C++中的关联容器multimap/multiset用法

声明

multimap<string, string> authors;

元素的添加

//adds first element with key Barth

authors.insert(make_pair(string(“Barth, John”), string(“Sot-Weed Factor”)));

//ok: adds second element with key Barth

authors.insert(make_pair(string(“Barth, John”), string(“Lost in the Funhouse”)));

元素的删除

string search_item(“Kazuo Ishiguro”);

//erase all elements with this key; returns number of elements removed

multimap<string, string>::size_type cnt=authors.erase(search_item);

查找元素

方法1： find count

//author we’ll look for

string search_item(“Alain de Botton”);

//how many entries are there for this author

typedef multimap<string, string>::size_type sz_type;

sz_type entries=author.count(search_item);

//get iterator to the first entry for this author

multimap<string, string>::iterator iter=author.find(search_item);

//loop through the number of entries there are for this author

for(sz_type cnt=0; cnt<entries; ++cnt, ++iter)

  cout<<iter->second<<endl; //print each titile

首先调用count确定某作者所写的书籍数目，然后调用find获得指向第一个该键所关联的元素的迭代器。for循环迭代的次数依赖于count返回的值。在特殊情况下，如果count返回0值，则该循环永不执行。

方法2： lower_bound upper_bound

//definitions of authors and search_item as above

//beg and end denote range of elements for this author

typedef multimap<string, string>::iterator authors_it;

authors_it beg=authors.lower_bound(search_item),

        end=authors.upper_bound(search_item);

//loop through the number of entries there are for this author

while(beg!=end){

cout<<beg->second<<endl;//print each title

++beg;

}

在同一个键上调用lower_bound和upper_bound，将产生一个迭代器范围，指示出该键所关联的所有元素。如果该键在容器中存在，则会获得两不同的迭代器：lower_bound返回的迭代器指向该键关联的第一个实例，而upper_bound返回的迭代器则指向最后一个实例的下一位置。如果该键不在multimap中，这两个操作将返回同一个迭代器，指向依据元素的排列顺序该键应该插入的位置。

方法3： equal_range

//definitions of authors and search_item as above

//pos holds iterators that denote range of elements ofr this key

pair<authors_it, authors_it> pos=authors.equal_range(search_item);

//loop through the number of entries there are for this author

while(pos.first!=pos.second){

   cout<<pos.first->second<<endl;//print each titile

   ++pos.first;

}

pair对象的first成员存储lower_bound函数返回的迭代器，而second成员则记录upper_bound函数返回的迭代器。