Python: sort a list and change another one consequently

Question

I have two lists: one contains a set of x points, the other contains y points. Python somehow manages to mix the x points up, or the user could. I'd need to sort them by lowest to highest, and move the y points to follow their x correspondants. They are in two separate lists.. how do I do it?

Answer 1

You could zip the lists and sort the result. Sorting tuples should, by default, sort on the first member.

>>> xs = [3,2,1]
>>> ys = [1,2,3]
>>> points = zip(xs,ys)
>>> points
[(3, 1), (2, 2), (1, 3)]
>>> sorted(points)
[(1, 3), (2, 2), (3, 1)]

And then to unpack them again:

>>> sorted_points = sorted(points)
>>> new_xs = [point[0] for point in sorted_points]
>>> new_ys = [point[1] for point in sorted_points]
>>> new_xs
[1, 2, 3]
>>> new_ys
[3, 2, 1]

Answer 2

>>> xs = [5, 2, 1, 4, 6, 3]
>>> ys = [1, 2, 3, 4, 5, 6]
>>> xs, ys = zip(*sorted(zip(xs, ys)))
>>> xs
(1, 2, 3, 4, 5, 6)
>>> ys
(3, 2, 6, 4, 1, 5)

Answer 3

>>> import numpy

>>> sorted_index = numpy.argsort(xs)
>>> xs = [xs[i] for i in sorted_index]
>>> ys = [ys[i] for i in sorted_index]

if you can work with numpy.array

>>> xs = numpy.array([3,2,1])
>>> xs = numpy.array([1,2,3])
>>> sorted_index = numpy.argsort(xs)
>>> xs = xs[sorted_index]
>>> ys = ys[sorted_index]

Answer 4

If the x and the y are meant to be a single unit (such as a point), it would make more sense to store them as tuples rather than two separate lists.

Regardless, here's what you should do:

x = [4, 2, 5, 4, 5,…]
y = [4, 5, 2, 3, 1,…]

zipped_list = zip(x,y)
sorted_list = sorted(zipped_list)