Concatenate 2D numpy array with an empty array

Question

This Python 3.8 code snippet:

a = np.array([[1, 2], [3, 4]])
b = np.array([[5, 6]])
a = np.concatenate((a, b), axis=0)
print(a)

produces:

[[1 2]
 [3 4]
 [5 6]]

as expected. How do I define an empty array e, such that this code produces the same result:

# define en empty array e here
a = np.array([[1, 2], [3, 4]])
b = np.array([[5, 6]])
e = np.concatenate((e, a), axis=0)
e = np.concatenate((e, b), axis=0)
print(e)

The use case here is an algorithm, which starts with an empty array and concatenates additional arrays to it in a loop.

Does this answer your question? Concatenating empty array in Numpy — Mahrkeenerh
– Mahrkeenerh, Commented Nov 12, 2021 at 11:14
@DaniMesejo I have to start with an empty array for brevity and elegance. — Paul Jurczak
– Paul Jurczak, Commented Nov 12, 2021 at 11:17
Elegance is in the eye of the beholder... and the answer posted is for sure not more breve (in terms of LOC) that the one in the question. — Dani Mesejo
– Dani Mesejo, Commented Nov 12, 2021 at 11:19
@DaniMesejo Obviously not an elegance of the snippet I posted. Brevity of an algorithm, which assumes an empty array at the beginning. — Paul Jurczak
– Paul Jurczak, Commented Nov 12, 2021 at 11:27

Paul Jurczak · Accepted Answer · 2021-11-12 11:16:23Z

1

Found it:

e = np.empty((0, 2))
a = np.array([[1, 2], [3, 4]])
b = np.array([[5, 6]])
e = np.concatenate((e, a), axis=0)
e = np.concatenate((e, b), axis=0)
print(e)

answered Nov 12, 2021 at 11:16

Paul Jurczak

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4 Comments

hpaulj Over a year ago

better yet f = np.concatenate((e,a,b), axis=0). One concatenate on the full list is better than repeated ones. Less copying.

Paul Jurczak Over a year ago

@hpaulj Yes, but that wasn't the question.

hpaulj Over a year ago

Sorry, I mistook you for a beginner who didn't realize that list append is more efficient than repeated np.concatenate. List append works in-place, and just adds a reference. concatenate makes a new array each time, with all the copying that entails. So yes, if you want to concatenate a "empty" array it needs to have the same number of dimensions as the others, here (0,2), (2,2) , (1,2). Calling np.empty((0,2)) "empty" is a bit of a misnomer. It shouldn't be confused with the list [].

hpaulj Over a year ago

Watch the dtype with this code. e is a float, a is int. Conditional code that sets e=a on the first iteration may be safer, and a bit faster (one less concatenate), even it it isn't as pretty.

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