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", 0},
        {"beta", 0},
        {"gamma", 0}};

    mymap.at("alpha") = 10;
    mymap.at("beta") = 20;
    mymap.at("gamma") = 30;

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

    return 0;
}

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 (10,20);
  bar = std::make_pair (10.5,'A'); // ok: implicit conversion from pair<double,char>

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

  return 0;
}

３．　map::begin/end

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

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

  mymap['b'] = 100;
  mymap['a'] = 200;
  mymap['c'] = 300;

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

  return 0;
}

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', 100) );
    mymap.insert ( pair<char,int>('z', 200) );

    pair<map<char, int>::iterator, bool> ret;
    ret = mymap.insert (pair<char,int>('z',500));
    if (ret.second == false){
        cout<<"element 'z' already existed";
        cout<<"with a value of " << ret.first->second << '
';
    }

    //second insert function version (with hint position):
    map<char, int>::iterator it = mymap.begin();
    mymap.insert (it, pair<char, int>('b',300)); // max efficiency inserting
    mymap.insert (it, pair<char, int>('c',400)); // 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: 
";
    for (it = mymap.begin(); it!= mymap.end(); ++it)
        cout<<it->first<<"=>"<<it->second<<'
';

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

    return 0;
}