Reputation: 4497
For SciPy sparse matrix, one can use todense()
or toarray()
to transform to NumPy matrix or array. What are the functions to do the inverse?
I searched, but got no idea what keywords should be the right hit.
Upvotes: 118
Views: 186910
Reputation: 2708
In Python, the Scipy library can be used to convert the 2-D NumPy matrix into a Sparse matrix. SciPy 2-D sparse matrix package for numeric data is scipy.sparse
The scipy.sparse package provides different Classes to create the following types of Sparse matrices from the 2-dimensional matrix:
CSR (Compressed Sparse Row) or CSC (Compressed Sparse Column) formats support efficient access and matrix operations.
Example code to Convert Numpy matrix into Compressed Sparse Column(CSC) matrix & Compressed Sparse Row (CSR) matrix using Scipy classes:
import sys # Return the size of an object in bytes
import numpy as np # To create 2 dimentional matrix
from scipy.sparse import csr_matrix, csc_matrix
# csr_matrix: used to create compressed sparse row matrix from Matrix
# csc_matrix: used to create compressed sparse column matrix from Matrix
create a 2-D Numpy matrix
A = np.array([[1, 0, 0, 0, 0, 0],\
[0, 0, 2, 0, 0, 1],\
[0, 0, 0, 2, 0, 0]])
print("Dense matrix representation: \n", A)
print("Memory utilised (bytes): ", sys.getsizeof(A))
print("Type of the object", type(A))
Print the matrix & other details:
Dense matrix representation:
[[1 0 0 0 0 0]
[0 0 2 0 0 1]
[0 0 0 2 0 0]]
Memory utilised (bytes): 184
Type of the object <class 'numpy.ndarray'>
Converting Matrix A to the Compressed sparse row matrix representation using csr_matrix Class:
S = csr_matrix(A)
print("Sparse 'row' matrix: \n",S)
print("Memory utilised (bytes): ", sys.getsizeof(S))
print("Type of the object", type(S))
The output of print statements:
Sparse 'row' matrix:
(0, 0) 1
(1, 2) 2
(1, 5) 1
(2, 3) 2
Memory utilised (bytes): 56
Type of the object: <class 'scipy.sparse.csr.csc_matrix'>
Converting Matrix A to Compressed Sparse Column matrix representation using csc_matrix Class:
S = csc_matrix(A)
print("Sparse 'column' matrix: \n",S)
print("Memory utilised (bytes): ", sys.getsizeof(S))
print("Type of the object", type(S))
The output of print statements:
Sparse 'column' matrix:
(0, 0) 1
(1, 2) 2
(2, 3) 2
(1, 5) 1
Memory utilised (bytes): 56
Type of the object: <class 'scipy.sparse.csc.csc_matrix'>
As it can be seen the size of the compressed matrices is 56 bytes and the original matrix size is 184 bytes.
For a more detailed explanation and code examples please refer to this article: https://limitlessdatascience.wordpress.com/2020/11/26/sparse-matrix-in-machine-learning/
Upvotes: 5
Reputation: 641
As for the inverse, the function is inv(A)
, but I won't recommend using it, since for huge matrices it is very computationally costly and unstable. Instead, you should use an approximation to the inverse, or if you want to solve Ax = b you don't really need A-1.
Upvotes: -1
Reputation: 10139
There are several sparse matrix classes in scipy.
bsr_matrix(arg1[, shape, dtype, copy, blocksize]) Block Sparse Row matrix
coo_matrix(arg1[, shape, dtype, copy]) A sparse matrix in COOrdinate format.
csc_matrix(arg1[, shape, dtype, copy]) Compressed Sparse Column matrix
csr_matrix(arg1[, shape, dtype, copy]) Compressed Sparse Row matrix
dia_matrix(arg1[, shape, dtype, copy]) Sparse matrix with DIAgonal storage
dok_matrix(arg1[, shape, dtype, copy]) Dictionary Of Keys based sparse matrix.
lil_matrix(arg1[, shape, dtype, copy]) Row-based linked list sparse matrix
Any of them can do the conversion.
import numpy as np
from scipy import sparse
a=np.array([[1,0,1],[0,0,1]])
b=sparse.csr_matrix(a)
print(b)
(0, 0) 1
(0, 2) 1
(1, 2) 1
See http://docs.scipy.org/doc/scipy/reference/sparse.html#usage-information .
Upvotes: 32
Reputation: 18121
You can pass a numpy array or matrix as an argument when initializing a sparse matrix. For a CSR matrix, for example, you can do the following.
>>> import numpy as np
>>> from scipy import sparse
>>> A = np.array([[1,2,0],[0,0,3],[1,0,4]])
>>> B = np.matrix([[1,2,0],[0,0,3],[1,0,4]])
>>> A
array([[1, 2, 0],
[0, 0, 3],
[1, 0, 4]])
>>> sA = sparse.csr_matrix(A) # Here's the initialization of the sparse matrix.
>>> sB = sparse.csr_matrix(B)
>>> sA
<3x3 sparse matrix of type '<type 'numpy.int32'>'
with 5 stored elements in Compressed Sparse Row format>
>>> print sA
(0, 0) 1
(0, 1) 2
(1, 2) 3
(2, 0) 1
(2, 2) 4
Upvotes: 162