std :: multimap的使用案例 [英] Use cases of std::multimap

问题描述

我不太明白这个数据结构的目的。 std :: multimap< K，V> 和 std :: map< K，std :: vector< V& / code>。这同样适用于 std :: multiset - 它可以只是 std :: map< K，int> int计数了K的出现次数。我在这些结构的使用上缺少了什么？

解决方案

考虑在按名称分组的AdressList中的PhoneEntry。

  int AdressListCompare（const PhoneEntry& p1，const PhoneEntry& p2）{
 return p1.name< p2.name; 
} 
 
 multiset< PhoneEntry，AdressListCompare> adressList; 
 
 adressList.insert（PhoneEntry（Cpt.G，123-456，Cellular））; 
 adressList.insert（PhoneEntry（Cpt.G，234-567，Work））; 
 //获取条目
 addressList.equal_range（PhoneENtry（Cpt.G））; //所有数字
  

这不适用于set + count。如果不需要这种行为，你的Object + count方法似乎更快。例如multiset :: count（）成员状态

复杂性：大小对数对数+ / p>

I don't quite get the purpose of this data structure. What's the difference between std::multimap<K, V> and std::map<K, std::vector<V>>. The same goes for std::multiset- it could just be std::map<K, int> where the int counts the number of occurrences of K. Am I missing something on the uses of these structures?

解决方案

A counter-example seems to be in order.

Consider a PhoneEntry in an AdressList grouped by name.

int AdressListCompare(const PhoneEntry& p1, const PhoneEntry& p2){
    return p1.name<p2.name;
}

multiset<PhoneEntry, AdressListCompare> adressList;

adressList.insert( PhoneEntry("Cpt.G", "123-456", "Cellular") );    
adressList.insert( PhoneEntry("Cpt.G", "234-567", "Work") );
// Getting the entries
addressList.equal_range( PhoneENtry("Cpt.G") ); // All numbers

This would not be feasible with a set+count. Your Object+count approach seems to be faster if this behavior is not required. For instance the multiset::count() member states

"Complexity: logarithmic in size + linear in count."

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