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POWhy is it faster to process a sorted array than an unsorted array?
 

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    <p>Here is a piece of C++ code that seems very peculiar. For some strange reason, sorting the data miraculously makes the code almost six times faster.</p> <pre class="lang-cpp prettyprint-override"><code>#include &lt;algorithm&gt; #include &lt;ctime&gt; #include &lt;iostream&gt; int main() { // Generate data const unsigned arraySize = 32768; int data[arraySize]; for (unsigned c = 0; c &lt; arraySize; ++c) data[c] = std::rand() % 256; // !!! With this, the next loop runs faster std::sort(data, data + arraySize); // Test clock_t start = clock(); long long sum = 0; for (unsigned i = 0; i &lt; 100000; ++i) { // Primary loop for (unsigned c = 0; c &lt; arraySize; ++c) { if (data[c] &gt;= 128) sum += data[c]; } } double elapsedTime = static_cast&lt;double&gt;(clock() - start) / CLOCKS_PER_SEC; std::cout &lt;&lt; elapsedTime &lt;&lt; std::endl; std::cout &lt;&lt; "sum = " &lt;&lt; sum &lt;&lt; std::endl; } </code></pre> <ul> <li>Without <code>std::sort(data, data + arraySize);</code>, the code runs in 11.54 seconds.</li> <li>With the sorted data, the code runs in 1.93 seconds.</li> </ul> <p>Initially, I thought this might be just a language or compiler anomaly. So I tried it in Java.</p> <pre class="lang-java prettyprint-override"><code>import java.util.Arrays; import java.util.Random; public class Main { public static void main(String[] args) { // Generate data int arraySize = 32768; int data[] = new int[arraySize]; Random rnd = new Random(0); for (int c = 0; c &lt; arraySize; ++c) data[c] = rnd.nextInt() % 256; // !!! With this, the next loop runs faster Arrays.sort(data); // Test long start = System.nanoTime(); long sum = 0; for (int i = 0; i &lt; 100000; ++i) { // Primary loop for (int c = 0; c &lt; arraySize; ++c) { if (data[c] &gt;= 128) sum += data[c]; } } System.out.println((System.nanoTime() - start) / 1000000000.0); System.out.println("sum = " + sum); } } </code></pre> <p>With a somewhat similar but less extreme result.</p> <hr> <p>My first thought was that sorting brings the data into the cache, but then I thought how silly that is because the array was just generated.</p> <ul> <li>What is going on?</li> <li>Why is it faster to process a sorted array than an unsorted array?</li> <li>The code is summing up some independent terms, and the order should not matter.</li> </ul>
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