Put a list as element in 2D array in Python numpy

Question

I've got a 2D matrix from numpy (np.zeros) and I would like that every element of this matrix is a 1D list with 2 elements, this is my code:

import numpy as np


N = 3
x = np.linspace(0,10,N)
y = np.linspace(0,10,N)

temp = np.zeros((N, N))

for i in range(len(x)):
    for j in range(len(y)):
        temp[i, j] = [x[i], y[j]]

but I've got error:

TypeError: float() argument must be a string or a real number, not 'list'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File (...), line 15, in <module>
    temp[i, j] = [x[i], y[j]]
ValueError: setting an array element with a sequence.

And okay, I understand this problem, but I don't know how to solve it.

You can also use a 3 dimensional matrix (not sure if this is what you wanted). temp = np.zeros((N,N,2)) — NirF
– NirF, Commented Feb 26, 2023 at 17:16
Can you provide an example of your expected output? You have two arrays containing 6 entries between them - how would that be mapped into an array with 9 entries? — Brian61354270
– Brian61354270, Commented Feb 26, 2023 at 17:17
What are you trying to solve? Do you want a 3d array of floats, or a 2d with actual list elements. Those are very different arrays. — hpaulj
– hpaulj, Commented Feb 26, 2023 at 17:22

Deadly Pointer · Accepted Answer · 2023-02-26 17:24:00Z

2

You can set the dtype=object when constructing the array and store lists, but that's quite inefficient. It's possible to define a custom dtype:

import numpy as np

# custom type of 2 float64 fields
pair_type = np.dtype("f8, f8")
x = np.zeros((3, 3), dtype=pair_type)
x[0, 0] = (1.5, 2.5)

You can retrieve x[i, j] as a tuple.

answered Feb 26, 2023 at 17:24

Deadly Pointer

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Comments

RomanPerekhrest · Accepted Answer · 2023-02-26 18:00:44Z

0

The result you are searching for can be conveniently achieved with numpy.meshgrid + numpy.stack:

np.stack(np.meshgrid(x, y, indexing='ij'), axis=-1)

array([[[ 0.,  0.],
        [ 0.,  5.],
        [ 0., 10.]],

       [[ 5.,  0.],
        [ 5.,  5.],
        [ 5., 10.]],

       [[10.,  0.],
        [10.,  5.],
        [10., 10.]]])

answered Feb 26, 2023 at 18:00

RomanPerekhrest

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