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

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Sunny el 27 de Sept. de 2018
Editada: Sunny el 21 de Oct. de 2018
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 el 28 de Sept. de 2018
@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.
Sunny el 28 de Sept. de 2018
Thanks @Stephen. Yes, I used ytzhak answer for this issue and accepted his answer

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ytzhak goussha el 27 de Sept. de 2018
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 el 21 de Oct. de 2018
@yzthak,
You may want to read the definition of matricide. I believe you meant matrices
Sunny el 21 de Oct. de 2018
Editada: Sunny el 21 de Oct. de 2018
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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