Find different possible combinations of the rows of two matrices.

Question

Tuhin Choudhury 2020-9-8

0
链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/590545-find-different-possible-combinations-of-the-rows-of-two-matrices

评论： Adam Danz 2020-9-8

采纳的回答： Johannes Fischer

在 MATLAB Online 中打开

Consider two matrices:

a = [1 2 3; 4 5 6; 7 8 10]

a =

1 2 3

4 5 6

7 8 10

and b=a + 10

b =

11 12 13

14 15 16

17 18 20

first, I am trying to obtain all possible combination of rows (1st row of 'a' and all rows of 'b', second row of 'a' and all rows of 'b' and so on) such that the output is

c =

1 2 3 11 12 13

4 5 6 11 12 13

7 8 10 11 12 13

1 2 3 14 15 16

4 5 6 14 15 16

7 8 10 14 15 16

1 2 3 17 18 20

4 5 6 17 18 20

7 8 10 17 18 20

secondly for each row, I am storing the first 3 and the last 3 columns in a single index of a 3 index vector. So in this example the vector of combosets is 2x3x9 where element is

combosets(:,:,1) =

1 2 3

11 12 13

combosets(:,:,2) =

4 5 6

11 12 13

and so on. The code I am using is below. Is there any better and faster way to do this and prefarably step 1 and 2 combined?

Thanks in advance!

clc
clearvars;
a = [1 2 3; 4 5 6; 7 8 10]; % 3 by 3 matrix 
b=a + 10; % 3 by 3 matrix
r=size(a,1);
combosets=zeros(r-1,r,r*r);
d={NaN};
% Step 1: storing different combination of rows in cell
for k=1:1:r
    c=vertcat(horzcat(a(:,:),repmat(b(k,:),r,1)),horzcat(a(:,:),repmat(b(k,:),r,1)),horzcat(a(:,:),repmat(b(k,:),r,1)));
    d{k}=(unique(c,'rows'));
end
ddash=cell2mat(d'); % making it a matrix again
% Step 2: storing in a multidimensional array
for j=1:1:r*r
    rowset1=ddash(j,1:3);
    rowset2=ddash(j,4:6);    
    combosets(:,:,j)=[rowset1;rowset2];    
end

0 个评论
显示 -2更早的评论隐藏 -2更早的评论

请先登录，再进行评论。

请先登录，再回答此问题。

Answer 1

Johannes Fischer 2020-9-8

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/590545-find-different-possible-combinations-of-the-rows-of-two-matrices#answer_491440

在 MATLAB Online 中打开

a = [1 2 3; 4 5 6; 7 8 10]; % 3 by 3 matrix 
b=a + 10; % 3 by 3 matrix
r=size(a, 1);
% create repetitions of values
a = repmat(a, [r, 1]);
b = repelem(b, r, 1);
% create combination matrix
c = [a b];
% reshape into combosets
combosets = reshape(c', 3, 2, 9);
% permute to get dimensions right
combosets = permute(combosets, [2, 1, 3]);

1 个评论
显示 -1更早的评论隐藏 -1更早的评论

Tuhin Choudhury 2020-9-8

编辑：Tuhin Choudhury 2020-9-8

Thank you! works perfectly fine. Infact your method works fastest and so I accepted this answer . although all of them work perfectly fine

请先登录，再进行评论。

Answer 2

Adam Danz 2020-9-8

0
链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/590545-find-different-possible-combinations-of-the-rows-of-two-matrices#answer_491437

编辑：Adam Danz 2020-9-8

在 MATLAB Online 中打开

Fast & clean 2-liner:

a = [1 2 3; 4 5 6; 7 8 10];
b = a + 10;
c = [repmat(a,size(b,1),1), repelem(b, size(a,1),1)];
combosets = permute(reshape(c.',3,2,9),[2,1,3]);

5 个评论
显示 3更早的评论隐藏 3更早的评论

Tuhin Choudhury 2020-9-8

编辑：Tuhin Choudhury 2020-9-8

I completely agree both are identical and I wish I could accept more than 1 answer. True that the readability helps as well. However for the speed, my problem lies with the vector size instead of reps, so initially i tested with 'a' as rand(1050x3) and the results were like:

>> Tuhin

Elapsed time is 2.835849 seconds.

>> Stephen

Elapsed time is 0.124517 seconds.

>> Johannes

Elapsed time is 0.085730 seconds.

>> Adam

Elapsed time is 0.094663 seconds.

Then I realized that in this case the speed might depend on the values generated by rand. So I tested with a fixed matrix 'a' (size (7,3)).The resutls were:

>> Tuhin

Elapsed time is 0.031925 seconds.

>> Stephen

Elapsed time is 0.007881 seconds.

>> Johannes

Elapsed time is 0.002299 seconds.

>> Adam

Elapsed time is 0.002421 seconds.

Like I said, both of you have very identical speed (way faster than mine :) )

Thanks again for the answer!

Adam Danz 2020-9-8

"I wish I could accept more than 1 answer"

You accepted the right one if it works for your data and it make sense to you. The time difference is so small that readability and understandability are important factors. Plus, I like when newer contributors get recognition for their contributions and encouragement to continue doing so 🙂.

请先登录，再进行评论。