numpy combine arrays with index ordering

Question

I would like to combine multiple numpy arrays by the order of the index. Here is what I mean: Suppose I have two numpy arrays like

>>> a = [0, 1, 2, 3]
>>> b = [-5, -6, -7, -8]

How can I combine these arrays such that the output is

c = np.combination_function((a,b))
>>> c = [0 -5, 1, -6, 2, -7, 3, -8]

concatenate and append will place a and b "next" to each other, which is not quite what I want.

Numpy's repeat and tile functions serve as an analogy here. For example

np.repeat([3, 4, 2], 2)
>>> [3, 3, 4, 4, 2, 2] # two 3's, then two 4's, then two 2's. This is what I want
np.tile([3, 4, 2], 2)
>>> [3, 4, 2, 3, 4, 2] # [3,4,2] twice. This is not what I want

tile is to concatenate as repeat is to ???

A solution:

I've been able to accomplish this with

c = np.stack((a,b)).flatten('F')

Is there a better way to do this?

Paul Panzer · Accepted Answer · 2020-07-20 23:34:51Z

2

Your solution is pretty good. You can speed it up a bit by doing the stacking already in FORTRAN order:

timeit(lambda:np.stack([a,b]).flatten("F"))
# 11.13118030806072
timeit(lambda:np.array([a,b],order="F").ravel(order="K"))
# 3.0580440941266716

answered Jul 20, 2020 at 23:34

Paul Panzer

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