CODEWITHSUNDEEP
pythonarraysnumpy
astrofrog
astrofrog

Reputation: 34091

Numpy object array of numerical arrays

I want to create an array with dtype=np.object, where each element is an array with a numerical type, e.g int or float. For example:

>>> a = np.array([1,2,3])
>>> b = np.empty(3,dtype=np.object)
>>> b[0] = a
>>> b[1] = a
>>> b[2] = a

Creates what I want:

>>> print b.dtype
object

>>> print b.shape
(3,)

>>> print b[0].dtype
int64

but I am wondering whether there isn't a way to write lines 3 to 6 in one line (especially since I might want to concatenate 100 arrays). I tried

>>> b = np.array([a,a,a],dtype=np.object)

but this actually converts all the elements to np.object:

>>> print b.dtype
object

>>> print b.shape
(3,)

>>> print b[0].dtype
object

Does anyone have any ideas how to avoid this?

Upvotes: 5

Views: 3659

Answers (4)

joeln
joeln

Reputation: 3643

a = np.array([1,2,3])
b = np.empty(3, dtype='O')
b[:] = [a] * 3

should suffice.

Upvotes: 3

tom10
tom10

Reputation: 69182

It's not exactly pretty, but...

import numpy as np

a = np.array([1,2,3])
b = np.array([None, a, a, a])[1:]

print b.dtype, b[0].dtype, b[1].dtype
# object int32 int32

Upvotes: 3

astrofrog
astrofrog

Reputation: 34091

I can't find any elegant solution, but at least a more general solution to doing everything by hand is to declare a function of the form:

def object_array(*args):
    array = np.empty(len(args), dtype=np.object)
    for i in range(len(args)):
        array[i] = args[i]
    return array

I can then do:

a = np.array([1,2,3])
b = object_array(a,a,a)

I then get:

>>> a = np.array([1,2,3])
>>> b = object_array(a,a,a)
>>> print b.dtype
object
>>> print b.shape
(3,)
>>> print b[0].dtype
int64

Upvotes: 0

SiggyF
SiggyF

Reputation: 23145

I think anyarray is what you need here:

b = np.asanyarray([a,a,a])
>>> b[0].dtype
dtype('int32')

not sure what happened to the other 32bits of the ints though.

Not sure if it helps but if you add another array of a different shape, it converts back to the types you want:

import numpy as np
a = np.array([1,2,3])
b = np.array([1,2,3,4])
b = np.asarray([a,b,a], dtype=np.object)
print(b.dtype)
>>> object
print(b[0].dtype)
>>> int32

Upvotes: -1

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