Permutations function in Matlab

Question

In Wolfram Mathematica，there is a function called Permutations(http://reference.wolfram.com/mathematica/ref/Permutations.html). It can gives all permutations containing exactly n elements.

For example: Permutations[{1,2,3,4}, {2}] gives

{{1, 2}, {1, 3}, {1, 4}, {2, 1}, {2, 3}, {2, 4}, {3, 1}, {3, 2}, {3, 4}, {4, 1}, {4, 2}, {4, 3}}

I know Matlab have a similar function perms, but it only receive one parameter and gives all possible permutations. Is there other function like Mathematica's Permutations[list,{n}]？

There is one on the FEX: http://www.mathworks.com/matlabcentral/fileexchange/11462-npermutek/ — Dan, Oct 22 '13 at 11:50

score 2 · Accepted Answer · edited May 23 '17 at 10:25

2

If order doesn't matter, take a look at nchoosek.

If it does (which seems to be the case), there is an inefficient, ugly, but non-toolbox dependent one-liner:

>> unique(builtin('_paren', perms(1:4), :,1:2), 'rows')
ans =
      1     2
      1     3
      1     4
      2     1
      2     3
      2     4
      3     1
      3     2
      3     4
      4     1
      4     2
      4     3

(which is really a hacked two-liner).

I'd suggest you just use combnk from the Statistics toolbox.

edited May 23 '17 at 10:25

Community

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answered Oct 22 '13 at 11:58

Rody Oldenhuis

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+1 but I suggest you add the 2 line equivalent as well for legibility. – Dan Oct 22 '13 at 12:02

score 2 · Answer 2 · answered Oct 22 '13 at 15:32

You can get all the combinations of your desired size using nchoosek and then permute them using perms. That means for permutations of length k from vector v you can use

A=nchoosek(v,k);
P=reshape(A(:,perms(1:k)), [], k);

Note that the rows of P will not be sorted and you can use sortrows to sort it:

P=sortrows(reshape(A(:,perms(1:k)), [], k));

Using your example

v = 1:4;
k = 2;
A=nchoosek(v,k);
P=sortrows(reshape(A(:,perms(1:k)), [], k))

returns:

Permutations function in Matlab

2 Answers2