Added one of the most important machine learning algorithm

dynamic_programming/k_means_clustering_tensorflow.py

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+import tensorflow as tf
+from random import choice, shuffle
+from numpy import array
+
+
+def TFKMeansCluster(vectors, noofclusters):
+    """
+    K-Means Clustering using TensorFlow.
+    'vectors' should be a n*k 2-D NumPy array, where n is the number
+    of vectors of dimensionality k.
+    'noofclusters' should be an integer.
+    """
+
+    noofclusters = int(noofclusters)
+    assert noofclusters < len(vectors)
+
+    #Find out the dimensionality
+    dim = len(vectors[0])
+
+    #Will help select random centroids from among the available vectors
+    vector_indices = list(range(len(vectors)))
+    shuffle(vector_indices)
+
+    #GRAPH OF COMPUTATION
+    #We initialize a new graph and set it as the default during each run
+    #of this algorithm. This ensures that as this function is called
+    #multiple times, the default graph doesn't keep getting crowded with
+    #unused ops and Variables from previous function calls.
+
+    graph = tf.Graph()
+
+    with graph.as_default():
+
+        #SESSION OF COMPUTATION
+
+        sess = tf.Session()
+
+        ##CONSTRUCTING THE ELEMENTS OF COMPUTATION
+
+        ##First lets ensure we have a Variable vector for each centroid,
+        ##initialized to one of the vectors from the available data points
+        centroids = [tf.Variable((vectors[vector_indices[i]]))
+                     for i in range(noofclusters)]
+        ##These nodes will assign the centroid Variables the appropriate
+        ##values
+        centroid_value = tf.placeholder("float64", [dim])
+        cent_assigns = []
+        for centroid in centroids:
+            cent_assigns.append(tf.assign(centroid, centroid_value))
+
+        ##Variables for cluster assignments of individual vectors(initialized
+        ##to 0 at first)
+        assignments = [tf.Variable(0) for i in range(len(vectors))]
+        ##These nodes will assign an assignment Variable the appropriate
+        ##value
+        assignment_value = tf.placeholder("int32")
+        cluster_assigns = []
+        for assignment in assignments:
+            cluster_assigns.append(tf.assign(assignment,
+                                             assignment_value))
+
+        ##Now lets construct the node that will compute the mean
+        #The placeholder for the input
+        mean_input = tf.placeholder("float", [None, dim])
+        #The Node/op takes the input and computes a mean along the 0th
+        #dimension, i.e. the list of input vectors
+        mean_op = tf.reduce_mean(mean_input, 0)
+
+        ##Node for computing Euclidean distances
+        #Placeholders for input
+        v1 = tf.placeholder("float", [dim])
+        v2 = tf.placeholder("float", [dim])
+        euclid_dist = tf.sqrt(tf.reduce_sum(tf.pow(tf.sub(
+            v1, v2), 2)))
+
+        ##This node will figure out which cluster to assign a vector to,
+        ##based on Euclidean distances of the vector from the centroids.
+        #Placeholder for input
+        centroid_distances = tf.placeholder("float", [noofclusters])
+        cluster_assignment = tf.argmin(centroid_distances, 0)
+
+        ##INITIALIZING STATE VARIABLES
+
+        ##This will help initialization of all Variables defined with respect
+        ##to the graph. The Variable-initializer should be defined after
+        ##all the Variables have been constructed, so that each of them
+        ##will be included in the initialization.
+        init_op = tf.initialize_all_variables()
+
+        #Initialize all variables
+        sess.run(init_op)
+
+        ##CLUSTERING ITERATIONS
+
+        #Now perform the Expectation-Maximization steps of K-Means clustering
+        #iterations. To keep things simple, we will only do a set number of
+        #iterations, instead of using a Stopping Criterion.
+        noofiterations = 100
+        for iteration_n in range(noofiterations):
+
+            ##EXPECTATION STEP
+            ##Based on the centroid locations till last iteration, compute
+            ##the _expected_ centroid assignments.
+            #Iterate over each vector
+            for vector_n in range(len(vectors)):
+                vect = vectors[vector_n]
+                #Compute Euclidean distance between this vector and each
+                #centroid. Remember that this list cannot be named
+                #'centroid_distances', since that is the input to the
+                #cluster assignment node.
+                distances = [sess.run(euclid_dist, feed_dict={
+                    v1: vect, v2: sess.run(centroid)})
+                             for centroid in centroids]
+                #Now use the cluster assignment node, with the distances
+                #as the input
+                assignment = sess.run(cluster_assignment, feed_dict = {
+                    centroid_distances: distances})
+                #Now assign the value to the appropriate state variable
+                sess.run(cluster_assigns[vector_n], feed_dict={
+                    assignment_value: assignment})
+
+            ##MAXIMIZATION STEP
+            #Based on the expected state computed from the Expectation Step,
+            #compute the locations of the centroids so as to maximize the
+            #overall objective of minimizing within-cluster Sum-of-Squares
+            for cluster_n in range(noofclusters):
+                #Collect all the vectors assigned to this cluster
+                assigned_vects = [vectors[i] for i in range(len(vectors))
+                                  if sess.run(assignments[i]) == cluster_n]
+                #Compute new centroid location
+                new_location = sess.run(mean_op, feed_dict={
+                    mean_input: array(assigned_vects)})
+                #Assign value to appropriate variable
+                sess.run(cent_assigns[cluster_n], feed_dict={
+                    centroid_value: new_location})
+
+        #Return centroids and assignments
+        centroids = sess.run(centroids)
+        assignments = sess.run(assignments)
+        return centroids, assignments
+