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POFinding the Nth key in a TreeMap most efficiently
 

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  1. POFinding the Nth key in a TreeMap most efficiently
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    <p>I have a Scala TreeMap that automatically sorts the keys. What I would like to know is if there is a more performant way of finding the Nth key in the map than the following example:</p> <pre><code>treeMap.take(N).lastKey </code></pre> <p>Thanks, Bruce</p> <p><strong>EDIT:</strong><br> I created a small test using the following code:</p> <pre><code>class Test { var treeMap = new scala.collection.immutable.TreeMap[Double,String]() val numberOfEntries = 1000 (0 until numberOfEntries) map { i =&gt; {treeMap += {i.toDouble -&gt; i.toString}}} val iterations = 2000 var N = 1 while(N &lt; numberOfEntries) { // my original version var i = 0 val start1 = System.nanoTime() while(i &lt; iterations) { i += 1 val v = treeMap.take(N).lastKey } val end1 = System.nanoTime() val elapsed1 = end1 - start1 // Daniel's suggestion i = 0 val start2 = System.nanoTime() while(i &lt; iterations) { i += 1 val v = treeMap.keysIterator.drop(N - 1).next } val end2 = System.nanoTime() val elapsed2 = end2 - start2 println("N = %d, elapsed1 = %d, elapsed2 = %d".format(N,elapsed1,elapsed2)) N += 50 } } object Test { def main(args:Array[String]) { val test = new Test } } </code></pre> <p>It appears that Daniel's suggestion is indeed better</p> <p><strong>Results</strong></p> <pre><code>N = 1, elapsed1 = 956492000, elapsed2 = 700300000 N = 51, elapsed1 = 1103271000, elapsed2 = 936045000 N = 101, elapsed1 = 1286896000, elapsed2 = 1041744000 N = 151, elapsed1 = 1368854000, elapsed2 = 1199766000 N = 201, elapsed1 = 1584878000, elapsed2 = 1333284000 N = 251, elapsed1 = 1790965000, elapsed2 = 1468806000 N = 301, elapsed1 = 2052298000, elapsed2 = 1649021000 N = 351, elapsed1 = 2294625000, elapsed2 = 1819525000 N = 401, elapsed1 = 2529855000, elapsed2 = 1961699000 N = 451, elapsed1 = 2762582000, elapsed2 = 2100127000 N = 501, elapsed1 = 2977613000, elapsed2 = 2232108000 N = 551, elapsed1 = 3211812000, elapsed2 = 2384940000 N = 601, elapsed1 = 3437116000, elapsed2 = 2539431000 N = 651, elapsed1 = 3652749000, elapsed2 = 2650910000 N = 701, elapsed1 = 3900431000, elapsed2 = 2807085000 N = 751, elapsed1 = 4123141000, elapsed2 = 2934904000 N = 801, elapsed1 = 4337909000, elapsed2 = 3060158000 N = 851, elapsed1 = 4554490000, elapsed2 = 3188378000 N = 901, elapsed1 = 4768488000, elapsed2 = 3306528000 N = 951, elapsed1 = 4978839000, elapsed2 = 3413813000 </code></pre>
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    1. USBruce Ferguson
    1. USBruce Ferguson
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