Permutation of an array

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Doobie 2017-5-8

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/339203-permutation-of-an-array

评论： Walter Roberson 2017-5-8

在 MATLAB Online 中打开

Is there a built-in matlab function that gives a certain number of permutations of an array? For example:

Take array:

A = [1 2 3 4];

Now there are 24 different permutations. I know that perms(A) would give me all 24 permutations of array A, but I don't need all 24. I only want 4 out of 24. So the function I'm looking for could give me the following matrices:

2 3 4            3 2 1 4        1 2 3 4
4 3 1            2 1 3 4        3 2 1 4
3 1 2            2 3 4 1        3 4 2 1
1 2 3            4 1 2 3        4 3 2 1

The important thing is that the selection of the 4 has to be random. So whenever I run my function, I expect to get different 4 (of course, because it is random, I could get the exact same 4 but you get the idea).

Is there a function that does this? If not, could someone direct me towards a code?

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Answer 1

Walter Roberson 2017-5-8

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/339203-permutation-of-an-array#answer_266009

编辑：Walter Roberson 2017-5-8

在 MATLAB Online 中打开

A( randperm(length(A)) )

which you can repeat 4 times.

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Doobie 2017-5-8

编辑：Doobie 2017-5-8

This is not what I'm looking for. Because this is a random process repeated 4 times, it could give the same permutation twice. I want 4 distinct permutations.

Walter Roberson 2017-5-8

在 MATLAB Online 中打开

There is no Mathworks provided function for this purpose.

rows_needed = 4;
cols_needed = length(A);
permidx = [];
more_needed = rows_needed;
while more_needed > 0
  [~, trial_idx] = sort( rand(more_needed, cols_needed), 2 );
  permidx = unique([permidx; trial_idx], 'rows', 'stable');
  more_needed = rows_needed - size(permidx,1);
end
randperms = A(permidx);