How i create a matrix from a nested loop?

Question

DIMITRIOS THEODOROPOULOS 2018-6-26

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/407490-how-i-create-a-matrix-from-a-nested-loop

编辑： Adam Danz 2018-6-26

For this simple loop

     for i=1:5
       for j=1:5
       A=[i j]
        end
         end

the result i get is 25 arrays ,2x1 each one. But i want them gathered as a matrix of 25x2.How can i simply do it?

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Adam Danz 2018-6-26

On each loop you're overwriting the value of A so you won't get 25 arrays, you'll get one array, 'A' that is of size [1-by-2] and A will equal [5,5] which are the last values of i and j.

You expect to get an array that is [25-by-2] but what values do you expect to be in that array?

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Answer 1

Adam Danz 2018-6-26

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/407490-how-i-create-a-matrix-from-a-nested-loop#answer_326349

编辑：Adam Danz 2018-6-26

在 MATLAB Online 中打开

If this is what you're looking for

A = zeros(5*5,2);
c = 0;
for i = 1:5
    for j = 1:5
        c=c+1;
        A(c, :) = [i,j];
    end
end

You can do this instead

A = [reshape(repmat(1:5,5,1),25,1), repmat(1:5,1,5)'];

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Stephen23 2018-6-26

Note that it is recommended to preallocate arrays before loops:

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

Array preallocation is a habit that all beginners need to learn.

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Answer 2

Adam 2018-6-26

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https://ww2.mathworks.cn/matlabcentral/answers/407490-how-i-create-a-matrix-from-a-nested-loop#answer_326350

编辑：Adam 2018-6-26

在 MATLAB Online 中打开

A = zeros( 25, 2 );
for i=1:5
   for j=1:5
      idx = 5 * ( i - 1 ) + j;
      A(idx,:) =[i, j];
   end
end

gives you what you want for your example, although there are much more efficient ways to create the resultant matrix than using a nested for loop, e.g.

A = [ repelem( 1:5, 5 ); repmat( 1:5, 1, 5 ) ]';

to throw in yet another variant!