Joining two 2D numpy arrays into a single 2D array of 2-tuples

Question

I have two 2D numpy arrays like this, representing the x/y distances between three points. I need the x/y distances as tuples in a single array.

So from:

x_dists = array([[ 0, -1, -2],
                 [ 1,  0, -1],
                 [ 2,  1,  0]])

y_dists = array([[ 0, -1, -2],
                 [ 1,  0, -1],
                 [ 2,  1,  0]])

I need:

dists = array([[[ 0,  0], [-1, -1], [-2, -2]],
               [[ 1,  1], [ 0,  0], [-1, -1]],
               [[ 2,  2], [ 1,  1], [ 0,  0]]])

I've tried using various permutations of dstack/hstack/vstack/concatenate, but none of them seem to do what I want. The actual arrays in code are liable to be gigantic, so iterating over the elements in python and doing the rearrangement "manually" isn't an option speed-wise.

Edit: This is what I came up with in the end: https://gist.github.com/807656

Is is just a coincidence that x_dist and y_dist are the same? Also what is the meaning of negative distance in your app? Would _dist allways be 'symmetric', but uppertridiagonal part just negative? How many points are you expecting to handle? What is the purpose of this stacking of arrays? Would the elements be in 'optimal' order for further calculations? Just some toughts, thanks — eat
– eat, Commented Feb 2, 2011 at 13:26
It's coincidence. The example points are (0, 0), (1, 1), (2, 2) for sanity's sake. _dist is indeed symmetric. The purpose of stacking the arrays is that I can use them to move the points around. See linked gist. — Zarkonnen
– Zarkonnen, Commented Feb 2, 2011 at 14:07

eumiro · Accepted Answer · 2011-02-02 12:02:04Z

13

import numpy as np
dists = np.vstack(([x_dists.T], [y_dists.T])).T

returns dists like you wanted them. Afterwards it is not "a single 2D array of 2-tuples", but a normal 3D array where the third axis is the concatenation of the two original arrays.

You see:

dists.shape # (3, 3, 2)

answered Feb 2, 2011 at 12:02

eumiro

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1 Comment

Zarkonnen Over a year ago

That's indeed what I wanted. I fear that my usage of the term "2D array of 2-tuples" was rather misleading. :(

jfs · Accepted Answer · 2011-02-02 11:37:40Z

8

numpy.rec.fromarrays([x_dists, y_dists], names='x,y')

answered Feb 2, 2011 at 11:37

jfs

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2 Comments

Zarkonnen Over a year ago

Brilliant! That's exactly what I wanted. So, um, seeing as you seem to know about numpy, is the function I edited in a sane way of calculating these pairwise distances?

Zarkonnen Over a year ago

Edit: Actually, this doesn't give me an array I know how to do anything with.

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