C++ 学习小程序之 map 的用法

１．　map::at

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

 int main(){
     map<string, int> mymap = {
         {"alpha", },
         {"beta", },
         {"gamma", }};

     mymap.at("alpha") = ;
     mymap.at("beta") = ;
     mymap.at("gamma") = ;

     for (auto& x:mymap){
         cout<<x.first<<": "<<x.second<<'\n';
     }

     return ;
 }

２．　make_pair example

 // make_pair example
 #include <utility>      // std::pair
 #include <iostream>     // std::cout

 int main () {
   std::pair <int,int> foo;
   std::pair <int,int> bar;

   foo = std::make_pair (,);
   bar = std::make_pair (10.5,'A'); // ok: implicit conversion from pair<double,char>

   std::cout << "foo: " << foo.first << ", " << foo.second << '\n';
   std::cout << "bar: " << bar.first << ", " << bar.second << '\n';

   return ;
 }

３．　map::begin/end

 // map::begin/end
 #include <iostream>
 #include <map>

 int main ()
 {
   std::map<char,int> mymap;

   mymap['b'] = ;
   mymap['a'] = ;
   mymap['c'] = ;

   // show content:
   for (std::map<char,int>::iterator it=mymap.begin(); it!=mymap.end(); ++it)
     std::cout << it->first << " => " << it->second << '\n';

   return ;
 }

4.   map::insert(C++98)

 // map::insert(C++98)
 #include <iostream>
 #include <map>
 using namespace std;
 int main ()
 {
     map<char,int> mymap;

     // first insert function version (single parameter):
     mymap.insert ( pair<char,int>('a', ) );
     mymap.insert ( pair<char,int>('z', ) );

     pair<map<char, int>::iterator, bool> ret;
     ret = mymap.insert (pair<char,int>('z',));
     if (ret.second == false){
         cout<<"element 'z' already existed";
         cout<<"with a value of " << ret.first->second << '\n';
     }

     //second insert function version (with hint position):
     map<char, int>::iterator it = mymap.begin();
     mymap.insert (it, pair<char, int>('b',)); // max efficiency inserting
     mymap.insert (it, pair<char, int>('c',)); // no max efficiency inserting

     //third insert function version (range insertion):
     map<char,int> anothermap;
     anothermap.insert(mymap.begin(),mymap.find('c'));

     // showing contents:
     cout<<"mymap contains: \n";
     for (it = mymap.begin(); it!= mymap.end(); ++it)
         cout<<it->first<<"=>"<<it->second<<'\n';

     cout<<"anothermap contains: \n";
     for(it=anothermap.begin(); it!=anothermap.end();++it)
         cout<<it->first<<"=>"<<it->second<<'\n';

     return ;
 }