.
>
> Now, I would like to create a distribution function called
> twocombsLstDistribution which I could call and it would give me back
> elements of twocombs with the probability as they occur in distro, that
> is for on average I would get twice as much {c,a}s as {d,a}s and never
> get {d.c} or {d,d}.
>
> How can I craft that ?
>
> /Of course I need it for an arbitrary but finite length string lst over
> a fixed length alphabet {a,b,c,d,....} for k-length elements of kcombs,
> and it has to be super fast  :).  My real lst is between 30,000 and
> 70,000 element long over a four element alphabet and I am looking for k
> between 5 and a few hundred. /
>
> Thanks ahead,
> János
> -------------------------------------------------
> People never lie so much as after a
> hunt, during a war or before an election
> - Otto von Bismarck -
>
>



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