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POWhy sorting a randomly populated array gets progressively faster
 

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  1. POWhy sorting a randomly populated array gets progressively faster
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    <p>For a programming exercise, we were told to implement insertion, selection, and bubble sorts (in java). I wanted to test how fast the sorts executed, so I wrote a loop to randomly populate and sort an array 10 times. The first two sorts take about twice as long as the last 8 iterations.. why?</p> <p>Here I have put relevant portions of code</p> <pre><code>// class fields public static final int POPULATE_MAX = 1000000000; public static final int POPULATE_MIN = -1000000000; public static void populateRandom(int[] toPopulate) { // populate array with random integers within bounds set by class fields for (int i = 0; i &lt; toPopulate.length; i++) { toPopulate[i] = (int)(Math.random() * (POPULATE_MAX - POPULATE_MIN)) + POPULATE_MIN; } } // end of method populateRandom(int[] toPopulate) public static void insertionSort(int[] toSort) throws IllegalArgumentException { if (toSort == null) { throw new IllegalArgumentException(); } if (toSort.length &lt;= 1) return; int temp; // Index i points to the next unsorted element; assigned to temp for (int i = 1; i &lt; toSort.length; i++) { temp = toSort[i]; // This loop searches through the sorted portion of the array and // determines where to put the aforementioned unsorted element for (int j = i - 1; j &gt;= 0; j--) { if (temp &lt; toSort[j]) { toSort[j + 1] = toSort[j]; if(j == 0) toSort[j] = temp; } else { toSort[j + 1] = temp; break; } // end of if (temp &lt; toSort[j]) } // end of for (int j = i - 1; j &gt;= 0; j--) } // end of for (int i = 1; i &lt; toSort.length; i++) } // end of method insertionSort(int[] toSort) throws IllegalArgumentException public static void main(String[] args) { long time; for (int testRun = 0; testRun &lt; 10; testRun++) { int[] array = new int[100000]; System.out.print(testRun + "..."); populateRandom(array); time = System.currentTimeMillis(); insertionSort(array); time = System.currentTimeMillis() - time; for (int i = 0; i &lt; array.length - 1; i++) { if (array[i] &gt; array[i+1]) System.out.println(i + ". Bad"); } System.out.println(time + " Done"); } System.out.println("ABS Done"); } </code></pre> <p>I am guessing that it has to do with branch prediction, but I am not sure as to why subsequent sorts are very noticeably faster.</p>
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    1. USDavid Rönnqvist
    1. USithenoob
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    1. COCan you post the code that make the test?
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      1. USjcrada
    2. COAll relevant code is in main(String[])... It creates the arrays and times the sorting
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      1. USithenoob
    3. COhttp://4groundtechsolutions.com/jvm-warmup/
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      1. USAndreas Fester
 

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