C++ unordered_map

unordered_map与hash_map对比：

1. unordered_map原来属于boost分支和std::tr1中，而hash_map属于非标准容器。
2. unordered_map感觉速度和hash_map差不多，但是支持string做key，也可以使用复杂的对象作为key。

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HashMap的定义

整形定义

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

int main(){
    unordered_map <int, int> inthash;
    inthash[1] = 123;
    inthash[2] = 456;
    int val1 = inthash[1];
    int val2 = inthash[2];
    cout << val1 << endl;
    cout << val2 << endl;
}

字符型定义

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

int main(){
    unordered_map <string, int> strhash;
    strhash["a"] = 123;
    strhash["b"] = 456;
    int val1 = strhash["a"];
    int val2 = strhash["b"];
    cout << val1 << endl;
    cout << val2 << endl;
}

HashMap的遍历

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

int main(){
    unordered_map<int, int> intHash;
    intHash[1] = 123;
    intHash[2] = 456;
    intHash[3] = 789;
    for (auto it = intHash.begin(); it != intHash.end(); ++it)
        cout << " " << it->first << ":" << it->second << endl;
}

HashMap的查找

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

int main(){
    unordered_map<int, string> strHash;
    strHash[12] = "123";
    strHash[25] = "456";
    strHash[39] = "789";
    unordered_map<int, string>::iterator it = strHash.begin();
    it = strHash.find(25);
    if(it != strHash.end()){
        cout << it->second << endl;
    }else
    {
        cout << "not find" << endl;
    }
}

显示HashMap的大小

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

int main(){
    unordered_map<string, string> strHash;
    strHash["a"] = "123";
    strHash["b"] = "456";
    strHash["c"] = "789";
    cout << strHash.size() << endl;
}

判断HashMap是否为空

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

int main(){
    unordered_map<const char*, string> strHash;
    strHash["a"] = "123";
    strHash["b"] = "456";
    strHash["c"] = "789";
    cout << strHash.empty() << endl;
}

删除HashMap中的元素

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

int main(){
    unordered_map<string, string> strHash;
    strHash["a"] = "123";
    strHash["b"] = "456";
    strHash["c"] = "789";

    strHash.erase("a");

    unordered_map<string, string>::iterator it = strHash.begin();
    while (it != strHash.end()){
        cout<< it->first <<"\t" << it++->second <<endl;
    }
}