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MathGroup Archive 1996

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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
Fax:+32/51/33 21 75
vdmcc at vandemoortele.be



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