A= [eye(2,1), 3*ones(2,3); linspace(3,9,4); zeros(1,4)] 
d=diag(A)' /// \sleftarrow{\normalfont main diagonal of \verb+A+ as a row matrix}
B=diag(d)   /// \sleftarrow{\normalfont builds a diagonal matrix  }
C=matrix(d,2,2) /// \sleftarrow{\normalfont reshape vector \verb+d+}
/// @@prerequisite  D=[1,2,3].*.ones(2,2) /// \sleftarrow{\normalfont Kronecker product}