sort eigenvalues and associated eigenvectors after using numpy.linalg.eig in python
I'm using numpy.linalg.eig to obtain a list of eigenvalues and eigenvectors:
A = someMatrixArray
from numpy.linalg import eig as eigenValuesAndVectors
solution = eigenValuesAndVectors(A)
eigenValues = solution[0]
eigenVectors = solution[1]
I would like to sort my eigenvalues (e.g. from lowest to highest), in a way I know what is the associated eigenvector after the sorting.
I'm not finding any way of doing that with python functions. Is there any simple way or do I have to code my sort version?
Answer
Use numpy.argsort. It returns the indices one would use to sort the array.
import numpy as np
import numpy.linalg as linalg
A = np.random.random((3,3))
eigenValues, eigenVectors = linalg.eig(A)
idx = eigenValues.argsort()[::-1]
eigenValues = eigenValues[idx]
eigenVectors = eigenVectors[:,idx]
If the eigenvalues are complex, the sort order is lexicographic (that is, complex numbers are sorted according to their real part first, with ties broken by their imaginary part).