C++ 为什么实现的二进制搜索比std:：binary_search（）慢得多？

在检测到std:：upper_bound之前，我实现了自己版本的binarySearch，以确定所需元素的索引。该实现可以工作，但与线性搜索相比，我的binarySearch只快了一点点。我的实现和std库之间的因素随着搜索区域的增长而增加

为了快速进行自检，我在本文末尾插入了完整的代码。要快速浏览一下我的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;
}

模板T searchBinary（常量std:：向量向量向量列表，常量std:：向量比较向量）{
长迭代=0；
大小\u t leftIndex=0；
size\u t rightIndex=vectorList.size（）-1；
尺寸和位置；
while（leftIndex向量列表[pos]）{
leftIndex=pos+1；
}否则{
这个
模板T searchBinary（const std:：vector vectorList，const std:：vector compareVector）
制作了一个输入向量的副本（当您通过值传递它时），它在时间上是线性的。所以您得到的结果实际上是预期的

顺便说一句，一个开玩笑的回答可能是，标准库是由相当优秀的开发人员编写的，预计它的性能很难超越

将函数原型更改为
模板T searchBinary（const std:：vector&vectorList，const std:：vector&compareVector）{

i、 通过常量引用而不是通过值传递。这将避免两个向量副本

您可以使用单个条件测试进行重构，向量通过值传递给
searchBinary
，因此将创建副本，这需要时间

如果您将签名更改为

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

模板T searchBinary（常量std:：vector&vectorList、常量std:：vector&compareVector）

它与std实现一样快：
我没有时间仔细查看，但是您的
searchBinary
创建了您传递的两个向量的副本，因此这将至少是一个瓶颈。因为许多人在算法标准化之前花了很长时间研究算法。-）您的项目2不仅仅是一个“微型”优化。这几乎是二的一个因素。除非我误解了，否则您仍然需要对正在排序的值进行<测试，同时对索引进行一些条件测试。但是取消对正在排序的值的>测试可以节省近一半的时间。然后在这段时间之后，您需要额外的测试，以查看leftIndex-1是否是您需要的项搜索（如果leftIndex初始化为1而不是0，则更安全、更快）。如果打印的迭代次数确实有趣，则整个想法都失败了。但我认为不是。你当然是对的。我已经改进了我的语言。如果你在回答中提供代码，我肯定会支持它。
#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;
}

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