Examples of Creating Map of Maps in C++ or Multilevel Maps in C++

To create map of map in C++ is very simple.
Map is a sorted associative array of unique keys and associated data. The elements of the map are internally sorted from lowest to highest key value.

Example 1:
//Program to create multi level map in C++
#include <iostream>
#include <map> // header file needed for to use MAP STL

using namespace std;

int main(void)
{
//Define multilevel map. map within a map.
map<int, map<int,char> > myMap;
myMap[1][1] = 'A';
myMap[1][2] = 'B';
myMap[2][1] = 'C';
myMap[2][2] = 'D';

cout << "[1][1] = " << myMap[1][1] << endl;
cout << "[1][2] = " << myMap[1][2] << endl;
cout << "[2][1] = " << myMap[2][1] << endl;
cout << "[2][2] = " << myMap[2][2] << endl;

return 0;
}

//Save a file as multimap.cpp

To Compile and execute it on Ubuntu use following commands:
$ g++ multimap.cpp -o multimap
$ ./multimap

[1][1] = A
[1][2] = B
[2][1] = C
[2][2] = D


Example 2: 

#include <iostream>
#include <string>
#include <map>

using namespace std;

typedef map<int, string> innerMap;
typedef map<double, innerMap> mainMap;

mainMap m;


void print()
{
  map<double, innerMap >::iterator it;
  map<int, string>::iterator inner_it;

  for ( it=m.begin() ; it != m.end(); it++ ) {
    cout << "\n\nNew element\n" << (*it).first << endl;
    for( inner_it=(*it).second.begin(); inner_it != (*it).second.end(); inner_it++)
      cout << (*inner_it).first << " => " << (*inner_it).second << endl;
  }
}

int main (int argc, char *argv[])
{

  // First insert an element into outermap
  m.insert (make_pair (2.4, innerMap ()));
  // Now insert elements into innermap
  m[2.4].insert (make_pair (2, "two"));
  m[2.4].insert (make_pair (5, "five"));

  // First insert an element into outermap
  m.insert (make_pair (0.6, innerMap ()));
  // Now insert elements into innermap
  m[0.6].insert (make_pair (5, "five"));
  m[0.6].insert (make_pair (1, "one"));
  m[0.6].insert (make_pair (2, "two"));

  // print elements in the mainMap
  print();
    
}

Output of above program (Example 2):

New element

0.6

1 => one

2 => two

5 => five

New element

2.4

2 => two

5 => five

Example 3: Uses "multimap" STL
Note that, "mutimap" STL does not support [], therefore we cannot use syntax "m[].insert( make_pair())"  we used in Example 2 to insert element.


#include <iostream>
#include <string>
#include <map>

using namespace std;

typedef multimap<int, string> innerMap;
multimap<double, innerMap> mainMap;

void print()
{
  multimap<double, innerMap >::iterator it;
  multimap<int, string>::iterator inner_it;

  for ( it=mainMap.begin() ; it != mainMap.end(); it++ ) {
    cout << "\n\nNew element\n" << (*it).first << endl;
    for( inner_it=(*it).second.begin(); inner_it != (*it).second.end(); inner_it++)
      cout << (*inner_it).first << " => " << (*inner_it).second << endl;
  }
}

void insert1(multimap<double, innerMap >::iterator it)
{
  it->second.insert (make_pair (2, "two"));
  it->second.insert (make_pair (5, "five"));
  it->second.insert (make_pair (2, "two"));

}

int main (int argc, char *argv[])
{
  multimap<double, innerMap >::iterator it;

  // First insert an element into outermap
  it = mainMap.insert (make_pair (2.4, innerMap ()));
  // Now insert elements into innermap
  insert1(it);

  // First insert an element into outermap
  it = mainMap.insert (make_pair (0.6, innerMap ()));
  // Now insert elements into innermap
  it->second.insert (make_pair (5, "five"));
  it->second.insert (make_pair (1, "one"));
  it->second.insert (make_pair (2, "two"));
  it->second.insert (make_pair (5, "five"));
  it->second.insert (make_pair (5, "five"));

  // print elements in the mainMap
  print();  
}




Output of the above program (Example 3):

New element
0.6
1 => one
2 => two
5 => five
5 => five
5 => five


New element
2.4
2 => two
2 => two
5 => five

Example 4:
how to find out if a particular key exists in the map or not?



#include <iostream>
#include <string>
#include <map>


using namespace std;


typedef multimap<int, string> innerMap;
multimap<string, innerMap> mainMap;


void insert1(multimap<string, innerMap >::iterator it)
{
  it->second.insert (make_pair (2, "two"));
  it->second.insert (make_pair (5, "five"));
  it->second.insert (make_pair (2, "two"));


}


int main (int argc, char *argv[])
{
  multimap<string, innerMap >::iterator it;


  // First insert an element into outermap
  it = mainMap.insert (make_pair ("1st Element Key", innerMap ()));
  // Now insert elements into innermap
  insert1(it);


  // First insert an element into outermap
  it = mainMap.insert (make_pair ("2nd element Key", innerMap ()));
  // Now insert elements into innermap
  it->second.insert (make_pair (5, "five"));
  it->second.insert (make_pair (1, "one"));
  it->second.insert (make_pair (2, "two"));
  it->second.insert (make_pair (5, "five"));
  it->second.insert (make_pair (5, "five"));


  // Find the key in the mainMap
  it = mainMap.find("2nd element Key");
  if (it != mainMap.end()) {
    cout << "\n Key Found!" << endl;
  } else {
    cout << "\n Key Not Found!" << endl;
  }
}

Output of the above program (Example 4):


 Key Found!