Lexicographic sort float array python

Question

Okay, so, I have a 4x2 numpy ndarray, and I want to sort it lexicographically. That is, if I have the array

[[0,0],
[1,1],
[0,1],
[1,0]]

I want it to become

[[0,0],
[0,1],
[1,0],
[1,1]]

How do I do this?

Doesn't seem to be doing it. Going l.sort() turns [[2,1], [0,2]] into [[1,2], [0,2]] instead of [[0,2], [2,1]] (when it's an ndarray). — Red
– Red, Commented Jan 21, 2015 at 18:27
l.sort() converts l = [[2,1], [0,2]] into [[0, 2], [2, 1]] only for a normal list and not for an numpy array — Bhargav Rao
– Bhargav Rao, Commented Jan 21, 2015 at 18:31

tom10 · Accepted Answer · 2015-01-21 18:35:31Z

5

You can use numpy's lexsort. Lexsort, though, sorts using the last column as the primary key. One way to get what you want is to specify the columns explicitly:

 x[np.lexsort((x[:,1], x[:,0]))]

 # array([[0, 0],
 #   [0, 1],
 #   [1, 0],
 #   [1, 1]])

answered Jan 21, 2015 at 18:35

tom10

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3 Comments

Red Over a year ago

Do you know what the time complexity of this is?

user2357112 Over a year ago

@PedroCarvalho: For an h-by-w array, the worst case performance is O(h*w*log(h)), since it's a comparison sort on an input of length h where comparisons can take worst-case O(w) time. If rows usually differ in the first few columns, the expected performance will be O(h*log(h)), since comparisons will take expected O(1) time.

user2357112 Over a year ago

Also, rather than listing columns individually, you can do x[np.lexsort(x.T[::-1])]. (That's still way too many intermediate steps to just do a lexicographic sort, though. NumPy's sorting API seems to be put together in a really weird way.)

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