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POPoint at which insertion sort beats quicksort
 

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  1. POPoint at which insertion sort beats quicksort
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    <p>I'm writing a quicksort that switches to insertion sort when the length of the array being sorted is small. I've run some tests and from what I see, when the array being sorted is 200 elements or less, it is faster to switch to insertion sort. This seems really large, I was expecting something closer to 20. Is 200 reasonable or is one of my implementations way off? I used the basic Wikipedia algorithms.</p> <pre><code>void quicksort(int list[asize], int left, int right) { int pivotindex; int newpivotindex; if (left &lt; right) { if ((right-left)&lt;=200) { insertionsort(list,left,right); } } else { pivotindex = r_rand(left,right); newpivotindex = partition(list, left, right, pivotindex); quicksort(list, left, newpivotindex-1); quicksort(list, newpivotindex+1, right); } return; } int partition(int* list,int left, int right, int pivotindex) { int pivotvalue = list[pivotindex]; int temp; int i; int storeindex; temp=list[pivotindex]; list[pivotindex]=list[right]; list[right]=temp; storeindex=left; for (i=left;i&lt;right;i++) { if (list[i] &lt; pivotvalue) { temp=list[i]; list[i]=list[storeindex]; list[storeindex]=temp; storeindex++; } } temp=list[storeindex]; list[storeindex]=list[right]; list[right]=temp; return storeindex; } void insertionsort(int* list, int left, int right) { int i; int item; int ihole; for (i=left;i&lt;=right;i++) { item=list[i]; ihole=i; while (ihole &gt; 0 &amp;&amp; list[ihole-1] &gt; item) { list[ihole]=list[ihole-1]; ihole--; } list[ihole]=item; } return; } </code></pre>
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    1. USJonathan Leffler
    1. USFree_D
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      1. VTUpMod
    1. COWhat I heard last was ~100, so that sounds reasonable.
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      1. USKevin
    2. CODoes this even work? It looks like it only ever invokes the quicksort part if `right` is less than or equal to `left`.
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      1. POPoint at which insertion sort beats quicksort
      1. USpaddy
    3. COOff-hand, I suspect your quicksort is broken or poorly implemented. Unless you hit a worst-case by choosing bad pivots, quicksort should not be significantly worse than insertion sort...
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      1. USR..
 

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