Optimized way to split the <numpy ndarray> to two columns in Python

Question

Let's consider the following numpy ndarray,

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

I want to make it to two column like,

    A     B
0   1     0
1   1     0
2   1     0
3   1     0
4   1     0
5   0     1
6   1     0

Now, I'm doing in the following way,

a = []
b = []
for i in lb_results:
    if i == 0:
        a.append(1)
        b.append(0)
    else:
        a.append(0)
        b.append(1)

lb_results = np.column_stack((a, b))
print(lb_results)

but I'm expecting something more optimized way, (less number of code lines are better, without using much more loops) Any suggestions would be grateful, Thanks.

There's no need for any loops, just create a second 1D array using the methods here and then stack (all you're doing is flipping 0's and 1's) — roganjosh
– roganjosh, Commented Oct 18, 2018 at 14:27

Paul Panzer · Accepted Answer · 2018-10-18 14:49:26Z

1

You could use xor like so:

>>> c = (np.c_[:7] == 5).astype(int)
>>> c
array([[0],
       [0],
       [0],
       [0],
       [0],
       [1],
       [0]])
>>> c ^ (1, 0)
array([[1, 0],
       [1, 0],
       [1, 0],
       [1, 0],
       [1, 0],
       [0, 1],
       [1, 0]])

I guess this is about as short as it gets ;-)

The magic behind this is numpy broadcasting. Briefly, xor operator ^ gets applied to each pair between an element of the column c and an element of the the 1D sequence (1, 0) leading to the full "xor table" between the two.

edited Oct 18, 2018 at 14:49

answered Oct 18, 2018 at 14:33

Paul Panzer

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4 Comments

Markus Over a year ago

Maybe unnecessarily short... I would suggest np.column_stack((np.logical_not(b), b))

Jeyakumar Kasi Over a year ago

@paul Thanks, I think, it would be difficult, if once we don't know the index of 1's (i.e, in your example it is '5')

Paul Panzer Over a year ago

@JaiK It works for any column c consisting of ones and zeros. The first line is just a shortcut to recreate your example vector. Please try it out: create a different c and do c ^ (1, 0).

Jeyakumar Kasi Over a year ago

@PaulPanzer Yes, Paul. Good, Working nice.. sorry, I understood mistakenly.

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