87 lines
4.2 KiB
Python
87 lines
4.2 KiB
Python
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# Natural Language Toolkit: Clusterers
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#
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# Copyright (C) 2001-2018 NLTK Project
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# Author: Trevor Cohn <tacohn@cs.mu.oz.au>
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# URL: <http://nltk.org/>
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# For license information, see LICENSE.TXT
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"""
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This module contains a number of basic clustering algorithms. Clustering
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describes the task of discovering groups of similar items with a large
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collection. It is also describe as unsupervised machine learning, as the data
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from which it learns is unannotated with class information, as is the case for
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supervised learning. Annotated data is difficult and expensive to obtain in
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the quantities required for the majority of supervised learning algorithms.
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This problem, the knowledge acquisition bottleneck, is common to most natural
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language processing tasks, thus fueling the need for quality unsupervised
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approaches.
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This module contains a k-means clusterer, E-M clusterer and a group average
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agglomerative clusterer (GAAC). All these clusterers involve finding good
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cluster groupings for a set of vectors in multi-dimensional space.
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The K-means clusterer starts with k arbitrary chosen means then allocates each
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vector to the cluster with the closest mean. It then recalculates the means of
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each cluster as the centroid of the vectors in the cluster. This process
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repeats until the cluster memberships stabilise. This is a hill-climbing
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algorithm which may converge to a local maximum. Hence the clustering is
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often repeated with random initial means and the most commonly occurring
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output means are chosen.
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The GAAC clusterer starts with each of the *N* vectors as singleton clusters.
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It then iteratively merges pairs of clusters which have the closest centroids.
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This continues until there is only one cluster. The order of merges gives rise
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to a dendrogram - a tree with the earlier merges lower than later merges. The
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membership of a given number of clusters *c*, *1 <= c <= N*, can be found by
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cutting the dendrogram at depth *c*.
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The Gaussian EM clusterer models the vectors as being produced by a mixture
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of k Gaussian sources. The parameters of these sources (prior probability,
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mean and covariance matrix) are then found to maximise the likelihood of the
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given data. This is done with the expectation maximisation algorithm. It
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starts with k arbitrarily chosen means, priors and covariance matrices. It
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then calculates the membership probabilities for each vector in each of the
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clusters - this is the 'E' step. The cluster parameters are then updated in
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the 'M' step using the maximum likelihood estimate from the cluster membership
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probabilities. This process continues until the likelihood of the data does
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not significantly increase.
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They all extend the ClusterI interface which defines common operations
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available with each clusterer. These operations include.
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- cluster: clusters a sequence of vectors
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- classify: assign a vector to a cluster
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- classification_probdist: give the probability distribution over cluster memberships
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The current existing classifiers also extend cluster.VectorSpace, an
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abstract class which allows for singular value decomposition (SVD) and vector
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normalisation. SVD is used to reduce the dimensionality of the vector space in
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such a manner as to preserve as much of the variation as possible, by
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reparameterising the axes in order of variability and discarding all bar the
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first d dimensions. Normalisation ensures that vectors fall in the unit
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hypersphere.
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Usage example (see also demo())::
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from nltk import cluster
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from nltk.cluster import euclidean_distance
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from numpy import array
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vectors = [array(f) for f in [[3, 3], [1, 2], [4, 2], [4, 0]]]
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# initialise the clusterer (will also assign the vectors to clusters)
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clusterer = cluster.KMeansClusterer(2, euclidean_distance)
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clusterer.cluster(vectors, True)
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# classify a new vector
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print(clusterer.classify(array([3, 3])))
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Note that the vectors must use numpy array-like
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objects. nltk_contrib.unimelb.tacohn.SparseArrays may be used for
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efficiency when required.
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"""
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from nltk.cluster.util import (VectorSpaceClusterer, Dendrogram,
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euclidean_distance, cosine_distance)
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from nltk.cluster.kmeans import KMeansClusterer
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from nltk.cluster.gaac import GAAClusterer
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from nltk.cluster.em import EMClusterer
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