Map

The STL has a class called "pair" that can hold 2 elements of different types. It had public data members "first" and "second" .

File: pair1.cpp

#include <iostream>
#include <string>
#include <utility> // Required for pair and make_pair

using namespace std ;

int main()
{
    // Declaring and initializing a pair
    pair<string, string> author("Scott", "Meyers") ;

    // Accessing elements
    cout << "Author's first name: " << author.first << endl;
    cout << "Author's last name: " << author.second << endl;

    // Modifying elements
    author.first = "Herbert" ;
    author.second = "Schildt"  ;

    cout << "Author's first name: " << author.first << endl;
    cout << "Author's last name: " << author.second << endl;

    // Using make_pair
    pair<string, int > student = make_pair( "John", 22 )  ;

    cout << "Name: " << student.first << endl;
    cout << "Age: " << student.second << endl;



    return 0;
}

The "map" class holds two values; a key and a value. The following program shows how it can be used. The map uses the pair class to hold the 2 elements.

File: map1.cpp

// C++ program to illustrate the begin and end iterator
#include <iostream>
#include <map>
#include <string>
using namespace std;

int main()
{
    // Create a map of strings to integers
    map<string, int> mapObject ;

    // Insert some values into the map
    mapObject["one"] = 1 ;
    mapObject["two"] = 2 ;
    mapObject["three"] = 3 ;

    // Get an iterator pointing to the first element in the
    // map
    map<string, int>::iterator it = mapObject.begin()  ;

    // Iterate through the map and print the elements
    while (  it != mapObject.end()  )
    {
        cout << "Key: " << it->first
            << ", Value: " << it->second << endl;
        ++it;
    }

    cout << "Range loop to access map objects." << endl ;
    //Range loop
    for( pair<string, int>  p1 :  mapObject )
     {
           cout << "Key: " << p1.first
            << ", Value: " << p1.second << endl;


     } //for

    return 0;
}

$ g++ map1.cpp ; ./a.exe
Key: one, Value: 1
Key: three, Value: 3
Key: two, Value: 2
Range loop to access map objects.
Key: one, Value: 1
Key: three, Value: 3
Key: two, Value: 2

The below program shows some operations on maps.

File: map2.cpp

#include <iostream>
#include <iterator>
#include <map>

using namespace std;

int main()
{

    // empty map container
    map<int, int> mapObject1 ;

    // insert elements in random order
    mapObject1.insert( pair<int, int>(1, 40) );
    mapObject1.insert( pair<int, int>(2, 30) );
    mapObject1.insert( pair<int, int>(3, 60) );
    mapObject1.insert( pair<int, int>(4, 20) );
    mapObject1.insert( pair<int, int>(5, 50) );
    mapObject1.insert( pair<int, int>(6, 50) );

    // another way of inserting a value in a map
    mapObject1[7] = 10;

    // printing map mapObject1
    map<int, int>::iterator itr;
    cout << "\nThe map mapObject1 is : \n";
    cout << "\tKEY\tELEMENT\n";
    for (itr = mapObject1.begin(); itr != mapObject1.end(); ++itr)
    {
        cout << '\t' << itr->first << '\t' << itr->second
            << '\n';
    }
    cout << endl;

    // assigning the elements from mapObject to mapObject2
    map<int, int> mapObject2(mapObject1.begin(), mapObject1.end());

    // print all elements of the map mapObject2
    cout << "\nThe map mapObject2 after"
        << " assign from mapObject is : \n";
    cout << "\tKEY\tELEMENT\n";
    for (itr = mapObject2.begin(); itr != mapObject2.end(); ++itr)
    {
        cout << '\t' << itr->first << '\t' << itr->second
            << '\n';
    }
    cout << endl;

    // remove all elements up to
    // element with key=3 in mapObject2
    cout << "\nmapObject2 after removal of"
            " elements less than key=3 : \n";
    cout << "\tKEY\tELEMENT\n";
    mapObject2.erase(mapObject2.begin(), mapObject2.find(3));
    for (itr = mapObject2.begin(); itr != mapObject2.end(); ++itr) {
        cout << '\t' << itr->first << '\t' << itr->second
            << '\n';
    }

    // remove all elements with key = 4
    int num;
    num = mapObject2.erase(4);
    cout << "\nmapObject2.erase(4) : ";
    cout << num << " removed \n";
    cout << "\tKEY\tELEMENT\n";
    for (itr = mapObject2.begin(); itr != mapObject2.end(); ++itr) {
        cout << '\t' << itr->first << '\t' << itr->second
            << '\n';
    }

    cout << endl;

    // lower bound and upper bound for map mapObject key = 5
    //lower_bound first key that is not less than 5

    //upper_bound first key that is greater than 5
    cout << "mapObject.lower_bound(5) : "
        << "\tKEY = ";
    cout << mapObject1.lower_bound(5)->first << '\t';
    cout << "\tELEMENT = " << mapObject1.lower_bound(5)->second
        << endl;
    cout << "mapObject.upper_bound(5) : "
        << "\tKEY = ";
    cout << mapObject1.upper_bound(5)->first << '\t';
    cout << "\tELEMENT = " << mapObject1.upper_bound(5)->second
        << endl;

    return 0;
}

Exercise

1)

File: map_ex1.cpp

#include <iostream>
#include <map>
#include <unordered_map>

using namespace std ;

void printFreq( string inputString )
{
    map<char,int> mapObject1 ;
    for( char ch1 :  inputString )
      {
        if ( ch1 == ' ' ) continue ;
        if ( mapObject1.count( ch1 ) > 0 )
           mapObject1[ch1] = mapObject1[ch1] + 1 ;
        else
           mapObject1[ch1] =  1 ;
      }
    for( pair<char,int> p1 : mapObject1 )
      cout << "( " << p1.first << " " << p1.second << " )" ;
    cout << endl ;

}


int main ()
{
    printFreq( "aabbcb" ) ;
    printFreq( "George Chuvalo" ) ;





  return 0;
}

File: map_ex1s.cpp