The Diagonal(A) command returns the main diagonal of the Matrix A as a (column) Vector.

$\mathrm{with}\left(\mathrm{Student}\left[\mathrm{LinearAlgebra}\right]\right)\:$

$A≔⟨⟨1\,2\,3⟩\left|⟨4\,5\,6⟩\right|⟨7\,8\,9⟩|⟨10\,11\,12⟩⟩$

$A≔\left[\begin{array}{cccc}1& 4& 7& 10\\ 2& 5& 8& 11\\ 3& 6& 9& 12\end{array}\right]$

$\mathrm{Diagonal}\left(A\right)$

$\left[\begin{array}{c}1\\ 5\\ 9\end{array}\right]$