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authorPatrick Wendell <pwendell@apache.org>2014-07-11 17:23:23 +0000
committerPatrick Wendell <pwendell@apache.org>2014-07-11 17:23:23 +0000
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+ <h1 class="title"><a href="mllib-guide.html">MLlib</a> - Clustering</h1>
+
+
+ <ul id="markdown-toc">
+ <li><a href="#clustering">Clustering</a></li>
+ <li><a href="#examples">Examples</a></li>
+</ul>
+
+<h2 id="clustering">Clustering</h2>
+
+<p>Clustering is an unsupervised learning problem whereby we aim to group subsets
+of entities with one another based on some notion of similarity. Clustering is
+often used for exploratory analysis and/or as a component of a hierarchical
+supervised learning pipeline (in which distinct classifiers or regression
+models are trained for each cluster). </p>
+
+<p>MLlib supports
+<a href="http://en.wikipedia.org/wiki/K-means_clustering">k-means</a> clustering, one of
+the most commonly used clustering algorithms that clusters the data points into
+predefined number of clusters. The MLlib implementation includes a parallelized
+variant of the <a href="http://en.wikipedia.org/wiki/K-means%2B%2B">k-means++</a> method
+called <a href="http://theory.stanford.edu/~sergei/papers/vldb12-kmpar.pdf">kmeans||</a>.
+The implementation in MLlib has the following parameters: </p>
+
+<ul>
+ <li><em>k</em> is the number of desired clusters.</li>
+ <li><em>maxIterations</em> is the maximum number of iterations to run.</li>
+ <li><em>initializationMode</em> specifies either random initialization or
+initialization via k-means||.</li>
+ <li><em>runs</em> is the number of times to run the k-means algorithm (k-means is not
+guaranteed to find a globally optimal solution, and when run multiple times on
+a given dataset, the algorithm returns the best clustering result).</li>
+ <li><em>initializationSteps</em> determines the number of steps in the k-means|| algorithm.</li>
+ <li><em>epsilon</em> determines the distance threshold within which we consider k-means to have converged. </li>
+</ul>
+
+<h2 id="examples">Examples</h2>
+
+<div class="codetabs">
+<div data-lang="scala">
+ <p>Following code snippets can be executed in <code>spark-shell</code>.</p>
+
+ <p>In the following example after loading and parsing data, we use the
+<a href="api/scala/index.html#org.apache.spark.mllib.clustering.KMeans"><code>KMeans</code></a> object to cluster the data
+into two clusters. The number of desired clusters is passed to the algorithm. We then compute Within
+Set Sum of Squared Error (WSSSE). You can reduce this error measure by increasing <em>k</em>. In fact the
+optimal <em>k</em> is usually one where there is an &#8220;elbow&#8221; in the WSSSE graph.</p>
+
+ <div class="highlight"><pre><code class="scala"><span class="k">import</span> <span class="nn">org.apache.spark.mllib.clustering.KMeans</span>
+<span class="k">import</span> <span class="nn">org.apache.spark.mllib.linalg.Vectors</span>
+
+<span class="c1">// Load and parse the data</span>
+<span class="k">val</span> <span class="n">data</span> <span class="k">=</span> <span class="n">sc</span><span class="o">.</span><span class="n">textFile</span><span class="o">(</span><span class="s">&quot;data/kmeans_data.txt&quot;</span><span class="o">)</span>
+<span class="k">val</span> <span class="n">parsedData</span> <span class="k">=</span> <span class="n">data</span><span class="o">.</span><span class="n">map</span><span class="o">(</span><span class="n">s</span> <span class="k">=&gt;</span> <span class="nc">Vectors</span><span class="o">.</span><span class="n">dense</span><span class="o">(</span><span class="n">s</span><span class="o">.</span><span class="n">split</span><span class="o">(</span><span class="sc">&#39; &#39;</span><span class="o">).</span><span class="n">map</span><span class="o">(</span><span class="k">_</span><span class="o">.</span><span class="n">toDouble</span><span class="o">)))</span>
+
+<span class="c1">// Cluster the data into two classes using KMeans</span>
+<span class="k">val</span> <span class="n">numClusters</span> <span class="k">=</span> <span class="mi">2</span>
+<span class="k">val</span> <span class="n">numIterations</span> <span class="k">=</span> <span class="mi">20</span>
+<span class="k">val</span> <span class="n">clusters</span> <span class="k">=</span> <span class="nc">KMeans</span><span class="o">.</span><span class="n">train</span><span class="o">(</span><span class="n">parsedData</span><span class="o">,</span> <span class="n">numClusters</span><span class="o">,</span> <span class="n">numIterations</span><span class="o">)</span>
+
+<span class="c1">// Evaluate clustering by computing Within Set Sum of Squared Errors</span>
+<span class="k">val</span> <span class="nc">WSSSE</span> <span class="k">=</span> <span class="n">clusters</span><span class="o">.</span><span class="n">computeCost</span><span class="o">(</span><span class="n">parsedData</span><span class="o">)</span>
