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MathGroup Archive 2004

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Re: UnrankPermutation newbie problem .. Combinatorica Package

  • To: mathgroup at smc.vnet.net
  • Subject: [mg49978] Re: UnrankPermutation newbie problem .. Combinatorica Package
  • From: BV <dont at bug.me>
  • Date: Mon, 9 Aug 2004 04:29:16 -0400 (EDT)
  • References: <cf22sg$7tf$1@smc.vnet.net> <cf4t32$lid$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

BV wrote:
> BV wrote:
> 
> 
>>I found that calling NthPermutation via MathLink in C++ is rather costly 
>>  for my purposes, therefore I'm inquiring to  re-write the function 
>>entirely in C++. I hope by eliminating unnecessary calls to MathLink 
>>this would speed up my program significantly.
>>
>>As, I admit, I am not very versed in Mathematics itself can anyone point 
>>me in the direction where to start; how this function works; what 
>>formula is used, etc.
>>Thank you
>>
> 
> I found in the "combinatorica.m" file that this function is obsolete and 
> that UnrankPermutation[] function should be used instead. This is the 
> code as it should work (from the "combinatorica.m" file):
> 
> UnrankPermutation[r_Integer, {}] := {}
> UnrankPermutation[r_Integer, l_List] :=
>          Module[{s = l, k, t, p = UP[Mod[r, Length[l]!], Length[l]]},
>                 Table[k = s[[t = p[[i]] ]];
>                       s = Delete[s, t];
>                       k,
>                       {i, Length[ p ]}
>                 ]
>          ]
> 
> UnrankPermutation[r_Integer, n_Integer?Positive] :=
>          UnrankPermutation[r, Range[n]]
> 
> Can someone please write this as an algorithm or a formula as I don't 
> have any books on Mathematica and this looks complicated; a plain 
> formula for this would do!
> 
I've just solved it no help needed anymore :)


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