c++ unordered_map 哈希表使用

1. find()函数

C++中的 unordered_map使用时通过键值对进行插入，并通过find(key)进行寻找键值是否存在。

//iterator find (key) : 找到则返回对应键值得迭代器
//没有找到则返回unordered_map::end.
// unordered_map::find

  if ( got == mymap.end() )
    std::cout << "not found";
  else
    std::cout << got->first << " is " << got->second;

不能插入重复元素，即具有同样键值(key)的元素。

#include <iostream>
#include <string>
#include <unordered_map>
using namespace std;

int main()
{
    unordered_map<string, string> mymap = {
        {"house", "maison"},
        {"apple", "pomme"},
        {"apple", "pomme"},
        {"apple", "pomme1"},
        {"tree", "arbre"},
        {"book", "livre"},
        {"door", "porte"},
        {"grapefruit", "pamplemousse"}
    };
    unsigned n = mymap.bucket_count();

    cout << "mymap has "  << n << "buckets.
";
    for(unsigned i = 0; i < n; i++)
    {
        cout << "bucket #" << i  << "contains: ";
        for(auto it = mymap.begin(i); it != mymap.end(i); ++it)
        {
            cout << "[" << it->first << ":" << it->second << "]";
        }
        cout << "
";
    } 
    return 0;

}
/* 输出
*mymap has 11buckets.
*bucket #0contains: 
*bucket #1contains: [book:livre]
*bucket #2contains: [grapefruit:pamplemousse]
*bucket #3contains: [house:maison]
*bucket #4contains: 
*bucket #5contains: 
*bucket #6contains: 
*bucket #7contains: 
*bucket #8contains: 
*bucket #9contains: [apple:pomme]
*bucket #10contains: [door:porte][tree:arbre]

 */

 2. 自己写的一个类测试unordered_map的使用，感觉键值映射的value值好像没什么用。

/*typedef pair<const Key, T> value_type;
 *unordered_map<Key, T>::iterator it;
 (*it).first; | it->first //the key value 
 *(*it).second;| it->second  //the mapped value
 */
#include <iostream>
#include <fstream>
#include <sstream>
#include <unordered_map>
#include <string>
#include <ctime>
using namespace std;
/* compute the number of target value t int the interval[-10000,10000]
 * such that distinct number x, y int the input file that 
 * satisfy x+y=t.
 * 
 */
class TwoSum
{
public:

    TwoSum()
    {
        get_data();
        for(long long  x = -10000; x <= 10000; x++)
        {
            calc_times(x);
        }
        
    }
/************读入数据 ***********/
    void get_data()
    {
        ifstream fin("algo1-programming_prob-2sum.txt");
        string line;
        long long  count = 0;
        stringstream stream;
        while(getline(fin,line))
        {
            long long data;
            stream.clear();
            stream << line;
            stream >> data;
/* 插入键值对，感觉映射的value有时候没有什么用 */
            map.insert(make_pair( data,count ));    
            count ++;
        }    
    }
/****************计算x+y=sum对的个数***************/
    void calc_times(long long  sum)
    {
        for(auto &x : map)
        {
            unordered_map<long long, long long >::iterator got = map.find(sum-(x.first));
            if( ( got != map.end() ) && (sum-x.first != x.first) )
            {
                totalSum ++;
            }
        }
    
    }


public:
    unordered_map<long long , long long > map; //hashtable 
    long long totalSum = 0; 

};

int main()
{
    clock_t start, end;
    start = clock();
    TwoSum twosum;
    end = clock();
    cout << twosum.totalSum << endl;
    cout << "running time:" << (double)(end-start) / CLOCKS_PER_SEC  << "s" << endl;
    return 0;

}