Combine elements from two arrays by pairs

Question

So I want to concatenate two arrays but by pairs. The input is as follows:

a = array([1, 1, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1])
b = array([0, 0, 1, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0])

And the output should be as follows:

out_put = 
[[1, 0],
[1, 0],
[0, 1],
[1, 0],
[0, 1],
[0, 1],
[0, 1],
[0, 1],
[0, 1],
[0, 1],
[0, 1],
[0, 1],
[0, 1],
[0, 1],
[1, 0]]

I managed to get such result by iterating over the two arrays

out_put = [[a[i],b[i]] for i in range(len(a)]

but I wonder if there any faster way .

Thank you

jpp · Accepted Answer · 2018-07-05T13:56:54.693

8

For a vectorised solution, you can stack and transpose:

a = np.array([1, 1, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1])
b = np.array([0, 0, 1, 0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0])

c = np.vstack((a, b)).T
# or, c = np.dstack((a, b))[0]

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

edited Jul 05 '18 at 13:56

answered Jul 05 '18 at 13:16

jpp

159,742
34
281
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1

Or just `np.dstack((a, b))`, though it's only a wrapper around concatenate – Brad Solomon Jul 05 '18 at 13:24
@BradSolomon, Thanks, think you'll also need to index `[0]` with that solution. – jpp Jul 05 '18 at 13:26
2

@BradSolomon `column_stack` is `dstack` without adding the additional dimension for 2 1D arrays – user3483203 Jul 05 '18 at 13:55

score 5 · Answer 2 · answered Jul 05 '18 at 13:55

Using np.column_stack

Stack 1-D arrays as columns into a 2-D array.

np.column_stack((a, b))

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

score 3 · Answer 3 · answered Jul 05 '18 at 13:21

3

You can use the zip function to combine any two iterables like this. It will continue until it reaches the end of the shorter iterable

list(zip(a, b))
# [(1, 0), (1, 0), (0, 1), (1, 0), (0, 1), (0, 1), (0, 1), (0, 1), (0, 1), (0, 1), (0, 1), (0, 1), (0, 1), (0, 1), (1, 0)]

answered Jul 05 '18 at 13:21

Patrick Haugh

59,226
13
88
96

Note: `zip` + NumPy arrays are actually inefficient: see [this answer](https://stackoverflow.com/a/50399219/9209546). – jpp Jul 05 '18 at 13:23

Combine elements from two arrays by pairs

3 Answers3