Inserting values to an empty multidim. numpy array [duplicate]

Question

I need to create an empty numpy array of a shape (?, 10, 10, 3). ? means I don't know how many elements will be inserted. Then I have many numpy arrays of a shape (1, 10, 10, 3) which I want to be inserted to the prepared array one by one, so the ? mark will be increasing with inserted elements.

I'm trying all variations of numpy array methods like empty, insert, concatenate, append... but I'm not able to achieve this. Could you please give me a hand with this?

Don't do that; NumPy arrays are fixed-size, and trying to extend them dynamically requires making a new array every time, with horribly expensive, quadratic runtime costs. — user2357112
– user2357112, Commented Dec 6, 2017 at 18:33
If you have some idea on the max possible length along the first axis, simply pre-allocate with out = (M,10,10,3), with M being that max number and index and assign at each iteration : out[i] = .. keeping the count of iterations. At the end, simply slice out out[:C] with Cbeing that last iteration number/count. — Divakar
– Divakar, Commented Dec 6, 2017 at 18:39
Thanks, I didn't know that. So I did it by removing the first single-dimensional entry of the inserted element, appended it to a regular python array and that made it a NumPy array. It works that way, not sure about effectiveness. — T.Poe
– T.Poe, Commented Dec 6, 2017 at 18:43
If you really have to do something like this, don't accumulate into an array, accumulate into a plain list object using my_list.append(array_x), then, do final_array = np.array(my_list) — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Dec 6, 2017 at 18:43
@juanpa.arrivillaga Yes I meant a list, sry. I did it the way you suggested. — T.Poe
– T.Poe, Commented Dec 6, 2017 at 18:48

Jacques Gaudin · Accepted Answer · 2017-12-06 20:52:08Z

1

Using np.append works:

import numpy as np

mat = np.empty((0,10,10,3))

array = np.random.rand(1,10,10,3)

mat = np.append(mat, array, axis=0)
mat = np.append(mat, array, axis=0)

print(mat.shape)

>>>(2,10,10,3)

edited Dec 6, 2017 at 20:52

answered Dec 6, 2017 at 20:46

Jacques Gaudin

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4 Comments

T.Poe Over a year ago

Thanks. But what about this "horribly expensive, quadratic runtime costs" as someone mentioned in the comments under the question? Is it true or not?

Jacques Gaudin Over a year ago

I believe it is. You'd be better off appending to a list and then form the array if you can. I was just answering your question!

T.Poe Over a year ago

Ok, so I just use the list list appending, thanks again.

hpaulj Over a year ago

If mat and array match in dimensions, I prefer to use concatenate. np.append looks too much like list.append, confusing beginners in many ways.

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