爲何處理排序數組要比處理未排序數組快？ - Why is processing a sorted array faster than processing an unsorted array?

問題：

Here is a piece of C++ code that shows some very peculiar behavior. 這是一段C ++代碼，顯示了一些很是特殊的行爲。 For some strange reason, sorting the data miraculously makes the code almost six times faster: 出於某些奇怪的緣由，奇蹟般地對數據進行排序使代碼快了將近六倍：

#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)
    {
        // Primary loop
        for (unsigned c = 0; c < arraySize; ++c)
        {
            if (data[c] >= 128)
                sum += data[c];
        }
    }

    double elapsedTime = static_cast<double>(clock() - start) / CLOCKS_PER_SEC;

    std::cout << elapsedTime << std::endl;
    std::cout << "sum = " << sum << std::endl;
}

• Without std::sort(data, data + arraySize); 沒有std::sort(data, data + arraySize); , the code runs in 11.54 seconds. ，代碼將在11.54秒內運行。
• With the sorted data, the code runs in 1.93 seconds. 使用排序的數據，代碼將在1.93秒內運行。

---

Initially, I thought this might be just a language or compiler anomaly, so I tried Java: 最初，我認爲這可能只是語言或編譯器異常，因此我嘗試了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. 具備類似但不太極端的結果。

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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. 我首先想到的是排序將數據帶入緩存，可是後來我想到這樣作是多麼愚蠢，由於剛剛生成了數組。

• What is going on? 究竟是怎麼回事？
• Why is processing a sorted array faster than processing an unsorted array? 爲何處理排序數組要比處理未排序數組快？

The code is summing up some independent terms, so the order should not matter. 該代碼總結了一些獨立的術語，所以順序可有可無。

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解決方案：

參考一： https://stackoom.com/question/l6rh/爲何處理排序數組要比處理未排序數組快
參考二： https://oldbug.net/q/l6rh/Why-is-processing-a-sorted-array-faster-than-processing-an-unsorted-array