How can I calculate the nearest positive semi-definite matrix?

Question

I\'m coming to Python from R and trying to reproduce a number of things that I\'m used to doing in R using Python. The Matrix library for R has a very nifty function called

Answer 1

This is perhaps a silly extension to DomPazz answer to consider both correlation and covariance matrices. It also has an early termination if you are dealing with a large number of matrices.

def near_psd(x, epsilon=0):
    '''
    Calculates the nearest postive semi-definite matrix for a correlation/covariance matrix

    Parameters
    ----------
    x : array_like
      Covariance/correlation matrix
    epsilon : float
      Eigenvalue limit (usually set to zero to ensure positive definiteness)

    Returns
    -------
    near_cov : array_like
      closest positive definite covariance/correlation matrix

    Notes
    -----
    Document source
    http://www.quarchome.org/correlationmatrix.pdf

    '''

    if min(np.linalg.eigvals(x)) > epsilon:
        return x

    # Removing scaling factor of covariance matrix
    n = x.shape[0]
    var_list = np.array([np.sqrt(x[i,i]) for i in xrange(n)])
    y = np.array([[x[i, j]/(var_list[i]*var_list[j]) for i in xrange(n)] for j in xrange(n)])

    # getting the nearest correlation matrix
    eigval, eigvec = np.linalg.eig(y)
    val = np.matrix(np.maximum(eigval, epsilon))
    vec = np.matrix(eigvec)
    T = 1/(np.multiply(vec, vec) * val.T)
    T = np.matrix(np.sqrt(np.diag(np.array(T).reshape((n)) )))
    B = T * vec * np.diag(np.array(np.sqrt(val)).reshape((n)))
    near_corr = B*B.T    

    # returning the scaling factors
    near_cov = np.array([[near_corr[i, j]*(var_list[i]*var_list[j]) for i in xrange(n)] for j in xrange(n)])
    return near_cov