How to Create a Sparse Matrix with SciPy

If most of the elements of the matrix have 0 value, then it is called a sparse matrix. The two major benefits of using sparse matrix instead of a simple matrix are:

• Storage: There are lesser non-zero elements than zeros and thus lesser memory can be used to store only those elements.
• Computing time: Computing time can be saved by logically designing a data structure traversing only non-zero elements.

Sparse matrices are generally utilized in applied machine learning such as in data containing data-encodings that map categories to count and also in entire subfields of machine learning such as natural language processing (NLP).

Example:

0 0 3 0 4
0 0 5 7 0
0 0 0 0 0
0 2 6 0 0

Representing a sparse matrix by a 2D array leads to wastage of lots of memory as zeroes in the matrix are of no use in most of the cases. So, instead of storing zeroes with non-zero elements, we only store non-zero elements. This means storing non-zero elements with triples- (Row, Column, value).

Create a Sparse Matrix with SciPy

The scipy.sparse module in Python provides efficient ways to store and work with large sparse matrices using various formats, each optimized for specific operations. Let's explore the different methods this module offers:

Examples

Example 1: csr_matrix (Compressed Sparse Row)

import numpy as np from scipy.sparse import csr_matrix d = np.array([3, 4, 5, 7, 2, 6]) # data r = np.array([0, 0, 1, 1, 3, 3]) # rows c = np.array([2, 4, 2, 3, 1, 2]) # cols csr = csr_matrix((d, (r, c)), shape=(4, 5)) print(csr.toarray()) 

Output

[[0 0 3 0 4]
[0 0 5 7 0]
[0 0 0 0 0]
[0 2 6 0 0]]

Explanation: Creates a Compressed Sparse Row (CSR) matrix using non-zero values d and their row r and column c indices. csr_matrix stores only the non-zero elements efficiently and toarray() converts it back to a full 2D NumPy array.

Example 2: csc_matrix (Compressed Sparse Column)

import numpy as np from scipy.sparse import csc_matrix d = np.array([3, 4, 5, 7, 2, 6]) # data r = np.array([0, 0, 1, 1, 3, 3]) # rows c = np.array([2, 4, 2, 3, 1, 2]) # cols csc = csc_matrix((d, (r, c)), shape=(4, 5)) print(csc.toarray()) 

Output

[[0 0 3 0 4]
[0 0 5 7 0]
[0 0 0 0 0]
[0 2 6 0 0]]

Explanation: Creates a Compressed Sparse Column (CSC) matrix using non-zero values d with their row r and column c indices. csc_matrix efficiently stores data column-wise and toarray() converts it back to a full 2D NumPy array.

Example 3: coo_matrix (Coordinate Format)

import numpy as np from scipy.sparse import coo_matrix d = np.array([3, 4, 5, 7, 2, 6]) # data r = np.array([0, 0, 1, 1, 3, 3]) # rows  c = np.array([2, 4, 2, 3, 1, 2]) # cols coo = coo_matrix((d, (r, c)), shape=(4, 5)) print(coo.toarray()) 

Output

[[0 0 3 0 4]
[0 0 5 7 0]
[0 0 0 0 0]
[0 2 6 0 0]]

Explanation: Creates a COO matrix using non-zero values and their row-column positions. Suitable for quick construction and toarray() returns the full matrix.

Example 4: lil_matrix (List of Lists)

import numpy as np from scipy.sparse import lil_matrix lil = lil_matrix((4, 5)) lil[0, 2] = 3 lil[0, 4] = 4 lil[1, 2] = 5 lil[1, 3] = 7 lil[3, 1] = 2 lil[3, 2] = 6 print(lil.toarray()) 

Output

[[0. 0. 3. 0. 4.]
[0. 0. 5. 7. 0.]
[0. 0. 0. 0. 0.]
[0. 2. 6. 0. 0.]]

Explanation: Creates a List of Lists (LIL) matrix by assigning non-zero values directly to specified row and column positions. lil_matrix allows efficient row-wise insertion

Example 5: dok_matrix (Dictionary of Keys)

import numpy as np from scipy.sparse import dok_matrix dok = dok_matrix((4, 5)) dok[0, 2] = 3 dok[0, 4] = 4 dok[1, 2] = 5 dok[1, 3] = 7 dok[3, 1] = 2 dok[3, 2] = 6 print(dok.toarray()) 

Output

[[0. 0. 3. 0. 4.]
[0. 0. 5. 7. 0.]
[0. 0. 0. 0. 0.]
[0. 2. 6. 0. 0.]]

Example 6: dia_matrix (Diagonal Matrix)

import numpy as np from scipy.sparse import dia_matrix d = np.array([[3, 0, 0, 0, 0], [0, 5, 0, 0, 0]]) offsets = np.array([0, -1]) dia = dia_matrix((d, offsets), shape=(4, 5)) print(dia.toarray()) 

Output

[[3 0 0 0 0]
[0 0 0 0 0]
[0 5 0 0 0]
[0 0 0 0 0]]