i want to extract rows from a matrix

Question

Mahmoud Chawki 2022-5-16

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回答： Animesh Gupta 2022-6-8

i have a matrix that the number of rows are always even.

i want a code that extracts 2 rows and put them together

for example:

s=[Row1;Row2;Row3;Row4;Row5;Row6]

s is a matrix that has six rows

i want to extract row 1 and row 2, put them together.

row 3 and row 4, put them together.

row 5 and row 6, put them together.

how can i achieve this?

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Mahmoud Chawki 2022-5-16

The problem is that this matrix varies in rows, it depends on a number which is number of plies if number of plies is 3 this matrix will have 6 rows if number of lies is 4 this matrix will have 8 rows so you see it is alternating and depends on the input that the user uses

DGM 2022-5-16

编辑：DGM 2022-5-16

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You're going to have to define the factors either way, so the fact that the array may differ in size doesn't matter. The problem is that we still don't know what you're actually trying to multiply with what and what the final output is.

For instance, if you want every pair of rows multiplied by a scalar:

A = [7 9 6; 9 1 5; 2 9 6; 9 2 1; 4 7 5; 7 8 8]
A = 6×3
     7     9     6
     9     1     5
     2     9     6
     9     2     1
     4     7     5
     7     8     8
k = repelem(1:size(A,1)/2,1,2).'
k = 6×1
     1
     1
     2
     2
     3
     3
B = A.*k
B = 6×3
     7     9     6
     9     1     5
     4    18    12
    18     4     2
    12    21    15
    21    24    24

Note that this example works regardless of how many rows A has. Generating k as a simple linear ramp is probably not what you want, but you haven't said what you want the factors to be.

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Answer 1

Animesh Gupta 2022-6-8

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Hi,

It is my understanding that you want to extract adjacent rows of a matrix.

You may refer the following code snippet that demonstrates a procedure to extract adjacent rows.

mat = rand(10,5); % creating an array using rand method
disp(mat);
num_of_rows = size(mat,1); % using size method to get the dimensions of matrix along axis 1
new_mat = [];
for i = 1:num_of_rows-1
    if mod(i,2) == 1
        new_mat = cat(3, new_mat, [mat(i,:); mat(i+1,:)]); % using cat method to append along the 3rd dimension of the new matrix
    end
end
% You can access the individual 2d sub-arrays as:
disp(new_mat(:,:,1));
disp(new_mat(:,:,2));