+<span class="n">println</span><span class="o">(</span><span class="s">&quot;Within Set Sum of Squared Errors = &quot;</span> <span class="o">+</span> <span class="nc">WSSSE</span><span class="o">)</span>
+</code></pre></div>
+
+ </div>
+
+<div data-lang="java">
+ <p>All of MLlib&#8217;s methods use Java-friendly types, so you can import and call them there the same
+way you do in Scala. The only caveat is that the methods take Scala RDD objects, while the
+Spark Java API uses a separate <code>JavaRDD</code> class. You can convert a Java RDD to a Scala one by
+calling <code>.rdd()</code> on your <code>JavaRDD</code> object.</p>
+ </div>
+
+<div data-lang="python">
+ <p>Following examples can be tested in the PySpark shell.</p>
+
+ <p>In the following example after loading and parsing data, we use the KMeans object to cluster the
+data into two clusters. The number of desired clusters is passed to the algorithm. We then compute
+Within Set Sum of Squared Error (WSSSE). You can reduce this error measure by increasing <em>k</em>. In
+fact the optimal <em>k</em> is usually one where there is an &#8220;elbow&#8221; in the WSSSE graph.</p>
+
+ <div class="highlight"><pre><code class="python"><span class="kn">from</span> <span class="nn">pyspark.mllib.clustering</span> <span class="kn">import</span> <span class="n">KMeans</span>
+<span class="kn">from</span> <span class="nn">numpy</span> <span class="kn">import</span> <span class="n">array</span>
+<span class="kn">from</span> <span class="nn">math</span> <span class="kn">import</span> <span class="n">sqrt</span>
+
+<span class="c"># Load and parse the data</span>
+<span class="n">data</span> <span class="o">=</span> <span class="n">sc</span><span class="o">.</span><span class="n">textFile</span><span class="p">(</span><span class="s">&quot;data/kmeans_data.txt&quot;</span><span class="p">)</span>
+<span class="n">parsedData</span> <span class="o">=</span> <span class="n">data</span><span class="o">.</span><span class="n">map</span><span class="p">(</span><span class="k">lambda</span> <span class="n">line</span><span class="p">:</span> <span class="n">array</span><span class="p">([</span><span class="nb">float</span><span class="p">(</span><span class="n">x</span><span class="p">)</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="n">line</span><span class="o">.</span><span class="n">split</span><span class="p">(</span><span class="s">&#39; &#39;</span><span class="p">)]))</span>
+
+<span class="c"># Build the model (cluster the data)</span>
+<span class="n">clusters</span> <span class="o">=</span> <span class="n">KMeans</span><span class="o">.</span><span class="n">train</span><span class="p">(</span><span class="n">parsedData</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="n">maxIterations</span><span class="o">=</span><span class="mi">10</span><span class="p">,</span>
+ <span class="n">runs</span><span class="o">=</span><span class="mi">10</span><span class="p">,</span> <span class="n">initializationMode</span><span class="o">=</span><span class="s">&quot;random&quot;</span><span class="p">)</span>
+
+<span class="c"># Evaluate clustering by computing Within Set Sum of Squared Errors</span>
+<span class="k">def</span> <span class="nf">error</span><span class="p">(</span><span class="n">point</span><span class="p">):</span>
+ <span class="n">center</span> <span class="o">=</span> <span class="n">clusters</span><span class="o">.</span><span class="n">centers</span><span class="p">[</span><span class="n">clusters</span><span class="o">.</span><span class="n">predict</span><span class="p">(</span><span class="n">point</span><span class="p">)]</span>
+ <span class="k">return</span> <span class="n">sqrt</span><span class="p">(</span><span class="nb">sum</span><span class="p">([</span><span class="n">x</span><span class="o">**</span><span class="mi">2</span> <span class="k">for</span> <span class="n">x</span> <span class="ow">in</span> <span class="p">(</span><span class="n">point</span> <span class="o">-</span> <span class="n">center</span><span class="p">)]))</span>
+
+<span class="n">WSSSE</span> <span class="o">=</span> <span class="n">parsedData</span><span class="o">.</span><span class="n">map</span><span class="p">(</span><span class="k">lambda</span> <span class="n">point</span><span class="p">:</span> <span class="n">error</span><span class="p">(</span><span class="n">point</span><span class="p">))</span><span class="o">.</span><span class="n">reduce</span><span class="p">(</span><span class="k">lambda</span> <span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">:</span> <span class="n">x</span> <span class="o">+</span> <span class="n">y</span><span class="p">)</span>
+<span class="k">print</span><span class="p">(</span><span class="s">&quot;Within Set Sum of Squared Error = &quot;</span> <span class="o">+</span> <span class="nb">str</span><span class="p">(</span><span class="n">WSSSE</span><span class="p">))</span>
+</code></pre></div>
+
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