C++ 字典 map详解

C++ 字典 map详解

2019-01-18 23:32:08 lh_lyh 阅读数 1123更多

文章目录

• 容器重要属性
• 操作
  • 迭代器
    • 1. begin 指向起始
    • 2. end 指向末尾
    • 3. rbegin 指向倒序起始（即末尾）
    • 4. rend 指向倒序末尾（即起始）
  • 大小
    • 1. empty 判断容器是否为空
    • 2. size 返回容器大小
    • 3. max_size 返回容器最大尺寸
  • 元素访问
    • 1. operater[ ]
    • 2. at
  • 修改元素
    • 1. insert 插入元素
    • 2. erase 删除元素
    • 3. swap 交换两个map容器内容
    • 4. clear 清除容器
    • 5. emplace 构造并插入元素
  • 操作
    • 1. 赋值
    • 2. find 获得指向元素的迭代器
    • 3. count 对某个键的元素计数
    • 4. lower_bound 返回下边界的迭代器
    • 5. upper_bound 返回上边界的迭代器
    • 6. equal_range 获得相同元素的范围

---

容器重要属性

• 通过键值访问，而不是位置
• 按Key有序排列
• 键与值一 一对应
• 键唯一，不存在相同的键对应不同的值

---

操作

• 迭代器

  1. begin 指向起始

  2. end 指向末尾

  3. rbegin 指向倒序起始（即末尾）

  4. rend 指向倒序末尾（即起始）

  // 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 << '\n';
  
    return 0;
  }
  // Output:
  //	a => 200
  //	b => 100
  //	c => 300
  

• 大小

  1. empty 判断容器是否为空

  // map::empty
  #include <iostream>
  #include <map>
  
  int main ()
  {
    std::map<char,int> mymap;
  
    mymap['a']=10;
    mymap['b']=20;
    mymap['c']=30;
  
    while (!mymap.empty())
    {
      std::cout << mymap.begin()->first << " => " << mymap.begin()->second << '\n';
      mymap.erase(mymap.begin());
    }
  
    return 0;
  }
  
  //Output:
  //a => 10
  //b => 20
  //c => 30
  

  2. size 返回容器大小

  // map::size
  #include <iostream>
  #include <map>
  
  int main ()
  {
    std::map<char,int> mymap;
    mymap['a']=101;
    mymap['b']=202;
    mymap['c']=302;
  
    std::cout << "mymap.size() is " << mymap.size() << '\n';
  
    return 0;
  }
  //Output:
  //mymap.size() is 3
  

  3. max_size 返回容器最大尺寸

• 元素访问

  1. operater[ ]

  2. at

  // accessing mapped values: at / operator[]
  #include <iostream>
  #include <string>
  #include <map>
  
  int main ()
  {
   std::map<std::string,int> mymap = {
                 { "alpha", 0 },
                 { "beta", 0 },
                 { "gamma", 0 } };
  
   mymap.at("alpha") = 10;
   mymap["beta"] = 20;
   mymap.at("gamma") = 30;
  
   for (auto& x: mymap) {
     std::cout << x.first << ": " << x.second << '\n';
   }
  
   return 0;
  }
  //Output:
  //alpha: 10
  //beta: 20
  //gamma: 30
  

• 修改元素

  1. insert 插入元素

  方法一：直接插入键值对mymap.insert ( std::pair<char,int>('a',100) );
  方法二：通过迭代器指定位置并插入std::map<char,int>::iterator it = mymap.begin(); mymap.insert (it, std::pair<char,int>('b',300)); // max efficiency inserting
  方法三：通过迭代器范围插入，区间半包含（[ , )）
  std::map<char,int> anothermap; anothermap.insert(mymap.begin(),mymap.find('c'));

  // map::insert (C++98)
  #include <iostream>
  #include <map>
  
  int main ()
  {
   std::map<char,int> mymap;
  
   // first insert function version (single parameter):
   mymap.insert ( std::pair<char,int>('a',100) );
   mymap.insert ( std::pair<char,int>('z',200) );
  
   std::pair<std::map<char,int>::iterator,bool> ret;
   ret = mymap.insert ( std::pair<char,int>('z',500) );
   if (ret.second==false) {
     std::cout << "element 'z' already existed";
     std::cout << " with a value of " << ret.first->second << '\n';
   }
  
   // second insert function version (with hint position):
   std::map<char,int>::iterator it = mymap.begin();
   mymap.insert (it, std::pair<char,int>('b',300));  // max efficiency inserting
   mymap.insert (it, std::pair<char,int>('c',400));  // no max efficiency inserting
  
   // third insert function version (range insertion):
   std::map<char,int> anothermap;
   anothermap.insert(mymap.begin(),mymap.find('c'));
  
   // showing contents:
   std::cout << "mymap contains:\n";
   for (it=mymap.begin(); it!=mymap.end(); ++it)
     std::cout << it->first << " => " << it->second << '\n';
  
   std::cout << "anothermap contains:\n";
   for (it=anothermap.begin(); it!=anothermap.end(); ++it)
     std::cout << it->first << " => " << it->second << '\n';
  
   return 0;
  }
  //Output:
  //element 'z' already existed with a value of 200
  //mymap contains:
  //a => 100
  //b => 300
  //c => 400
  //z => 200
  //anothermap contains:
  //a => 100
  //b => 300
  

  2. erase 删除元素

  // erasing from map
  #include <iostream>
  #include <map>
  
  int main ()
  {
   std::map<char,int> mymap;
   std::map<char,int>::iterator it;
  
