Numpy array insert every second element from second array

Question

I have two arrays of the same shape and now want to combine them by making every odd element and 0 one of the first array and every even one of the second array in the same order. E.g.:

a = ([0,1,3,5])
b = ([2,4,6])

c = ([0,1,2,3,4,5,6])

I tried something including modulo to identify uneven indices:

a = ([0,1,3,5])
b = ([2,4,6])

c = a

i = 0
j = 2
l = 0

for i in range(1,22):
    k = (i+j) % 2
    if k > 0:
        c = np.insert(c, i, b[l])
        l+=1
    else: 
        continue

I guess there is some easier/faster slicing option, but can't figure it out.

Chapmacl · Accepted Answer · 2020-09-01 20:55:22Z

2

np.insert would work well:

>>> A = np.array([1, 3, 5, 7])
>>> B = np.array([2, 4, 6, 8])
>>> np.insert(B, np.arange(len(A)), A)
array([1, 2, 3, 4, 5, 6, 7, 8])

However, if you don't rely on sorted values, try this:

>>> A = np.array([5, 3, 1])
>>> B = np.array([1, 2, 3])
>>> C = [ ]
>>> for element in zip(A, B):
        C.extend(element)
>>> C
[5, 1, 3, 2, 1, 3]

edited Sep 1, 2020 at 20:55

answered Sep 1, 2020 at 20:46

Chapmacl

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LongToeBoy · Accepted Answer · 2020-09-01 20:45:32Z

0

read the documentation of the range

for i in range(0,10,2):
  print(i)

will print [0,2,4,6,8]

answered Sep 1, 2020 at 20:45

LongToeBoy

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ilmarinen · Accepted Answer · 2020-09-01 21:05:38Z

0

From what I understand, the first element in a is always first the rest are just intereleaved. If that is the case, then some clever use of stacking and reshaping is probably enough.

a = np.array([0,1,3,5])
b = np.array([2,4,6])
c = np.hstack([a[:1], np.vstack([a[1:], b]).T.reshape((-1, ))])

answered Sep 1, 2020 at 21:05

ilmarinen

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tamr · Accepted Answer · 2020-09-01 21:28:02Z

0

You could try something like this

import numpy as np

A = [0,1,3,5]
B = [2,4,6]
lst = np.zeros(len(A)+len(B))

lst[0]=A[0]
lst[1::2] = A[1:]
lst[2::2] = B

Even though I don't understand why you would make it so complicated

answered Sep 1, 2020 at 21:28

tamr

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