Coming from a Matlab background, I found sparse matrices to be easy to use and well integrated into the language. However, when transitioning to python’s scientific computing ecosystem, I had a harder time using sparse matrices. This post is intended to help Matlab refugees and those interested in using sparse matricies in python (in particular, for clustering)
Requirements:
scikit-learn (2.10+)
numpy (refer to scikit-learn version requirements)
scipy (refer to scikit-learn version requirements)
Sparse Matrix Types:
There are six types of sparse matrices implemented under scipy:
- bsr_matrix -- block sparse row matrix
- coo_matrix -- sparse matrix in coordinate format
- csc_matrix -- compressed sparse column matrix
- csr_matrix -- compressed sparse row matrix
- dia_matrix -- sparse matrix with diagonal storage
- dok_matrix -- dictionary of keys based sparse matrix
- lil_matrix -- row-based linked list sparse matrix
For more info see:
(http://docs.scipy.org/doc/scipy/reference/sparse.html):
When to use which matrix:The following are scenarios when you would want to choose one sparse matrix type over the another:- Fast Arithmetic Operation: csc_matrix, csr_matrix
- Fast Column Slicing (e.g., A[:, 1:2]): csc_matrix
- Fast Row Slicing (e.g., A[1:2, :]) csr_matrix
- Fast Matrix vector products: csr_matrix, bsr_matrix, csc_matrix
- Fast Changing of sparsity (e.g., adding entries to matrix): lil_matrix, dok_matrix
- Fast conversion to other sparse formats: coo_matrix
- Constructing Large Sparse Matrices: coo_matrix
Clustering with scikit-learn:
With the release of scikit-learn 2.10, one of the useful new features is the support for sparse matrices with the k-means algorithm. The following is how you would use sparse matrices with k-means:
Full Matrix to Sparse Matrix
Example-1
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from numpy.random import random from scipy.sparse import *
from sklearn.cluster import KMeans
# create a 30x1000 dense matrix random matrix.
D = random((30,1000))
# keep entries with value < 0.10 (10% of entries in matrix will be non-zero)
# X is a "full" matrix that is intrinsically sparse.
X = D*(D<0.10) # note: element wise mult
# convert D into a sparse matrix (type coo_matrix)
# note: we can initialize any type of sparse matrix.
# There is no particular motivation behind using
# coo_matrix for this example.
S = coo_matrix(X)
labeler = KMeans(k=3)
# convert coo to csr format
# note: Kmeans currently only works with CSR type sparse matrix
labeler.fit(S.tocsr())
# print cluster assignments for each row
for (row, label) in enumerate(labeler.labels_):
print "row %d has label %d"%(row, label)
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One of the issues with Example-1 is that we are constructing a sparse matrix from a full matrix. It will often be the case that we will not be able to fit a full (although intrinsically sparse) matrix in memory. For example, if the matrix X was a 100000x1000000000 full matrix, there could be some issues. One solution to this is to somehow extract out the non-zero entries of X and to use a smarter constructor for the sparse matrix.
Sparse Matrix Construction
In Example-2, we will assume that we have X's data stored on some file on disk. In particular, we will assume that X is stored in a csv file and that we are able to extract out the non-zero data efficiently.
Example-2
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import csv
from scipy.sparse import *
from sklearn.cluster import KMeans
def extract_nonzero(fname):
"""
extracts nonzero entries from a csv file
input: fname (str) -- path to csv file
output: generator<(int, int, float)> -- generator
producing 3-tuple containing (row-index, column-index, data)
"""
for (rindex,row) in enumerate(csv.reader(open(fname))):
for (cindex, data) in enumerate(row):
if data!="0":
yield (rindex, cindex, float(data))
def get_dimensions(fname):
"""
determines the dimension of a csv file
input: fname (str) -- path to csv file
output: (nrows, ncols) -- tuple containing row x col data
"""
rowgen = (row for row in csv.reader(open(fname)))
# compute col size
colsize = len(rowgen.next())
# compute row size
rowsize = 1 + sum(1 for row in rowgen)
return (rowsize, colsize)
# obtain dimensions of data
(rdim, cdim) = get_dimensions("X.csv")
# allocate a lil_matrix of size (rdim by cdim)
# note: lil_matrix is used since we be modifying
# the matrix a lot.
S = lil_matrix((rdim, cdim))
# add data to S
for (i,j,d) in extract_nonzero("X.csv"):
S[i,j] = d
# perform clustering
labeler = KMeans(k=3)
# convert lil to csr format
# note: Kmeans currently only works with CSR type sparse matrix
labeler.fit(S.tocsr())
# print cluster assignments for each row
for (row, label) in enumerate(labeler.labels_):
print "row %d has label %d"%(row, label)
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What to do when Sparse Matrices aren't supported:
When sparse matrices aren't supported, one solution is to convert the matrix to a full matrix. To do this, simply invoke the todense() method.
