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AttributeError using scipy.sparse.toarray()

import scipy as sp
import numpy as np

a=sp.sparse.coo_matrix(np.random.randint(0,9,[4,5]))
b=sp.sparse.coo_matrix(np.random.randint(0,9,[4,2]))
sp.hstack([a,b]).toarray()


is giving me

AttributeError: 'numpy.ndarray' object has no attribute 'toarray'


could you help me with my silly mistake here?

1 Answer
1

sp.hstack (i.e. numpy.hstack) is the ordinary, dense hstack, which won't combine the sparse arrays correctly. It builds a 1D numpy array already (of object dtype; in other words, it just wraps the Python-level objects and crams them in there.) You want scipy.sparse.hstack:

sp.hstack

numpy.hstack

scipy.sparse.hstack

In [332]: sp.hstack([a, b])
Out[332]:
array([<4x5 sparse matrix of type '<class 'numpy.int64'>'
with 17 stored elements in COOrdinate format>,
<4x2 sparse matrix of type '<class 'numpy.int64'>'
with 7 stored elements in COOrdinate format>], dtype=object)

In [333]: sp.hstack([a, b]).toarray()
---------------------------------------------------------------------------
AttributeError Traceback (most recent call last)
<ipython-input-333-4f469eb98b12> in <module>()
----> 1 sp.hstack([a, b]).toarray()

AttributeError: 'numpy.ndarray' object has no attribute 'toarray'

In [334]: sp.sparse.hstack([a, b])
Out[334]:
<4x7 sparse matrix of type '<class 'numpy.int64'>'
with 24 stored elements in COOrdinate format>

In [335]: sp.sparse.hstack([a, b]).toarray()
Out[335]:
array([[3, 2, 7, 0, 5, 5, 1],
[7, 1, 2, 1, 7, 0, 8],
[6, 1, 6, 1, 8, 6, 2],
[7, 6, 0, 5, 0, 8, 8]], dtype=int64)


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