为什么我的机器上的hash_map和unordered_map非常慢？ [英] Why hash_map and unordered_map on my machine are extremely slow?

问题描述

我用这段代码测试了这些代码（在Visual Studio 2010 sp1上）：

I tested them with this code (on Visual Studio 2010 sp1):

#include <ctime>
#include <iostream>
#include <map>
#include <unordered_map>
#include <hash_map>

int main()
{ 
    clock_t time;
    int LOOP = (1 << 16);
    std::map<int, int> my_map;
    std::unordered_map<int, int> map_unordered_map;
    std::hash_map<int, int> my_hash_map;

    time = clock();
    for (int i = 0; i != LOOP; ++i)
    {
        my_map[i] = i;
    }
    std::cout << "map: " << ((double)(clock() - time) / CLOCKS_PER_SEC) << std::endl;

    time = clock();
    for (int i = 0; i != LOOP; ++i)
    {
        map_unordered_map[i] = i;
    }
    std::cout << "unordered_map: " << ((double)(clock() - time) / CLOCKS_PER_SEC) << std::endl;

    time = clock();
    for (int i = 0; i != LOOP; ++i)
    {
        my_hash_map[i] = i;
    }
    std::cout << "hash_map: " << ((double)(clock() - time) / CLOCKS_PER_SEC) << std::endl;

    system("PAUSE");
    return EXIT_SUCCESS;
}

结果很奇怪：

在DEBUG：
地图：0.289
unordered_map：10.738
hash_map：10.58
按任意键继续。 。 。

In DEBUG: map: 0.289 unordered_map: 10.738 hash_map: 10.58 Press any key to continue . . .

在RELEASE中：
地图：0.101
unordered_map：0.463
hash_map：0.429
按任意键继续。 。

In RELEASE: map: 0.101 unordered_map: 0.463 hash_map: 0.429 Press any key to continue . . .

推荐答案

1. 您只在每张地图中插入65536个项目 - 不够大O（log N）和O（1）之间的差异意味着很多。


2. 您只是插入项目，之后不进行任何搜索。 / li>
   
3. 您的密钥全部是连续的整数，不适合通常使用任何地图。

1. You're only inserting 65536 items in each map -- not large enough for the difference between O(log N) and O(1) to mean a whole lot.
2. You're only inserting items, not doing any searching afterwards.
3. Your keys are all contiguous integers in increasing order -- doesn't fit how any map will typically be used.

底线：这不太可能告诉你有关数据结构的问题。

Bottom line: this isn't likely to tell you much about the data structures in question.

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