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PORemove from a HashSet failing after iterating over it
 

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  1. PORemove from a HashSet failing after iterating over it
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    <p>I'm writing an agglomerative clustering algorithm in java and having trouble with a remove operation. It seems to always fail when the number of clusters reaches half the initial number.</p> <p>In the sample code below, <code>clusters</code> is a <code>Collection&lt;Collection&lt;Integer&gt;&gt;</code>.</p> <pre><code> while(clusters.size() &gt; K){ // determine smallest distance between clusters Collection&lt;Integer&gt; minclust1 = null; Collection&lt;Integer&gt; minclust2 = null; double mindist = Double.POSITIVE_INFINITY; for(Collection&lt;Integer&gt; cluster1 : clusters){ for(Collection&lt;Integer&gt; cluster2 : clusters){ if( cluster1 != cluster2 &amp;&amp; getDistance(cluster1, cluster2) &lt; mindist){ minclust1 = cluster1; minclust2 = cluster2; mindist = getDistance(cluster1, cluster2); } } } // merge the two clusters minclust1.addAll(minclust2); clusters.remove(minclust2); } </code></pre> <p>After a few runs through the loop, <code>clusters.remove(minclust2)</code> eventually returns false, but I don't understand why.</p> <p>I tested this code by first creating 10 clusters, each with one integer from 1 to 10. Distances are random numbers between 0 and 1. Here's the output after adding a few println statements. After the number of clusters, I print out the actual clusters, the merge operation, and the result of clusters.remove(minclust2).</p> <pre><code>Clustering: 10 clusters [[3], [1], [10], [5], [9], [7], [2], [4], [6], [8]] [5] &lt;- [6] true Clustering: 9 clusters [[3], [1], [10], [5, 6], [9], [7], [2], [4], [8]] [7] &lt;- [8] true Clustering: 8 clusters [[3], [1], [10], [5, 6], [9], [7, 8], [2], [4]] [10] &lt;- [9] true Clustering: 7 clusters [[3], [1], [10, 9], [5, 6], [7, 8], [2], [4]] [5, 6] &lt;- [4] true Clustering: 6 clusters [[3], [1], [10, 9], [5, 6, 4], [7, 8], [2]] [3] &lt;- [2] true Clustering: 5 clusters [[3, 2], [1], [10, 9], [5, 6, 4], [7, 8]] [10, 9] &lt;- [5, 6, 4] false Clustering: 5 clusters [[3, 2], [1], [10, 9, 5, 6, 4], [5, 6, 4], [7, 8]] [10, 9, 5, 6, 4] &lt;- [5, 6, 4] false Clustering: 5 clusters [[3, 2], [1], [10, 9, 5, 6, 4, 5, 6, 4], [5, 6, 4], [7, 8]] [10, 9, 5, 6, 4, 5, 6, 4] &lt;- [5, 6, 4] false </code></pre> <p>The the [10, 9, 5, 6, 4, 5, 6, 4, ...] set just grows infinitely from there. </p> <p><strong>Edit: to clarify, I'm using a <code>HashSet&lt;Integer&gt;</code> for each cluster in clusters (a <code>HashSet&lt;HashSet&lt;Integer&gt;&gt;)</code>.</strong></p>
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    1. CO[10, 9, 5, 6, 4, 5, 6, 4, ...] clearly is not a set. Is it a list?
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      1. USTom Hawtin - tackline
    2. COYeah, good point. A HashSet should not be able to contain duplicate objects. Something is weird here.
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