我哪种情况会比排序的std：：><vector<std：std：:p air<A，B>>更快？

我在某些代码中使用了map来存储有序数据。我发现对于巨大的地图，破坏可能需要一段时间。在我有的这段代码中，将map替换为vector<pair>将处理时间减少了 10000...

非常惊讶，我决定将map表演与排序vector或pair进行比较。

我很惊讶，因为我找不到map比pair的排序vector快的情况(随机填充并稍后排序(......一定有一些情况map更快。否则提供这门课有什么意义？

这是我测试的内容：

测试一个，比较map填充和销毁与vector填充、排序(因为我想要一个分类容器(和销毁：

#include <iostream>
#include <time.h>
#include <cstdlib>
#include <map>
#include <vector>
#include <algorithm>
int main(void)
{
    clock_t tStart = clock();
    {
        std::map<float,int> myMap;
        for ( int i = 0; i != 10000000; ++i )
        {
            myMap[ ((float)std::rand()) / RAND_MAX ] = i;
        }
    }
    std::cout << "Time taken by map: " << ((double)(clock() - tStart)/CLOCKS_PER_SEC) << std::endl;
    tStart = clock();
    {
        std::vector< std::pair<float,int> > myVect;
        for ( int i = 0; i != 10000000; ++i )
        {
            myVect.push_back( std::make_pair( ((float)std::rand()) / RAND_MAX, i ) );
        }
        // sort the vector, as we want a sorted container:
        std::sort( myVect.begin(), myVect.end() );
    }
    std::cout << "Time taken by vect: " << ((double)(clock() - tStart)/CLOCKS_PER_SEC) << std::endl;
    return 0;
}

用g++ main.cpp -O3 -o main编译并得到：

Time taken by map: 21.7142
Time taken by vect: 7.94725

map慢了3倍...

然后，我说，"好吧，矢量填充和排序更快，但使用地图搜索会更快"......所以我测试了：

#include <iostream>
#include <time.h>
#include <cstdlib>
#include <map>
#include <vector>
#include <algorithm>
int main(void)
{
    clock_t tStart = clock();
    {
        std::map<float,int> myMap;
        float middle = 0;
        float last;
        for ( int i = 0; i != 10000000; ++i )
        {
            last = ((float)std::rand()) / RAND_MAX;
            myMap[ last ] = i;
            if ( i == 5000000 )
                middle = last; // element we will later search
        }
        std::cout << "Map created after " << ((double)(clock() - tStart)/CLOCKS_PER_SEC) << std::endl;
        float sum = 0;
        for ( int i = 0; i != 10; ++i )
            sum += myMap[ last ]; // search it
        std::cout << "Sum is " << sum << std::endl;
    }
    std::cout << "Time taken by map: " << ((double)(clock() - tStart)/CLOCKS_PER_SEC) << std::endl;
    tStart = clock();
    {
        std::vector< std::pair<float,int> > myVect;
        std::pair<float,int> middle;
        std::pair<float,int> last;
        for ( int i = 0; i != 10000000; ++i )
        {
            last = std::make_pair( ((float)std::rand()) / RAND_MAX, i );
            myVect.push_back( last );
            if ( i == 5000000 )
                middle = last; // element we will later search
        }
        std::sort( myVect.begin(), myVect.end() );
        std::cout << "Vector created after " << ((double)(clock() - tStart)/CLOCKS_PER_SEC) << std::endl;
        float sum = 0;
        for ( int i = 0; i != 10; ++i )
            sum += (std::find( myVect.begin(), myVect.end(), last ))->second; // search it
        std::cout << "Sum is " << sum << std::endl;
    }
    std::cout << "Time taken by vect: " << ((double)(clock() - tStart)/CLOCKS_PER_SEC) << std::endl;
    return 0;
}

用g++ main.cpp -O3 -o main编译并得到：

Map created after 19.5357
Sum is 1e+08
Time taken by map: 21.41
Vector created after 7.96388
Sum is 1e+08
Time taken by vect: 8.31741

使用vector甚至搜索速度显然更快(使用 map 进行 10 次搜索几乎需要 2 秒，而使用 vector 只花了半秒钟(....

所以：

• 我错过了什么吗？
• 我的测试不正确/不准确吗？
• map只是要避免的课程，还是真的存在map提供良好表现的情况？