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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?

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  • 4
    $\begingroup$ Can't you apply permutations to the results of IntegerPartitions ? $\endgroup$ – b.gates.you.know.what Feb 12 '13 at 14:00
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This seems to be quite quick :

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

output // Dimensions
(* {455126, 5} *)
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