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MathGroup Archive 2005

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Re: Why are permutations dupl

  • To: mathgroup at smc.vnet.net
  • Subject: [mg54691] Re: [mg54628] Why are permutations dupl
  • From: DrBob <drbob at bigfoot.com>
  • Date: Sun, 27 Feb 2005 01:29:16 -0500 (EST)
  • References: <200502251040.j1PAeSVa021379@ls413.htnet.hr>
  • Reply-to: drbob at bigfoot.com
  • Sender: owner-wri-mathgroup at wolfram.com

Here's a link that MAY help.

http://forums.wolfram.com/mathgroup/archive/2004/Aug/msg00180.html

Bobby

On Fri, 25 Feb 2005 11:40:28 CEST, <valentina.mikel at po.htnet.hr> wrote:

> Bob!
> Yes, I'm aware we're talking about 14! here, but this I got reduced from 24! and
> that is unsolvable!
> As I imagine, I would have to make a function in Mathematica that performs the
> functionality of NextPermutation;  it is called once for every sequential
> permutation, and it still must not return duplicates.
>
> The scenario is as follows:
>
> loop
> {
> C++ program calls Mathematica's NextPermutationModified function, which returns
> 1 (next) result.
> C++ program performs some operations
> } until no more permutations are left
>
> Can you make this function, as specified?
>
> Pseudo-code for this would be even better, as that would enable me to solve the
> problem directly in C++ without Mathematica at all!
>
> Can you produce a code-snippet so that Mathematica throws out _one result at the
> time_, so that I can call this from a C++ process in my program.
>
> THAnks Bob
>
>> You get duplicates because the two zeroes are treated as distinct items, as if
> you'd
> entered:
>>
>> Needs["DiscreteMath`Combinatorica`"]
>> LexicographicPermutations[{a, b, c, d}] /. {c -> 0, d -> 0}
>>
>> Is this what you wanted, instead?
>>
>> Union@LexicographicPermutations[{a,b,0,0}]
>>
>> {{0,0,a,b},{0,0,b,a},{0,a,0,b},{0,a,b,0},{0,b,0,a},{0,b,a,0},{
>>    a,0,0,b},{a,0,b,0},{a,b,0,0},{b,0,0,a},{b,0,a,0},{b,a,0,0}}
>>
>> or maybe...
>>
>> LexicographicPermutations[{a,b,0}]/.(0\[RuleDelayed]Sequence[0,0])
>>
>> {{a,b,0,0},{a,0,0,b},{b,a,0,0},{b,0,0,a},{0,0,a,b},{0,0,b,a}}
>>
>> as for
> LexicographicPermutations[{0,C,D,0,G,H,J,0,0,P,0,0,T,V,0,X,Y,0,0,4,0,7,8,0}], you
> have GOT to be kidding. Even with the most conservative assumption, look how many
> permutations there'd be:
>>
>> Length@Union@{0,C,D,0,G,H,J,0,0,P,0,0,T,V,0,X,Y,0,0,4,0,7,8,0}
>> %!
>>
>> 14
>>
>> 87178291200
>>
>> Bobby
>>
>> On Thu, 24 Feb 2005 03:21:32 -0500 (EST), Valentina Mikel
> <valentina.mikel at po.htnet.hr>
> wrote:
>>
>> > When I enter
>> >
>> > LexicographicPermutations[{a, b, 0, 0}]
>> >
>> > the result is following:
>> > {{a, b, 0, 0}, {a, b, 0, 0}, {a, 0, b, 0}, {a, 0, 0, b}, {
>> >    a, 0, b, 0}, {a, 0, 0, b}, {b, a, 0, 0}, {b, a, 0, 0}, {
>> >    b, 0, a, 0}, {b, 0, 0, a}, {b, 0, a, 0}, {b, 0, 0, a}, {
>> >    0, a, b, 0}, {0, a, 0, b}, {0, b, a, 0}, {0, b, 0, a}, {
>> >    0, 0, a, b}, {0, 0, b, a}, {0, a, b, 0}, {0, a, 0, b}, {
>> >    0, b, a, 0}, {0, b, 0, a}, {0, 0, a, b}, {0, 0, b, a}}
>> >
>> >
>> > Why does the result repeat duplicate cases?
>> > In my case I must perform exact functionality of
>> > LexicographicPermutations[{0,C,D,0,G,H,J,0,0,P,0,0,T,V,0,X,Y,0,0,4,0,7,8,0}];
>> > since there are lots of duplicates I'm trying to implement this in C++
>> > for maximum speed.
>> >
>> > Can this be avoided in Mathematica itself, and how?
>> >
>> >
>> >
>> >
>>
>>
>>
>> --
>> DrBob at bigfoot.com
>> www.eclecticdreams.net
>>
>
>
>
>
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>
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>
>



-- 
DrBob at bigfoot.com
www.eclecticdreams.net


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