Re:List Hepl Needed
- To: mathgroup at smc.vnet.net
- Subject: [mg5135] Re:[mg5065] List Hepl Needed
- From: Vandemoortele CC Group R&D Center <w.meeussen.vdmcc at vandemoortele.be>
- Date: Wed, 6 Nov 1996 01:32:54 -0500
- Sender: owner-wri-mathgroup at wolfram.com
after reading Todd Garley's and John Fultz'es suggestions, here is a way to
decompose the problem into simple steps :
Needs["DiscreteMath`Combinatorica`"]
?Partitions
Partitions[n] constructs all partitions of integer n in reverse
lexicographic order
lis=ToExpression[FromCharacterCode/@(64+32+Range[14])]
{a, b, c, d, e, f, g, h, i, j, k, l, m, n}
For all partitions of 14 elements into 7 sublists :
your=Select[Partitions[14],Length[#]==7&]
{{8, 1, 1, 1, 1, 1, 1}, {7, 2, 1, 1, 1, 1, 1}, {6, 3, 1, 1, 1, 1, 1},
{6, 2, 2, 1, 1, 1, 1}, {5, 4, 1, 1, 1, 1, 1}, {5, 3, 2, 1, 1, 1, 1},
{5, 2, 2, 2, 1, 1, 1}, {4, 4, 2, 1, 1, 1, 1}, {4, 3, 3, 1, 1, 1, 1},
{4, 3, 2, 2, 1, 1, 1}, {4, 2, 2, 2, 2, 1, 1}, {3, 3, 3, 2, 1, 1, 1},
{3, 3, 2, 2, 2, 1, 1}, {3, 2, 2, 2, 2, 2, 1}, {2, 2, 2, 2, 2, 2, 2}}
Up to here, the lists remain small ; further down, they inflate because all
permutations have to be taken into account :
?Permutations
Permutations[list] generates a list of all possible permutations of the
elements in list.
Timing[par=Flatten[Permutations/@your,1];]
{0.06 Second, Null}
Timing[ran=Range[#]&/@par;]
{2.494 Second, Null}
Timing[try=Map[Rest at FoldList[Last[#1]+#2&,{0},#]&,ran,1];]
{2.925 Second, Null}
Timing[result=Map[Part[lis,#]&,#]&/@try;]
{1.402 Second, Null}
Length[result]
1716
ByteCount[result]
370672
as you can see, going from "ran" to "try" costs a lot of trickery, although
at first glance it should be easy.
nice problem, this !
Wouter.
NV Vandemoortele Coordination Center
Group R&D Center
Prins Albertlaan 79
Postbus 40
B-8870 Izegem
Tel: +/32/51/33 21 11
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vdmcc at vandemoortele.be