侯捷STL学习(十)--容器hashtable探索(unordered set/map)

layout: post

title: 侯捷STL学习（十）

date: 2017-07-23

tag: 侯捷STL

---

第二十三节 容器hashtable探索

hashtable冲突（碰撞）处理

rehash时，篮子扩充两倍，找到其附近的质数，重新计算元素位置

内部扩充的数据已经预定好，53->97->....



hashtable实现

iterator要实现当当前node链表结束，要能进入到下一个buckets



hashtable使用

模板参数的形式



容器hashtable中hashfunction

hash{}的偏特化实现



hashtable使用

C++11--unordered容器

结构



test unordered_set

#include <unordered_set>
#include <stdexcept>
#include <string>
#include <cstdlib> //abort()
#include <cstdio>  //snprintf()
#include <iostream>
#include <ctime>
namespace jj15
{
void test_unordered_set(long& value)
{
cout << "\ntest_unordered_set().......... \n";

unordered_set<string> c;
char buf[10];

clock_t timeStart = clock();
for(long i=0; i< value; ++i)
{
try {
snprintf(buf, 10, "%d", rand());
c.insert(string(buf));
}
catch(exception& p) {
cout << "i=" << i << " " << p.what() << endl;
abort();
}
}
cout << "milli-seconds : " << (clock()-timeStart) << endl;
cout << "unordered_set.size()= " << c.size() << endl;
cout << "unordered_set.max_size()= " << c.max_size() << endl;  //357913941
cout << "unordered_set.bucket_count()= " << c.bucket_count() << endl;
cout << "unordered_set.load_factor()= " << c.load_factor() << endl;
cout << "unordered_set.max_load_factor()= " << c.max_load_factor() << endl;
cout << "unordered_set.max_bucket_count()= " << c.max_bucket_count() << endl;
for (unsigned i=0; i< 20; ++i) {
cout << "bucket #" << i << " has " << c.bucket_size(i) << " elements.\n";
}

string target = get_a_target_string();
{
timeStart = clock();
auto pItem = find(c.begin(), c.end(), target);  //比 c.find(...) 慢很多
cout << "std::find(), milli-seconds : " << (clock()-timeStart) << endl;
if (pItem != c.end())
cout << "found, " << *pItem << endl;
else
cout << "not found! " << endl;
}

{
timeStart = clock();
auto pItem = c.find(target);        //比 std::find(...) 快很多
cout << "c.find(), milli-seconds : " << (clock()-timeStart) << endl;
if (pItem != c.end())
cout << "found, " << *pItem << endl;
else
cout << "not found! " << endl;
}
}
}