MathGroup Archive 2000

[Date Index] [Thread Index] [Author Index]

Search the Archive

Re: Help on Partitions, Again!!!


Here's one idea.

Clear[allpartitions, partitions, subsetsplit]

partitions[a_List, spec_] := Union[Sort /@ allpartitions[a, spec]]

allpartitions[a_List, {n_}] := {{a}}
allpartitions[a_List, spec_] := Module[{},
    tmp = subsetsplit[a, spec[[1]]];
      Outer[Prepend, allpartitions[#[[2]], Rest[spec]], {#[[1]]}, 1] &
/@ tmp,

subsetsplit[a_, n_] := Module[{len = Length[a]},
    i[0] = 0;
        h[a[[Table[i[k], {k, n}]]], Delete[a, Table[{i[k]}, {k, n}]]],
        Evaluate[Sequence @@ Table[{i[k], i[k - 1] + 1, len - n + k},
{k, n}]]
    n - 1]]

Basically, subsetsplit forms h[a,b], where a is a subset of a certain
length, and b is the remaining elements from the list allpairings. The
function subsetsplit is similar in functionality to the function
KSubsets, except that subsetsplit also includes the expression b.

The function allpairings begins by calling subsetsplit to form all
possible h[a,b], and then applying allpairings to b, and then hooking up
a to each of the sets formed by applying allpairings to b. The function
allpairings will treat {{i1,i2},{i3,i4}} as different from
{{i3,i4},{i1,i2}}, so the final function pairings removes all these

For your test case we have

partitions[{i1, i2, i3, i4, i5, i6}, {3, 3}]

{{{i1, i2, i3}, {i4, i5, i6}},

  {{i1, i2, i4}, {i3, i5, i6}},

  {{i1, i2, i5}, {i3, i4, i6}},

  {{i1, i2, i6}, {i3, i4, i5}},

  {{i1, i3, i4}, {i2, i5, i6}},

  {{i1, i3, i5}, {i2, i4, i6}},

  {{i1, i3, i6}, {i2, i4, i5}},

  {{i1, i4, i5}, {i2, i3, i6}},

  {{i1, i4, i6}, {i2, i3, i5}},

  {{i1, i5, i6}, {i2, i3, i4}}}


Jose Prado de Melo wrote:

> Hello, MathGroup
> First of all, thanks for your attention.
> To be more specific:
> It's not too dificult to calculate the solution of the problem:
> How many ways, can the set {A,B,C,D,E,F} be separeted into two parts
> with three elements in each?
>  Answer:   x = 6!/(2!.3!.3!) = 10
>  I'm looking for a function to generate all the partitions using
> Mathematica 3.0 .
> I'm not sure, but I think the package Combinatorica doesn't have a
> function to do this.
> For example, I'm trying to think up a function f  like this one:
> In[ ] = f [ {A,B,C,D,E,F},{3,3}]
> Out [ ] = { { {A,B,C},{D,E,F} }, { {
> A,B,F},{C,D,E}},...................} and so on.
> In [ ] = Length[%]
> Out [ ] = 10
> Please, help me.
> Thanks!

  • Prev by Date: Re: Help on Partitions, Again!!!
  • Next by Date: Re: mathematica gets a simple limit wrong?
  • Previous by thread: Help on Partitions, Again!!!
  • Next by thread: Re: Help on Partitions, Again!!!