Question

If I have two parallel lists and want to sort them by the order of the elements in the first, it’s very easy:

>>> a = [2, 3, 1]
>>> b = [4, 6, 7]
>>> a, b = zip(*sorted(zip(a,b)))
>>> print a
(1, 2, 3)
>>> print b
(7, 4, 6)

How can I do the same using numpy arrays without unpacking them into conventional Python lists?

Answer 1

b[a.argsort()] should do the trick.

Here’s how it works. First you need to find a permutation that sorts a. argsort is a method that computes this:

>>> a = numpy.array([2, 3, 1])
>>> p = a.argsort()
>>> p
[2, 0, 1]

You can easily check that this is right:

>>> a[p]
array([1, 2, 3])

Now apply the same permutation to b.

>>> b = numpy.array([4, 6, 7])
>>> b[p]
array([7, 4, 6])