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I have a two matrix mm and nn. Matrix mm is of dimension 8 cross 8 and nn is of dimensions 4 cross 4. I want to replace the first four rows and columns in mm by nn. How to do this?

mm = ConstantArray[q, {8, 8}]
nn = ConstantArray[r, {4, 4}]
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    $\begingroup$ Try mm[[;; 4, ;; 4]] = nn. $\endgroup$ – J. M. is away Apr 4 at 7:07
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you can use Table and make any changes you like

Table[mm[[j, k]] = nn[[j, k]], {j, 4}, {k, 4}]   

{{r,r,r,r,q,q,q,q},{r,r,r,r,q,q,q,q},{r,r,r,r,q,q,q,q},{r,r,r,r,q,q,q,q},{q,q,q,q,q,q,q,q},{q,q,q,q,q,q,q,q},{q,q,q,q,q,q,q,q},{q,q,q,q,q,q,q,q}}

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  • $\begingroup$ Do makes more sense here than Table. $\endgroup$ – Roman May 4 at 10:03

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