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MathGroup Archive 2001

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Re: Algorithm Questions

  • To: mathgroup at smc.vnet.net
  • Subject: [mg29397] Re: Algorithm Questions
  • From: "Orestis Vantzos" <atelesforos at hotmail.com>
  • Date: Sat, 16 Jun 2001 22:43:47 -0400 (EDT)
  • Organization: National Technical University of Athens, Greece
  • References: <9gf01l$7lj$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

<< DiscreteMath`Permutations`

rndPerms[n_, k_] /; k <= n! :=
  Module[{P = {}},
    While[Length[P] < k,
      P = Union[P, {RandomPermutation[n]}]];
    P]

should do the trick fairly fast.
Orestis




"Hu Zhe" <huzhe at public3.sta.net.cn> wrote in message
news:9gf01l$7lj$1 at smc.vnet.net...
>
> Hi,
> I am wondering that if Mathematica has lazy-evaluation features. Say I
> am going to extract 100 lists from the permutation lists of {1,
> 2,...,100} (Permutation[Range[100]]) randomly.
>
> If I program like this:
>
> largeList = Permutation[Range[100]];
>
> indx = Table[Random[Integer, {1, 100!}], {100}];
>
> Part[largeList, indx]
>
> the program is very slow, since it works out all the 100! elements for
> the largeList, while I only need 100 items from it.
>
> Can I program like this ? Say,
>
> take 100 (somePermutationFunction 100)
>
> so that the permutation stops after 100 lists were taken. (I learned
> that it's called lazy-evaluation from Haskell.)
>
> The second question is that:
>
> I want the 100 lists randomly taken to be all different.
>
> So I program this way:
>
>  While[Length[a] != 100,
>       a = (Table[Random[Integer, {1, n!}], {100}] // Union)];
>
> there must be more efficient solutions, especially to combine the
> solution with the first question?
>
>
> So my slow program is like this, deeply welcome suggestions to improve
> it:
>
> randomList[n_, k_] :=
>   Module[{a},
>     While[Length[a] != k,
>       a = (Table[Random[Integer, {1, n!}], {k}] // Union)];
>     Part[Permutations[Range[n]], a]]
>
>
> Sincerely,
> Hu Zhe
>




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