Get original indices of a sorted Numpy array

Question

I have an array of distances a = np.array([20.5 ,5.3 ,60.7 ,3.0 ], 'double') and I need the indices of the sorted array (for example [3, 1, 0, 2] , for a.sort() ). Is there a function in Numpy to do that?

Answer 1

Yes, there's the x = numpy.argsort(a) function or x = numpy.ndarray.argsort(a) method. It does exactly what you're asking for. You can also call argsort as a method on an ndarray object like so: a.argsort() .

Here's a link to the documentation: http://docs.scipy.org/doc/numpy/reference/generated/numpy.argsort.html#numpy.argsort

Answer 2

Here's an example, for reference and convenience:

# create an array
a = np.array([5,2,3])

# np.sort - returns the array, sorted
np.sort(a)
>>> array([2, 3, 5])

# argsort - returns the original indexes of the sorted array
np.argsort(a)
>>> array([1, 2, 0])