Numpy Array of Lists

Question

I want to create a Numpy array, which contains two (Python-) lists. When i try to append an element to one of those lists, the code crashes.

import numpy as np
list0 = [0,0,0]
list1 = [1,1,1]
#list1.append(0)
print(type(list0))
print(type(list1))
array0 = np.array((list0, list1))
array0[0].append(42)
print(array0)

The confusing thing is that when i uncomment the 4th line, the code works just fine.

The error message i get:

File "test.py", line 10, in <module>
array0[0].append(3)
AttributeError: 'numpy.ndarray' object has no attribute 'append'

I run on python 3.5.1 and numpy 1.10.4

I just found out that np.array(((0,0,0),(1,1,1))) tries to create a 2d array. This does not happen, if the two lists (e.g. (0,0,0) and (1,1,1)) have different size (e.g. (0,0) and (1,1,1)). A way to initialize an array with two empty lists is to write array0 = np.empty(2, dtype=np.object) array0[:] = [], [] — The_Mad_Duck
– The_Mad_Duck, Commented Apr 13, 2016 at 9:51
Yes, np.array defaults to making a mulidimensional array. Making an object dtype is the 2nd class backup choice. It may be better to use plain lists, even faster. — hpaulj
– hpaulj, Commented Apr 13, 2016 at 12:53

James McCormac · Accepted Answer · 2016-04-14 08:50:25Z

0

How about using numpy's stack functions? You can use vstack (vertical stack) and hstack (horizontal stack) to append lists/arrays together. You can then also continue to stack more lists/arrays onto the newly created stack. I give three examples below.

Python (saved in file stackingArrays)

import numpy as np

list0 = [0,0,0]
list1 = [1,1,1]

# stack vertically
array_v=np.vstack((list0,list1))
print array_v

# stack horizontally
array_h=np.hstack((list0,list1))
print array_h

# stack more on to stacked array
array_v2=np.vstack((array_v,list1))
print array_v2

Output

> python -i stackingArrays.py
>>> [[0 0 0]
    [1 1 1]]
>>> [0 0 0 1 1 1]
>>> [[0 0 0]
    [1 1 1]
    [1 1 1]]

answered Apr 14, 2016 at 8:50

James McCormac

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itai schwed · Accepted Answer · 2020-04-16 19:03:35Z

0

the lists has both the same size that's why it become a 2d np array.

therefor you are trying to append a value to one of the rows of the np array (that you can't because it's not a list any more).

I recommend you to use a np array of lists as shown here.

good luck ;)

answered Apr 16, 2020 at 19:03

itai schwed

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