def primal_energy(u, f, lamda, coilSens, mask, nabla):
    ''' Compute the primal energy as described in Eq. (1) in the assignment
        sheet.
        @param u: current reconstruction
        @param f: raw input data
        @param lamda: tradeoff parameter between data fidelity and smoothness
    '''

    img_csr = sp.csc_matrix(u.reshape(-1, 1))

    dx = sp.csr_matrix.dot(nabla[0], img_csr)
    dy = sp.csr_matrix.dot(nabla[1], img_csr)

    dxd = np.abs(sp.csr_matrix.todense(dx).reshape([320, 320]))
    dyd = np.abs(sp.csr_matrix.todense(dy).reshape([320, 320]))

    tvd = np.sum(np.sqrt(dxd**2+dyd**2))

    return tvd+lamda/2*np.linalg.norm(op_A_mri(u, coilSens, mask)-f)