How to save and then append/augment a matrix into another already existing matrix?

Hi guys, I have got a brief question about saving a file. I have a file named myfile.mat, (meaning this file already exists) and it contains let's say a matrix M of size 20x4 with all-zero elements. Now I want to run a code that produces a matrix N inside a loop of sizes 3x4 where 3 is the number of cycles of that loop. So putting it into a code would look like:

for j = 1:3
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.
.
.
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N(j,:) = [a vector of length 4];
end

In the end I will get matrix N of size Cx4. Then I would like to save N into myfile.mat by replacing the elements in M in the rows whose indices match that of the loop indices. So in the above case I would want to fill from 1st to 3rd row of M with the elements of N leaving the other elements in another rows unaffected. Should I set the cycle indices to run from let's say 5 to 10 (j=5:10) then I will fill only elements in row 5 to 10 of M.

Any suggestion for how to do this? Thanks for the help.

Answer 1

[per isakson]

See matfile, Access and change variables directly in MAT-files, without loading into memory

Answer 2

[Imam]

Thanks to you both, that really describes the solution to my problem.