How to extract diagonal elements of multiple nxn square matrix and place them in single mat file or matrix?

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Sunny 2018-9-27

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编辑： Sunny 2018-10-21

Hello,

I have 500 mat files with 10x10 double dimension. I want to extract the 10 diagonal elements from each matrix and place them in a single matrix which will be 500x10 (500 rows and 10 columns) each row consists of diagonal elements of one matrix. How can I make a single variable with all the diagonal elements?

Thanks

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Stephen23 2018-9-28

@Varun Mandalapu: note that concatenating onto the output array is not very efficient. For a more efficient solution you should preallocate the output array and use indexing, as ytzhak goussha's answer shows.

https://www.mathworks.com/help/matlab/matlab_prog/preallocating-arrays.html

Sunny 2018-9-28

Thanks @Stephen. Yes, I used ytzhak answer for this issue and accepted his answer

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ytzhak goussha 2018-9-27

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在 MATLAB Online 中打开

Hope this helps

step1:creat a nXnXm with random elements for the example:

    %set dimensions of matrix 
    size_of_matrix=10;
    number_of_matricides=500;
    mat_input=randi(30,size_of_matrix,size_of_matrix,number_of_matricides);

step2: initialize an output matrix with nXm dimensions

mat_output=zeros(number_of_matricides,size_of_matrix);

step3: extract a diagonal from each matrix and place them in the output matrix

    for i=1:number_of_matricides
        mat_output(i,:)=diag(mat_input(:,:,i));
    end

Alternatively, though not recommended, you can simply concatenate:

     %initialize an empty matrix
    mat_output=[];
    for i=1:number_of_matricides
       mat_output=[mat_output;diag(mat_input(:,:,i))'];
    end

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Guillaume 2018-10-21

@yzthak,

Just reread your answer in more details, and I noticed something that made me chuckle:

You may want to read the definition of matricide. I believe you meant matrices

Sunny 2018-10-21

编辑：Sunny 2018-10-21

Thanks for the comment, I figured out that I need to create a struct with all the variables and access it. I already created a question and the link is New_Question

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