Diagonal Operation

Creates a matrix from only the diagonal entries of a matrix. $diag(A)$

Example

$$diag(\left[\begin{array}{ccc}1& 2& 3\\ 4& 5& 6\\ 7& 8& 9\end{array}\right])=\left[\begin{array}{ccc}1& 0& 0\\ 0& 5& 0\\ 0& 0& 9\end{array}\right]$$