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I want to partition an integer into $k$ integers all possible orderings. This can be done in the following way

integerPartitions[n_, {k_}] := Select[FrobeniusSolve[Table[1, {k}], n], FreeQ[#, 0] &]

For example,

integerPartitions[4, {2}]

gives 

{{1, 3}, {2, 2}, {3, 1}}

But this is very slow if the partition number is large as compared to IntegerPartitions, which gives partitions in reverse lexicographic order. For instance, integerPartitions[60, {5}] will take more than 4 seconds in my laptop in contrast to 0 seconds of IntegerPartitions[60, {5}].

So my question is: what would be the most efficient Mathematica code for this problem?

unstable
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2 Answers2

12

This seems to be quite quick :

output = Flatten[Permutations /@ IntegerPartitions[60, {5}], 1]; // AbsoluteTiming
(* {0.245025, Null} *)

output // Dimensions
(* {455126, 5} *)
Mr.Wizard
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b.gates.you.know.what
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0

If order does matter, you are generating integer compositions. Integer compositions are related to the operation multichoose. You can use the Resource Functions IntegerCompositions, where 0 s are allowed like {0,0,5} for 5 into 3 parts, and StrictIntegerCompositions for this, where 0 s are not allowed like {2,1,2} for 5 into 3 parts.

Peter Burbery
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