Can you concatenate multiple arrays from the output of a function without using temporal variables in Python?

Question

Problem:

The function f returns the numpy ndarrays np_array_1, np_array_2. Both arrays have the same length, but this length may be different for each call.
I want to call f several times and keep only the two arrays concatenated from the different calls.

Question: Is there a way to do it without using temporal variables? Using temporal variables:

def f(i):
   ...
   return np_array_1, np_array_2

np_array_1, np_array_2 = f(0)
for i in range(1, 5):
   np_array_1_t, np_array_2_t = f(i)
   np_array_1 = np.concatenate(np_array_1, np_array_1_t)
   np_array_2 = np.concatenate(np_array_2, np_array_2_t)
del np_array_1_t, np_array_2_t

@hpaulj What do you mean with this? Why would I use more than two arrays? — Puco4
– Puco4, Commented Jun 29, 2021 at 13:15

hpaulj · Accepted Answer · 2021-06-29 14:47:37Z

1

This may make the list operations of the other answer more understandable

alist1 = []; alist2 = []
np_array_1, np_array_2 = f(0)
for i in range(0, 5):
   a1, a2 = f(i)
   alist1.append(a1); alist2.append(a2)
np_array_1 = np.concatenate(alist1)
np_array_2 = np.concatenate(alist2)

concatenate and array (and variations) all take a list of arrays as input. It's better to collect the arrays in one list, and do just on concatenate. List append is more efficient and suitable for iterative application.

answered Jun 29, 2021 at 14:47

hpaulj

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Comments

orlp · Accepted Answer · 2021-06-29 10:51:25Z

0

If I understand your question correctly the following works:

a1s, a2s = zip(*(f(i) for i in range(5)))
a1 = np.concatenate(a1s)
a2 = np.concatenate(a2s)

Or even simpler if all arrays have the same size:

a1, a2 = np.array([f(i) for i in range(5)]).squeeze().T

answered Jun 29, 2021 at 10:51

orlp

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1 Comment

Puco4 Over a year ago

In general the arrays in my problem do not have the same size. Your first approach still uses temporal variables, but it is more concise than the method I thought. However, I do not completely understand it. So (f(i) for i in range(5)) is like a tuple of 5 tuples of 2 arrays? I am a bit lost with the asterisk and the zip...

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