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

Here is a piece of C++ code that shows some very peculiar behavior. For some strange reason, sorting the data (before the timed region) miraculously makes the loop almost six times faster.

#include <algorithm>
#include <ctime>
#include <iostream>
 
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)
    {
        for (unsigned c = 0; c < arraySize; ++c)
        {   // Primary loop
            if (data[c] >= 128)
                sum += data[c];
        }
    }
 
    double elapsedTime = static_cast<double>(clock()-start) / CLOCKS_PER_SEC;
 
    std::cout << elapsedTime << '\n';
    std::cout << "sum = " << sum << '\n';
}

Without std::sort(data, data + arraySize);, the code runs in 11.54 seconds.

With the sorted data, the code runs in 1.93 seconds.

(Sorting itself takes more time than this one pass over the array, so it's not actually worth doing if we needed to calculate this for an unknown array.)


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)
        {
            for (int c = 0; c < arraySize; ++c)
            {   // Primary loop
                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?

https://hkrtrainings.com/array-length-in-java

The code is summing up some independent terms, so the order should not matter.