FeatureHasher and DictVectorizer Comparison

Compares FeatureHasher and DictVectorizer by using both to vectorize text documents.

The example demonstrates syntax and speed only; it doesn?t actually do anything useful with the extracted vectors. See the example scripts {document_classification_20newsgroups,clustering}.py for actual learning on text documents.

A discrepancy between the number of terms reported for DictVectorizer and for FeatureHasher is to be expected due to hash collisions.

# Author: Lars Buitinck
# License: BSD 3 clause
 
from __future__ import print_function
from collections import defaultdict
import re
import sys
from time import time
 
import numpy as np
 
from sklearn.datasets import fetch_20newsgroups
from sklearn.feature_extraction import DictVectorizer, FeatureHasher
 
 
def n_nonzero_columns(X):
    """Returns the number of non-zero columns in a CSR matrix X."""
    return len(np.unique(X.nonzero()[1]))
 
 
def tokens(doc):
    """Extract tokens from doc.
 
    This uses a simple regex to break strings into tokens. For a more
    principled approach, see CountVectorizer or TfidfVectorizer.
    """
    return (tok.lower() for tok in re.findall(r"\w+", doc))
 
 
def token_freqs(doc):
    """Extract a dict mapping tokens from doc to their frequencies."""
    freq = defaultdict(int)
    for tok in tokens(doc):
        freq[tok] += 1
    return freq
 
 
categories = [
    'alt.atheism',
    'comp.graphics',
    'comp.sys.ibm.pc.hardware',
    'misc.forsale',
    'rec.autos',
    'sci.space',
    'talk.religion.misc',
]
# Uncomment the following line to use a larger set (11k+ documents)
#categories = None
 
print(__doc__)
print("Usage: %s [n_features_for_hashing]" % sys.argv[0])
print("    The default number of features is 2**18.")
print()
 
try:
    n_features = int(sys.argv[1])
except IndexError:
    n_features = 2 ** 18
except ValueError:
    print("not a valid number of features: %r" % sys.argv[1])
    sys.exit(1)
 
 
print("Loading 20 newsgroups training data")
raw_data = fetch_20newsgroups(subset='train', categories=categories).data
data_size_mb = sum(len(s.encode('utf-8')) for s in raw_data) / 1e6
print("%d documents - %0.3fMB" % (len(raw_data), data_size_mb))
print()
 
print("DictVectorizer")
t0 = time()
vectorizer = DictVectorizer()
vectorizer.fit_transform(token_freqs(d) for d in raw_data)
duration = time() - t0
print("done in %fs at %0.3fMB/s" % (duration, data_size_mb / duration))
print("Found %d unique terms" % len(vectorizer.get_feature_names()))
print()
 
print("FeatureHasher on frequency dicts")
t0 = time()
hasher = FeatureHasher(n_features=n_features)
X = hasher.transform(token_freqs(d) for d in raw_data)
duration = time() - t0
print("done in %fs at %0.3fMB/s" % (duration, data_size_mb / duration))
print("Found %d unique terms" % n_nonzero_columns(X))
print()
 
print("FeatureHasher on raw tokens")
t0 = time()
hasher = FeatureHasher(n_features=n_features, input_type="string")
X = hasher.transform(tokens(d) for d in raw_data)
duration = time() - t0
print("done in %fs at %0.3fMB/s" % (duration, data_size_mb / duration))
print("Found %d unique terms" % n_nonzero_columns(X))

Total running time of the script: (0 minutes 0.000 seconds)

Download Python source code: hashing_vs_dict_vectorizer.py

Download IPython notebook: hashing_vs_dict_vectorizer.ipynb

Links:

http://scikit-learn.org/stable/auto_examples/text/hashing_vs_dict_vectorizer.html

doc_scikit_learn

2025-01-10 15:47:30

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