CODEWITHSUNDEEP
pythonarraysnumpy
user2944669
user2944669

Reputation: 165

numpy array : can not find how to transform the array

I have 2 numpy arrays: b and s. And len(b)==2*len(s).

s has the form:

l r l r l r ...

I what to transform it into:

0 0 l r 0 0 l r 0 0 l r ...

I'm doing it the easy way:

ix = 0
jx = 0
while ix < len(b):
    b[ix] = 0
    ix += 1
    b[ix] = 0
    ix += 1 
    b[ix] = s[jx]
    ix += 1
    jx += 1
    b[ix] = s[jx]
    ix += 1
    jx += 1

For speed, I'd like to do the same using numpy api but can't get something working.

What numpy methods should I be using?

Upvotes: 2

Views: 117

Answers (2)

hpaulj
hpaulj

Reputation: 231385

Based on the repetition in b and s, I automatically see a 2d solution:

b = np.zeros(N,4)
b[:,2:] = s.reshape(N,2)
b = b.flatten()

or appending the 0s onto the reshaped s:

b = np.hstack([np.zeros((10,2)),s.reshape(10,2)]).flatten()

or if we want to think in 3d

np.einsum('ij,k->ikj',s.reshape(N,2),[0,1]).flatten()

Same but with broadcasting

(s.reshape(10,2)[:,None,:]*np.array([0,1])[None,:,None]).flatten()

Upvotes: 1

Lev Levitsky
Lev Levitsky

Reputation: 65791

May not be the smartest solution, but you can do it in two passes similar to this answer:

In [1]: import numpy as np

In [2]: s = np.array([1, 2] * 3)

In [3]: b = np.zeros(2 * s.size)

In [4]: b[2::4] = s[::2]

In [5]: b[3::4] = s[1::2]

In [6]: b
Out[6]: array([ 0.,  0.,  1.,  2.,  0.,  0.,  1.,  2.,  0.,  0.,  1.,  2.])

Upvotes: 3

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