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POWhy Does This Improve Performance?
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  1. POWhy Does This Improve Performance?
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    copied!<p>I've two for loops that basically look up in two different arrays (each having a size around 2-4k at peak) and set a value in a 3rd array based on these values. For some weird reason there is a factor two difference between the performance of this piece of code depending on in which order I put the two for loops.</p> <p>This is the first setup. It executes in ~150 milliseconds on my PC:</p> <pre><code>public static int[] SchoolMultiplication(int[] a, int[] b, int numberBase) { List&lt;double&gt; times = new List&lt;double&gt;(); TimeTest timeTest = new TimeTest(); int aLen = a.Length; int bLen = b.Length; int[,] resultMatrix = new int[a.Length + b.Length, aLen]; int[] result = new int[a.Length + b.Length]; timeTest.Start(); for (int horizontalIndex = 0; horizontalIndex &lt; b.Length; horizontalIndex++) { for (int verticalIndex = 0; verticalIndex &lt; a.Length; verticalIndex++) { resultMatrix[a.Length + b.Length - 1 - verticalIndex - horizontalIndex, verticalIndex] = a[a.Length - verticalIndex - 1] * b[b.Length - horizontalIndex - 1]; } } </code></pre> <p>Now if I change nothing but the order of the loops like this</p> <pre><code>for (int verticalIndex = 0; verticalIndex &lt; a.Length; verticalIndex++) { for (int horizontalIndex = 0; horizontalIndex &lt; b.Length; horizontalIndex++) { resultMatrix[a.Length + b.Length - 1 - verticalIndex - horizontalIndex, verticalIndex] = a[a.Length - verticalIndex - 1] * b[b.Length - horizontalIndex - 1]; } } </code></pre> <p>The total running time of the method drops to about ~400 milliseconds. How does a simple exchange of loop order improve performance by almost 300%? I suppose it is some kind of caching or pointer performance thing?</p>
 

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