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Re: generating submultisets with repeated elements

  • To: mathgroup at smc.vnet.net
  • Subject: [mg104028] Re: generating submultisets with repeated elements
  • From: Raffy <raffy at mac.com>
  • Date: Fri, 16 Oct 2009 07:19:04 -0400 (EDT)
  • References: <ha4r9k$d0h$1@smc.vnet.net>

On Oct 2, 5:22 am, David Bevan <david.be... at pb.com> wrote:
> I'm new to Mathematica, so if I've missed something obvious, my apologies.
>
> I want a function to generate a list of "submultisets" with up to k elements of a set s, allowing elements from s to be repeated.
>
> The following works, but is very inefficient since each multiset is generated multiple times and then sorted and then repeats deleted:
>
> coinSets[s_,k_]:=DeleteDuplicates[Sort/@Flatten[Tuples[s,#]&/@Range[k],1]]
>
> coinSets[{1,3,4},3]
>
> {{1},{3},{4},{1,1},{1,3},{1,4},{3,3},{3,4},{4,4},{1,1,1},{1,1,3},{1,1,4},{1 ,3,3},{1,3,4},{1,4,4},{3,3,3},{3,3,4},{3,4,4},{4,4,4}}
>
> I assumed there would be a suitable function in the Combinatorica package, but I can't see anything -- which would be a bit odd for a combinatorial package. What have I missed?
>
> Do I need to write my own (perhaps by looking at how KSubsets is implemented) or is there some easy way of generating these multisets?
>
> Thanks.
>
> David %^>

[I posted this yesterday but Google Groups is acting odd and placed my
reply in a different thread.]

Another way of looking at the "coin sets" is thinking about it in
terms of how each set combination is generated.

I'll start with a naive solution:

naive[k_, n_] := Union[Ceiling[#/n] & /@ Subsets[Range[k*n], {1, n}]];

naive[3, 3] = {{1}, {2}, {3}, {1, 1}, {1, 2}, {1, 3}, {2, 2}, {2, 3},
{3, 3}, {1, 1, 1}, {1, 1, 2}, {1, 1, 3}, {1, 2, 2}, {1, 2, 3}, {1, 3,
3}, {2, 2, 2}, {2, 2, 3}, {2, 3, 3}, {3, 3, 3}}

This set of sets can be analyzed by asking: for each set, how many 1's
are there, how many 2's, etc...

recipe[k_, n_] := Table[Count[v, i], {v, naive[k, n]}, {i, k}];

recipe[3, 3] = {{1, 0, 0}, {0, 1, 0}, {0, 0, 1}, {2, 0, 0}, {1, 1, 0},
{1, 0, 1}, {0, 2, 0}, {0, 1, 1}, {0, 0, 2}, {3, 0, 0}, {2, 1, 0}, {2,
0, 1}, {1, 2, 0}, {1, 1, 1}, {1, 0, 2}, {0, 3, 0}, {0, 2, 1}, {0, 1,
2}, {0, 0, 3}}

For example:

set 	  => recipe
{1}       => {1, 0, 0} 	"a list with 1x1"
{1, 1}    => {2, 0, 0} 	"a list with 2x1"
{2, 3, 3} => {0, 1, 2}  "a list with 1x2 + 2x3"

If you take the element-sorted union of these recipes, you'll see a
pattern:

basis[k_, n_] := Union[Sort /@ recipe[k, n]]

basis[3, 3] = {{0, 0, 1}, {0, 0, 2}, {0, 0, 3}, {0, 1, 1}, {0, 1, 2},
{1, 1, 1}}

You can reverse this operation by performing:

recipe[3, 3] === Join @@ (Permutations /@ basis[3, 3])
(note they might have different sortings)

So the process to generate all the coin sets is the following:

1. We need a function that generates the "basis" from above, given k
and n:

basis[len_, sum_] :=
  Reap[Do[If[Length[#1] == n, Sow[PadRight[#1, len]],
        Do[#0[Append[#1, i], i, #3 - i], {i, #2, #3}]] &[{}, 1,
      sum], {n, len}]][[2, 1]];

basis[3, 3] = {{1, 0, 0}, {2, 0, 0}, {3, 0, 0}, {1, 1, 0}, {1, 2, 0},
{1, 1, 1}}

2. We need to permutate each basis to generate all the recipes for
that combination.

recipes[k_, n_] := Join @@ Table[Permutations[Developer`ToPackedArray
[v]], {v, basis[k, n]}];

3. At this stage, we have our answer as a list of recipes, where each
recipe is a vector of length k, whose sum is between 1 and n, can be
converted into a coin set by using it as a recipe (see above).

{0, 1, 0} => 1x2             => {2}
{3, 0, 0} => 3x1             => {1, 1, 1}
{1, 1, 1} => 1x1 + 1x2 + 1x3 => {1, 2, 3}

Timing[Length[recipes[15, 12]]] => {4.62615, 17383859}
Timing[Length[recipes[15, 10]]] => {0.841442, 3268759}
Timing[Length[recipes[15, 7]]]  => {0.052076, 170543}

Another advantage of the recipe form is it makes Intersect/Union/
Complement/MemberQ/FreeQ fast to implement.


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