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Why is processing a sorted array faster than processing an unsorted array?

Digital Marketing Facebook Marketing API 2 years ago

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_x000D_ _x000D_ Here is a piece of C++ code that shows some very peculiar behavior. For some strange reason, sorting the data miraculously makes the code almost six times faster: #include #include #include int main() { // Generate data const unsigned arraySize = 32768; int data[arraySize]; for (unsigned c = 0; c < 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 < 100000; ++i) { // Primary loop for (unsigned c = 0; c < arraySize; ++c) { if (data[c] >= 128) sum += data[c]; } } double elapsedTime = static_cast(clock() - start) / CLOCKS_PER_SEC; std::cout << elapsedTime << std::endl; std::cout << "sum = " << sum << std::endl; } Without std::sort(data, data + arraySize);, the code runs in 11.54 seconds. With the sorted data, the code runs in 1.93 seconds. Initially, I thought this might be just a language or compiler anomaly, so I tried Java: 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 < 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 < 100000; ++i) { // Primary loop for (int c = 0; c < arraySize; ++c) { if (data[c] >= 128) sum += data[c]; } } System.out.println((System.nanoTime() - start) / 1000000000.0); System.out.println("sum = " + sum); } } With a similar but less extreme result. My first thought was that sorting brings the data into the cache, but then I thought how silly that was because the array was just generated. What is going on? Why is processing a sorted array faster than processing an unsorted array? The code is summing up some independent terms, so the order should not matter.
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Posted on 16 Aug 2022, this text provides information on Facebook Marketing API related to Digital Marketing. Please note that while accuracy is prioritized, the data presented might not be entirely correct or up-to-date. This information is offered for general knowledge and informational purposes only, and should not be considered as a substitute for professional advice.

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