Generate a matrix of combinations without repetition (array exceeds maximum array size preference)

Question

Reinforcement Learning 2021-10-25

0
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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1571208-generate-a-matrix-of-combinations-without-repetition-array-exceeds-maximum-array-size-preference

编辑： Jan 2021-10-25

Hey,

I am trying to generate a matrix, that has all unique combinations of [0 0 1 1], I wrote this code for this:

v1 = [0 0 1 1];
M1 = unique(perms([0 0 1 1]),'rows');

This isn't ideal, because perms() is seeing each vector element as unique and doing 4! = 4 * 3 * 2 * 1 = 24 combinations.

With unique() I tried to delete all the repetitive entries so I end up with the combination matrix M1 -> only [4!/ 2! * (4-2)!] = 6 combinations!

Now, when I try to do something very simple like:

n = 15;
i = 1;
v1 = [zeros(1,n-i) ones(1,i)];
M = unique(perms(vec_1),'rows');

Instead of getting [15!/ 1! * (15-1)!] = 15 combinations, the perms() function is trying to do 15! = 1.3077e+12 combinations and it's interrupted.

How would you go about doing in a much better way? Thanks in advance!

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Answer 1

Jan 2021-10-25

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1571208-generate-a-matrix-of-combinations-without-repetition-array-exceeds-maximum-array-size-preference#answer_816068

编辑：Jan 2021-10-25

Instead of permuting [0,0,1,1] and removing the duplicates, you can obtain the list of indices of the ones (or zeros):

M1 = unique(perms([0 0 1 1]), 'rows')
M1 = 6×4
     0     0     1     1
     0     1     0     1
     0     1     1     0
     1     0     0     1
     1     0     1     0
     1     1     0     0
nDigit = 4;
nOnes  = 2;
index = nchoosek(1:nDigit, nOnes)  % Indices of 2 ones in 4 elements
index = 6×2
     1     2
     1     3
     1     4
     2     3
     2     4
     3     4
n     = size(index, 1);
v     = (1:n).' + n * (index - 1);
M2    = zeros(n, nDigit);
M2(v(:)) = 1
M2 = 6×4
     1     1     0     0
     1     0     1     0
     1     0     0     1
     0     1     1     0
     0     1     0     1
     0     0     1     1