容器之分类与各种测试——unordered-multiset

Posted 2021-03-24 area-h-p

unordered-multiset是不定序关联式容器，其底部是通过哈希表实现功能。

（ps:黑色框就是bucket，白色框即为bucket上挂载的元素）

 

 

 为了提高查找效率，bucket（篮子）的数量应当大于元素的个数，这是因为在bucket上悬挂的元素是通过单链表串起来的，而且一个unordered-multiset上一定会有很多bucket是没有悬挂元素，所以为了提升查找效率，当 元素个数>=bucket个数 时，bucket会自动扩充为原来的两倍左右，并且将元素重新哈希碰撞后再悬挂到bucket上。

例程

#include<stdexcept>
#include<string>
#include<cstdlib>
#include<cstdio>
#include<algorithm>
#include<iostream>
#include<ctime>
#include<unordered_set>
using namespace std;
long get_a_target_long()
{
	long target = 0;
	cout<<"target(0~"<<RAND_MAX<<"):";
	cin>>target;
	return target;
}
string get_a_target_string()
{
	long target = 0;
	char buf[10];
	cout<<"target(0~"<<RAND_MAX<<"):";
	cin>>target;
	snprintf(buf, 10, "%ld", target);
	return string(buf);
}
int compareLongs(const void* a, const void* b)
{
	return (*(long*)a - *(long*)b);
}

int compareStrings(const void *a, const void *b)
{
	if(*(string*)a > *(string*)b)
		return 1;
	else if(*(string*)a < *(string*)b)
		return -1;
	else
		return 0;
}
void test_unordered_multiset(long& value)
{
	cout << "
test_unordered_multiset().......... 
";

	unordered_multiset<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_multiset.size()= " << c.size() << endl;
	cout << "unordered_multiset.max_size()= " << c.max_size() << endl;	//与计算机内存大小正相关
	cout << "unordered_multiset.bucket_count()= " << c.bucket_count() << endl;	//篮子个数
	cout << "unordered_multiset.load_factor()= " << c.load_factor() << endl;	//载重因子，元素个数/bucket个数
	cout << "unordered_multiset.max_load_factor()= " << c.max_load_factor() << endl;	//恒定为1
	cout << "unordered_multiset.max_bucket_count()= " << c.max_bucket_count() << endl;	//与计算机内存大小正相关			
	for (unsigned i=0; i< 20; ++i) {
		cout << "bucket #" << i << " has " << c.bucket_size(i) << " elements.
";
	}					

	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;	
	}		
}
int main()
{
	long int value;
	cout<<"how many elements: ";
	cin>>value;
	test_unordered_multiset(value);
	return 0;
}

 运行一下