append numpy arrays in a loop to an empty numpy array

Question

I have an empty numpy array. And I want to append arrays to it such that each appended array becomes an element.

import numpy as np

a = np.array([])

for i in range(3):
    a=np.append(a,np.array(['x','y','z']))

print(a)

My expected outcome is : a= [['x','y','z'],['x','y','z'],['x','y','z']] but this seems to be impossible without adding axis=1 and handling the first append differently. This adds to unnecessary if condition every time in loop. Same is the problem while using vstack also. The first insert to the array has to happen with a hstack and the subsequent ones with a vstack.

What is the best way to achieve this in numpy?

TIA :)

best is not to do any of this in a loop. You are trying imitate a list method. Stick with lists. — hpaulj
– hpaulj, Commented Apr 29, 2023 at 14:17
elaborate "handling the first append differently", how those subarrays differ? — RomanPerekhrest
– RomanPerekhrest, Commented Apr 29, 2023 at 14:20
But if you insist, You have to start with a correct shaped a, like (0,3) shape. And add a (1,3) shape each loop. And make sure each has the correct dtype. And use axis=0. The goal is a (n,3) array. Keep that in mind. — hpaulj
– hpaulj, Commented Apr 29, 2023 at 14:24

Mechanic Pig · Accepted Answer · 2023-04-29 14:24:27Z

2

You should not use any method of concatenating arrays repeatedly. Each concatenation will create a brand new array, which is a huge waste of time and space.

The best practice should be to create an array list and then use a single stack to build the target array:

>>> np.vstack([np.array(['x','y','z']) for _ in range(3)])
array([['x', 'y', 'z'],
       ['x', 'y', 'z'],
       ['x', 'y', 'z']], dtype='<U1')

Some other construction methods for this example:

>>> np.tile(np.array(['x', 'y', 'z']), (3, 1))
array([['x', 'y', 'z'],
       ['x', 'y', 'z'],
       ['x', 'y', 'z']], dtype='<U1')
>>> np.array(['x','y','z'])[None].repeat(3, 0)
array([['x', 'y', 'z'],
       ['x', 'y', 'z'],
       ['x', 'y', 'z']], dtype='<U1')

edited Apr 29, 2023 at 14:24

answered Apr 29, 2023 at 14:18

Mechanic Pig

7,7813 gold badges17 silver badges35 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

append numpy arrays in a loop to an empty numpy array

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related