How to write the code of eigenvalue decomposition (EVD) ?

If i have a matrix A,and ignore the rank-1 constraint for each matrix A,and A=a times a^H,that is ,(A=a a^H),and we can obtain a by using Eigenvalue Decomposition (EVD) of A.How do i write the code about this?

Does anyone know or have an example about eigenvalue decomposition (EVD) ? Or does anyone know how to write the code of my question?