determine combination of elements in matrix

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JL 2019-8-22

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https://ww2.mathworks.cn/matlabcentral/answers/477234-determine-combination-of-elements-in-matrix

编辑： Adam Danz 2019-8-22

采纳的回答： Adam Danz

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I have a matrix, X

x = [1 1 1 1]

and I know if I introduced one zero to x, possible combination 4^1=4, which gives me, y

y = [0 1 1 1;
0 1 1;
1 0 1;
1 1 0;];

if I introduced two zeros to x, what are possible combinations which gives me, z. Is this correct below? Is there a code to determine the combinations?

 z = [0 0 1 1
1 0 1
1 1 0
0 0 1
0 1 0
1 0 0]

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Answer 1

Adam Danz 2019-8-22

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编辑：Adam Danz 2019-8-22

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You can use combnk() to create the permuted column indices for each row of your matrix and then replace those values by 0.

% Generate some data
z = ones(6,4);  % your original matrix
nzeros = 2;     % number of zeros to add to each row

Solution:

permIdx = combnk(1:size(z,2), nzeros); %permutation index
linIdx = sub2ind(size(z),repmat(1:size(permIdx),nzeros,1).',sortrows(permIdx)); 
z(linIdx) = 0; 

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JL 2019-8-22

Thank you. I think this works better

Adam Danz 2019-8-22

编辑：Adam Danz 2019-8-22

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Glad I could help. If you need to know the number of combinations you can use

numCombo = nchoosek(4,2);   % for 4 columns, 2 zeros.
% = 6, so you would need 6 rows.  

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Answer 2

Bruno Luong 2019-8-22

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编辑：Bruno Luong 2019-8-22

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lgt = 4;    % number of elements
nz = 2;     % number of 0s
j = nchoosek(1:lgt,lgt-nz);
i = repmat((1:size(j,1))',[1 size(j,2)]);
z = accumarray([i(:),j(:)],1)