   // insert some values:
   mymap['a']=10;
   mymap['b']=20;
   mymap['c']=30;
   mymap['d']=40;
   mymap['e']=50;
   mymap['f']=60;
  
   it=mymap.find('b');
   mymap.erase (it);                   // erasing by iterator
  
   mymap.erase ('c');                  // erasing by key
  
   it=mymap.find ('e');
   mymap.erase ( it, mymap.end() );    // erasing by range
  
   // show content:
   for (it=mymap.begin(); it!=mymap.end(); ++it)
     std::cout << it->first << " => " << it->second << '\n';
  
   return 0;
  }
  //Output:
  //a => 10
  //d => 40
  

  3. swap 交换两个map容器内容

  // swap maps
  #include <iostream>
  #include <map>
  
  int main ()
  {
    std::map<char,int> foo,bar;
  
    foo['x']=100;
    foo['y']=200;
  
    bar['a']=11;
    bar['b']=22;
    bar['c']=33;
  
    foo.swap(bar);
  
    std::cout << "foo contains:\n";
    for (std::map<char,int>::iterator it=foo.begin(); it!=foo.end(); ++it)
      std::cout << it->first << " => " << it->second << '\n';
  
    std::cout << "bar contains:\n";
    for (std::map<char,int>::iterator it=bar.begin(); it!=bar.end(); ++it)
      std::cout << it->first << " => " << it->second << '\n';
  
    return 0;
  }
  //Output:
  //foo contains:
  //a => 11
  //b => 22
  //c => 33
  //bar contains:
  //x => 100
  //y => 200
  

  4. clear 清除容器

  5. emplace 构造并插入元素

  // map::emplace
  #include <iostream>
  #include <map>
  
  int main ()
  {
    std::map<char,int> mymap;
  
    mymap.emplace('x',100);
    mymap.emplace('y',200);
    mymap.emplace('z',100);
  
    std::cout << "mymap contains:";
    for (auto& x: mymap)
      std::cout << " [" << x.first << ':' << x.second << ']';
    std::cout << '\n';
  
    return 0;
  }
  //Output:
  //mymap contains: [x:100] [y:200] [z:100]
  

• 操作

  1. 赋值

  // assignment operator with maps
  #include <iostream>
  #include <map>
  
  int main ()
  {
   std::map<char,int> first;
   std::map<char,int> second;
  
   first['x']=8;
   first['y']=16;
   first['z']=32;
  
   second=first;                // second now contains 3 ints
   first=std::map<char,int>();  // and first is now empty
  
   std::cout << "Size of first: " << first.size() << '\n';
   std::cout << "Size of second: " << second.size() << '\n';
   return 0;
  }
  //Output:
  //Size of first: 0
  //Size of second: 3
  

  2. find 获得指向元素的迭代器

  // map::find
  #include <iostream>
  #include <map>
  
  int main ()
  {
   std::map<char,int> mymap;
   std::map<char,int>::iterator it;
  
   mymap['a']=50;
   mymap['b']=100;
   mymap['c']=150;
   mymap['d']=200;
  
   it = mymap.find('b');
   if (it != mymap.end())
     mymap.erase (it);
  
   // print content:
   std::cout << "elements in mymap:" << '\n';
   std::cout << "a => " << mymap.find('a')->second << '\n';
   std::cout << "c => " << mymap.find('c')->second << '\n';
   std::cout << "d => " << mymap.find('d')->second << '\n';
  
   return 0;
  }
  //Output:
  //elements in mymap:
  //a => 50
  //c => 150
  //d => 200
  

  3. count 对某个键的元素计数

  // map::count
  #include <iostream>
  #include <map>
  
  int main ()
  {
    std::map<char,int> mymap;
    char c;
  
    mymap ['a']=101;
    mymap ['c']=202;
    mymap ['f']=303;
  
    for (c='a'; c<'h'; c++)
    {
      std::cout << c;
      if (mymap.count(c)>0)
        std::cout << " is an element of mymap.\n";
      else 
        std::cout << " is not an element of mymap.\n";
    }
  
    return 0;
  }
  //Output:
  //a is an element of mymap.
  //b is not an element of mymap.
  //c is an element of mymap.
  //d is not an element of mymap.
  //e is not an element of mymap.
  //f is an element of mymap.
  //g is not an element of mymap.
  

  4. lower_bound 返回下边界的迭代器

  返回指向该元素的迭代器

  5. upper_bound 返回上边界的迭代器

  返回指向该元素的下一个元素的迭代器

  // map::lower_bound/upper_bound
  #include <iostream>
  #include <map>
  
  int main ()
  {
   std::map<char,int> mymap;
   std::map<char,int>::iterator itlow,itup;
  
   mymap['a']=20;
   mymap['b']=40;
   mymap['c']=60;
   mymap['d']=80;
   mymap['e']=100;
  
   itlow=mymap.lower_bound ('b');  // itlow points to b
   itup=mymap.upper_bound ('d');   // itup points to e (not d!)
  
   mymap.erase(itlow,itup);        // erases [itlow,itup)
  
   // print content:
   for (std::map<char,int>::iterator it=mymap.begin(); it!=mymap.end(); ++it)
     std::cout << it->first << " => " << it->second << '\n';
  
   return 0;
  }
  //Output:
  //a => 20
  //e => 100
  

  6. equal_range 获得相同元素的范围