Re: permutations
- To: mathgroup at smc.vnet.net
- Subject: [mg62473] Re: permutations
- From: "Dana" <Dana.OnTheBeach at comcastl.com>
- Date: Fri, 25 Nov 2005 02:25:28 -0500 (EST)
- Sender: owner-wri-mathgroup at wolfram.com
> I would need to do permutations of lists of Length 20, perhaps 25.
Here are some other ideas.
lst = Range[25];
The size is too big to do anything meaningful:
n = 25!
15,511,210,043,330,985,984,000,000
Perhaps you just want to work with a random sample from that "large" list:
Table[RandomPermutation[lst], {3}];
If you would like to see the Permutation that is half-way in that "large"
list, perhaps more efficiently would be:
NthPermutation[n/2, lst]
{13,14,1,2,3,4,5,6,7,8,9,10,11,12,15,16,
17,18,19,20,21,22,23,24,25}
The 2nd from the end of that "large" list:
NthPermutation[n - 2, lst]
{25,24,23,22,21,20,19,18,17,16,15,14,13,12,
11,10,9,8,7,6,5,4,3,1,2}
HTH. :>)
--
Dana DeLouis
Mathematica Windows, 5.2
"Francisco Gutierrez" <fgutiers2002 at yahoo.com> wrote in message
news:dm1jov$n4c$1 at smc.vnet.net...
>
> Dear Group:
> If I do for example:
> Permutations[{x1,x2,x3,x4,x5,x6,x7,x8,x9,x10,x11,x12}],
> my computer runs out of memory.
> This is not such a poor computer. Actually, I would need to do
permutations of lists of Length 20, perhaps 25. I am aware these
calculations are big (Length[x]!), but I wonder if there is some way around
the problem. Compilating Permutations? But then how? Can somebody help me?
> Francisco Gutiérrez