The multimap Type

As we know, both map and set can contain only a single instance of each key . The type multimap allow multiple instances of a key (1:n). In a phone directory, for example, some might wish to provide a separate listing for each phone number associated with an individual. The multimap type is defined in the same headers as the corresponding single-element versions: the map header .

 

[Note]

[1] The operations supported by multimap are the same as those on map, with one exception : multimap does not support subscripting . We cannot subscript a multimap because there may be more than one value associated with a given key.

 

[2] Because keys need not be unique, insert always adds an element .

 

[3] Finding elements in a multimap , we should note that maps store their elements in order, the multimap type does so as well . As a result, when a multimap has multiple instances of a given key, those instances will be adjacent elements within the container . It turns out that there are three strategies we might use to find and print all the elements by a given key. Each of these strategies relies on the face that we know that all the entries for a given key will be adjacent within the multimap . The strategies are as following.

(1) using find and count

(2) using lower_bound and upper_bound

(3) using equal_range (the best way )

 

[An example of multimap]

/* * My schedule of events. * 2010-4-13 wcdj VS2008 */ #include <iostream> #include <map> #include <string> using namespace std; int main() { multimap<string,string> schedule; string date,event,searchItem; // enter my schedule cout<<"***Welcome to using [personal schedule note]***"<<endl<<endl; while (1) { cout<<"Enter the date (Ctrl+Z to end): "<<endl; cin>>date; if (!cin) { break; } cout<<"Enter your events on this date (Ctrl+Z to end): "<<endl; while (cin>>event) { schedule.insert(make_pair(date,event)); } cin.clear(); } cin.clear(); // search the events of some date // firstly, check the multimap is empty or not typedef multimap<string,string>::const_iterator itType; itType iter=schedule.begin(); if (iter==schedule.end()) { cout<<"the schedule is empty! "<<endl; return 0; } else// not empty { while (1) { cout<<"search events, enter some date (Ctrl+Z to end): "<<endl; cin>>searchItem; if (!cin) { break; } pair<itType,itType> pos=schedule.equal_range(searchItem); if (pos.first==pos.second) { cout<<"can not find this date, "<<searchItem<<" is your free time :)"<<endl; } else// find it { // print the events on this date cout<<"the events on ["<<searchItem<<"] is: "<<endl; size_t index=1; while (pos.first!=pos.second) { cout<<"["<<index++<<"]:/t"<<pos.first->second<<endl; ++pos.first; } } } } return 0; }

output:

[The usage of find and equal_range ]

[1]

typedef multimap<type,type>::iterator itType;

itType iter=m.find(key);

/* multimap+find+erase */ #include <iostream> #include <map>// multimap #include <string> using namespace std; int main() { multimap<string,string> authors; string author,work,searchItem; // set multimap do { cout<<"Enter author's name (Ctrl+Z to end): "<<endl; cin>>author; if (!cin) { break; } cout<<"Enter author's work (Ctrl+Z to end): "<<endl; while (cin>>work) { authors.insert( make_pair(author,work)); } cin.clear(); } while (cin); cin.clear(); // search cout<<"Who is the author that you want to erase: "<<endl; cin>>searchItem; // find the first element about this author multimap<string,string>::const_iterator iter=authors.find(searchItem); if (iter!=authors.end()) { authors.erase(searchItem);// delete all elements about this author } else { cout<<"can not find this author!"<<endl; } // print multimap's objects cout<<"author/t/twork"<<endl; for (iter=authors.begin();iter!=authors.end();++iter) { cout<<iter->first<<"/t/t"<<iter->second<<endl; } return 0; }  

[2]

typedef multimap<type,type>::iterator itType;

pair<itType,itType> pos=m.equal_range(key);

/* multimap+equal_range+erase */ #include <iostream> #include <map>// multimap #include <string> using namespace std; int main() { multimap<string,string> authors; string author,work,searchItem; // set do { cout<<"Enter author's name (Ctrl+Z to end): "<<endl; cin>>author; if (!cin) { break; } cout<<"Enter author's work (Ctrl+Z to end): "<<endl; while (cin>>work) { authors.insert(make_pair(author,work)); } cin.clear(); } while (cin); cin.clear(); // search cout<<"Who is the author that you want erase: "<<endl; cin>>searchItem; // ensure the range of multimap typedef multimap<string,string>::const_iterator itType; pair<itType,itType> pos=authors.equal_range(searchItem); if (pos.first!=pos.second) { authors.erase(pos.first,pos.second);// delete all the elements of this key } else { cout<<"can not find this author!"<<endl; } // print cout<<"author/t/twork"<<endl; for (itType iter=authors.begin();iter!=authors.end();++iter) { cout<<iter->first<<"/t/t"<<iter->second<<endl; } return 0; }

output: