c++ - 为什么这个实现的二进制搜索比 std::binary_search() 慢得多？

Question

在我检测到 std::upper_bound 之前，我实现了我自己的 binarySearch 版本来确定所需元素的索引。该实现有效，但与线性搜索相比，我的 binarySearch 只快了一点点。随着搜索区域的扩大，我的实现与标准库之间的因素也会增加。

为了进行快速自测，我在本文末尾插入了完整代码。在这里快速浏览一下我的 searchBinary 实现:

template<typename T> T searchBinary(const std::vector<std::vector<T> > vectorList, const std::vector<T> compareVector) {
    long iteration = 0;
    size_t leftIndex = 0;
    size_t rightIndex = vectorList.size()-1;
    size_t pos;

    while (leftIndex <= rightIndex) {
        iteration++;
        pos = (leftIndex + rightIndex) / 2;

        if (compareVector < vectorList[pos]) {
            rightIndex = pos - 1;
        } else if (compareVector > vectorList[pos]) {
            leftIndex = pos + 1;
        } else {
            cout << "Match at binary search after " << iteration << " iterations.\n";
            return pos;
        }
    }

    cout << "No match at binary search after " << iteration << " iterations.\n";
    return -1;
}

这就是我弄乱运行时的方式:

void searchBinaryOwn_messure(std::vector<std::vector<u_char> > vectorList, std::vector<u_char> compareVector) {
    struct timeval begin, end;
    long seconds, useconds;

    if (gettimeofday(&begin,(struct timezone *)0)) {
        fprintf(stderr, "can not get time\n");
        exit(1);
    }

    searchBinary(vectorList, compareVector);

    if (gettimeofday(&end,(struct timezone *)0)) {
        fprintf(stderr, "can not get time\n");
        exit(1);
    }

    seconds = end.tv_sec - begin.tv_sec;
    useconds = end.tv_usec - begin.tv_usec;
    if(useconds < 0) {
        useconds += 1000000;
        seconds--;
    }

    printf("searchBinaryOwn(): %ld sec %ld usec\n\n", seconds, useconds);
    return;
}

在这里看不到任何问题。如果我用 8 000 运行这个程序 000 个元素:

• searchLinear() 耗时约 3.7 秒
• searchBinaryOwn() 需要大约 2.8 秒
• searchBinaryStd() 需要 ~0,0007 秒

那么，为什么两种二进制搜索之间存在如此巨大的差异？(使用 gcc 4.8.2 编译)注意:因为“cout...”大约需要 30usec，std::binarySearch 实际上比显示的要快

完整代码如下:

#include <iostream>
#include <vector>
#include <sys/time.h>
#include <algorithm>
#include <string>
#include <stdio.h>
using namespace std;


template<typename T> T searchBinary(const std::vector<std::vector<T> > vectorList, const std::vector<T> compareVector) {
    long iteration = 0;
    size_t leftIndex = 0;
    size_t rightIndex = vectorList.size()-1;
    size_t pos;


    while (leftIndex <= rightIndex) {
        iteration++;
        pos = (leftIndex + rightIndex) / 2;

        if (compareVector < vectorList[pos]) {
            rightIndex = pos - 1;
        } else if (compareVector > vectorList[pos]) {
            leftIndex = pos + 1;
        } else {
            cout << "Match at binary search after " << iteration << " iterations.\n";
            return pos;
        }
    }

    cout << "No match at binary search after " << iteration << " iterations.\n";
    return -1;
}

size_t searchLinear(std::vector<std::vector<u_char> > vectorList, std::vector<u_char> compareVector) {
    size_t vectorListSize = vectorList.size();
    for (size_t i = 0; i < vectorListSize; i++) {
        if (vectorList[i] == compareVector) {
            return i;
        }
    }
    return (size_t)-1;
}

void searchLinear_messure(std::vector<std::vector<u_char> > vectorList, std::vector<u_char> compareVector) {
    struct timeval begin, end;
    long seconds, useconds;

    if (gettimeofday(&begin,(struct timezone *)0)) {
        fprintf(stderr, "can not get time\n");
        exit(1);
    }

    //search
    cout << "\nPos: " << searchLinear(vectorList, compareVector) << endl;

    if (gettimeofday(&end,(struct timezone *)0)) {
        fprintf(stderr, "can not get time\n");
        exit(1);
    }

    seconds = end.tv_sec - begin.tv_sec;
    useconds = end.tv_usec - begin.tv_usec;
    if(useconds < 0) {
        useconds += 1000000;
        seconds--;
    }

    printf("searchLinear(): %ld sec %ld usec\n\n", seconds, useconds);
    return;
}

void searchBinaryStd_messure(std::vector<std::vector<u_char> > vectorList, std::vector<u_char> compareVector) {
    struct timeval begin, end;
    long seconds, useconds;

    if (gettimeofday(&begin,(struct timezone *)0)) {
        fprintf(stderr, "can not get time\n");
        exit(1);
    }

    //search
    cout << "found: " << std::binary_search(vectorList.begin(), vectorList.end(), compareVector) << endl;

    if (gettimeofday(&end,(struct timezone *)0)) {
        fprintf(stderr, "can not get time\n");
        exit(1);
    }

    seconds = end.tv_sec - begin.tv_sec;
    useconds = end.tv_usec - begin.tv_usec;
    if(useconds < 0) {
        useconds += 1000000;
        seconds--;
    }

    printf("searchBinaryStd(): %ld sec %ld usec\n\n", seconds, useconds);
    return;
}

void searchBinaryOwn_messure(std::vector<std::vector<u_char> > vectorList, std::vector<u_char> compareVector) {
    struct timeval begin, end;
    long seconds, useconds;

    if (gettimeofday(&begin,(struct timezone *)0)) {
        fprintf(stderr, "can not get time\n");
        exit(1);
    }

    searchBinary(vectorList, compareVector);

    if (gettimeofday(&end,(struct timezone *)0)) {
        fprintf(stderr, "can not get time\n");
        exit(1);
    }

    seconds = end.tv_sec - begin.tv_sec;
    useconds = end.tv_usec - begin.tv_usec;
    if(useconds < 0) {
        useconds += 1000000;
        seconds--;
    }

    printf("searchBinaryOwn(): %ld sec %ld usec\n\n", seconds, useconds);
    return;
}


int main() {
    std::vector<u_char> compareVector;
    compareVector.clear();
    compareVector.push_back(0xF8);
    compareVector.push_back(0xD1);
    compareVector.push_back(0x11);
    compareVector.push_back(0xFF);

    std::vector<std::vector<u_char> > vectorList;
    vectorList.clear();
    std::vector<u_char> temp;
    for (unsigned int i = 0; i < ((unsigned int)-1); i++) {
        if (i == 8000000) {
//      if (i == 15000000) {
            break;
        }
        temp.clear();
        temp.push_back(0x11);
        temp.push_back(0x22);
        temp.push_back(0x33);
        temp.push_back(0x44);
        vectorList.push_back(temp);
    }

    vectorList[7999999] = compareVector;

    cout << "Elements in vectorList: " << vectorList.size() << endl;

    searchLinear_messure(vectorList, compareVector);
    searchBinaryStd_messure(vectorList, compareVector);
    searchBinaryOwn_messure(vectorList, compareVector);

    return 0;
}

Answer 1

1. 将您的函数原型(prototype)更改为

template<typename T> T searchBinary(const std::vector<std::vector<T> >& vectorList, const std::vector<T>& compareVector) {

即通过常量引用而不是值传递。这将避免两个 vector 拷贝。

1. 您可以使用单个条件测试进行重构 <每次迭代。 (您还需要更改 while 条件)。

2. 是否 iteration需要是 long ？不能短点吗？收敛的最坏情况是什么？

第 1 点很重要。 2 非常重要，3 是微优化，在某些系统上可能根本不会产生任何影响。