Two Dimensional Python Array Sort: Sort the first having the same second values

Question

I am currently working on coordinates and somehow, I need to sort the x-coordinates in ascending order having the same y-coordinates without touching the y-coordinates.

Here's the sample input:

x = [213,212,213,214,215,216,215,216,217,216,218,217,219,220,219]
y = [352,333,332,330,328,327,327,326,325,325,324,324,323,322,322]
xy = np.array([x,y])

Here's the desired output:

x = [213,212,213,214,215,215,216,216,216,217,217,218,219,219,220]
y = [352,333,332,330,328,327,327,326,325,325,324,324,323,322,322]

Note: I have been searching for this one but I can't seem to find a direct answer. Thank you.

@Copperfield I'm not sure if sorting the x list first is the same idea because it might change the order of the y list. I only need to sort the x having the same y values without touching the y. — Melon Streams
– Melon Streams, Commented Jun 2, 2022 at 16:56
in your example the x list is independent from the y list, whatever you do the x list have no effect on the y list — Copperfield
– Copperfield, Commented Jun 2, 2022 at 17:01
@Copperfield sorry for the wrong example, i somehow updated it — Melon Streams
– Melon Streams, Commented Jun 2, 2022 at 17:35

Copperfield · Accepted Answer · 2022-06-02 18:51:27Z

1

you can first make a list of pair and order that by the first coordinate and them make it into a numpy array

>>> x = [213,212,213,214,215,216,215,216,217,216,218,217,219,220,219]
>>> y = [352,333,332,330,328,327,327,326,325,325,324,324,323,322,322]
>>> xy=sorted(zip(x,y),key=lambda pair:pair[0])
>>> xy
[(212, 333), (213, 352), (213, 332), (214, 330), (215, 328), (215, 327), (216, 327), (216, 326), (216, 325), (217, 325), (217, 324), (218, 324), (219, 323), (219, 322), (220, 322)]
>>> nxy = np.array(xy).T
>>> nxy
array([[212, 213, 213, 214, 215, 215, 216, 216, 216, 217, 217, 218, 219,
        219, 220],
       [333, 352, 332, 330, 328, 327, 327, 326, 325, 325, 324, 324, 323,
        322, 322]])
>>> nxy
array([[212, 213, 213, 214, 215, 215, 216, 216, 216, 217, 217, 218, 219,
        219, 220],
       [333, 352, 332, 330, 328, 327, 327, 326, 325, 325, 324, 324, 323,
        322, 322]])
>>>

after some good old trial and error find a way to sort them in numpy

>>> x = [0, 500, 100, 300, 200, 800, 400, 600, 700, 900]
>>> y = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> xy= np.array([x,y])
>>> xy
array([[  0, 500, 100, 300, 200, 800, 400, 600, 700, 900],
       [  0,   1,   2,   3,   4,   5,   6,   7,   8,   9]])
>>> np.argsort(xy[0]) #get an array with the index that correspond to the elements in a sorted fashion 
array([0, 2, 4, 3, 6, 1, 7, 8, 5, 9], dtype=int64)
>>> xy[:,np.argsort(xy[0])] #ask for the elements in the aforementioned order, while preserving their pairing
array([[  0, 100, 200, 300, 400, 500, 600, 700, 800, 900],
       [  0,   2,   4,   3,   6,   1,   7,   8,   5,   9]])
>>>

edited Jun 2, 2022 at 18:51

answered Jun 2, 2022 at 16:56

Copperfield

8,6353 gold badges26 silver badges32 bronze badges

Sign up to request clarification or add additional context in comments.

13 Comments

Kelly Bundy Over a year ago

Interesting. How does sorting xy[0] sort both rows correspondingly? Where do I look in the documentation for that?

Copperfield Over a year ago

I suppose it should be somewhere in the numpy documentation, I was testing to see if it worked or not

Kelly Bundy Over a year ago

Oh wait, I thought I had seen a difference in the second row before and after but I don't see it anymore, so I can't tell whether the second row was touched or not.

Copperfield Over a year ago

in the second example you can see that the second row wasn't affected by sorting only the first

Kelly Bundy Over a year ago

Maybe. The whole question is really unclear and weird. Among other issues, they say "having the same y-coordinates without touching the y-coordinates", which sounds like saying the same thing twice. So presumably the two things don't mean the same? But then what do they mean? It's a mystery...

|

Collectives™ on Stack Overflow

Two Dimensional Python Array Sort: Sort the first having the same second values

1 Answer 1

13 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

13 Comments

Your Answer

Sign up or log in

Post as a guest

Related