Cross 2 arrays

[paul18fr]

It's not straigtforward (and it might be improved), but it does the job quit fastly even for huge vectors :-).

import time
import numpy as np

n,m=1_000_000, 4_000_000
a=np.ones((n), dtype=int)
b=2*np.ones((m), dtype=int)

t0=time.time()
La=len(a)
Lb=len(b)
if (Lb<La):
    c=np.concatenate((a[:Lb], b))
    c=np.reshape(c, (Lb, 2), order='f')
    c=np.reshape(c, (2*Lb), order='c')
    c=np.concatenate((c, a[Lb::]))
elif (Lb>La):
    c=np.concatenate((a, b[:La]))
    c=np.reshape(c, (La, 2), order='f')
    c=np.reshape(c, (2*La), order='c')
    c=np.concatenate((c, b[La::]))
else:
    c=np.concatenate((a, b))
    c=np.reshape(c, (La, 2), order='f')
    c=np.reshape(c, (2*La), order='c')
t1=time.time()
print(f"Duration={t1-t0}")