How to append arrays to another numpy array?-CodePudding

I am trying to loop through a set of coordinates and 'stacking' these arrays of coordinates to another array (so in essence I want to have an array of arrays) using numpy.

This is my attempt:

import numpy as np
all_coordinates = np.array([[]])

for y in range(2):
    for x in range(2):
        coordinate = np.array([[x,y]])

        # append
        all_coordinates = np.append(all_coordinates,[coordinate])

print(all_coordinates)

But it's not working. It's just concatenating the individual numbers and not appending the array.

Instead of giving me (the output that I want to achieve):

[[0 0] [1 0] [0,1] [1,1]]

The output I get instead is:

[0 0 1 0 0 1 1 1]

Why? What I am doing wrong here?

CodePudding user response：

The problem that stack functions don't work, is that they need that the row added is of the same size of the already present rows. Using np.array([[]]), the first row is has a length of zero, which means that you can only add rows that also have length zero.

In order to solve this, we need to tell Numpy that the first row is of size two and not zero. The array thus needs to be of size (0, 2) and not (0, 0). This can be done using one of the array-initializing functions that accept size arguments, like empty, zeros or ones. Which function does not matter, as there are no spaces to fill.

Then you can use one of the functions mentioned in comments, like vstack or stack. The code thus becomes:

import numpy as np
all_coordinates = np.zeros((0, 2))

for y in range(2):
    for x in range(2):
        coordinate = np.array([[x,y]])

        # append
        all_coordinates = np.vstack((all_coordinates, coordinate))

print(all_coordinates)

CodePudding user response：

In such a case, I would use a list and only convert it into an array once you have appended all the elements you want. here is a suggested improvement

import numpy as np
all_coordinates = []

for y in range(2):
    for x in range(2):
        coordinate = np.array([x,y])

        # append
        all_coordinates.append(coordinate)

all_coordinates = np.array(all_coordinates)

print(all_coordinates)

The output of this code is indeed

array([[0, 0],
       [1, 0],
       [0, 1],
       [1, 1]])