Map of list and forward_list in C++ STL with Examples

Maps are associative containers that store elements in a mapped fashion. Each element has a key value and a mapped value. No two mapped values can have the same key values.

Lists are sequence containers that allow non-contiguous memory allocation. As compared to vector, the list has slow traversal, but once a position has been found, insertion and deletion are quick. A list simply means a doubly-linked list and for a singly linked list forward List is used. 

Map of List in STL

A map of lists can be very useful in designing complex data structures.

Syntax:

map<datatype, list<datatype>> map_of_list 
This stores a list corresponding to a datatype

or

map<list<datatype>, datatype> map_of_list
This stores a datatype corresponding to a list

Below is the implementation of the Map of List in C++-

C++

// C++ program to implement
// the above approach
#include <bits/stdc++.h>
using namespace std;

void printMapContent1(map<list<int>,
                          int>& mapOfList)
{
    cout << "   Key         Value"
         << "\n\n";
    for (auto p : mapOfList) {
        // Key is a list of integers
        list<int> ourList = p.first;

        // Value is an integer
        int val = p.second;

        // Printing list elements
        cout << "[ ";
        for (auto it = ourList.begin();
             it != ourList.end(); it++) {
            // Dereferencing value pointed by
            // iterator
            cout << (*it) << ' ';
        }

        cout << ']';

        cout << "     ";

        // Printing value
        cout << mapOfList[ourList] << '\n';
    }
}

void printMapContent2(map<int,
                          list<int> >& mapOfList)
{

    cout << "   Key         Value"
         << "\n\n";
    for (auto p : mapOfList) {
        // Key is an integer
        int key = p.first;

        // Value is a list of integers
        list<int> ourList = p.second;

        cout << "   ";
        cout << key << "          ";

        // Printing list elements
        cout << "[ ";
        for (auto it = ourList.begin();
             it != ourList.end(); it++) {
            // Dereferencing value pointed by
            // iterator
            cout << (*it) << ' ';
        }

        cout << ']';
        cout << '\n';
    }
}

// Driver code
int main()
{
    // Declaring a list of integers
    list<int> ourList1;

    // Inserting elements at the
    // back of list
    ourList1.push_back(2);
    ourList1.push_back(10);
    ourList1.push_back(13);

    // Declaring another list
    list<int> ourList2;

    // Inserting elements at the back
    // of list
    ourList2.push_back(7);
    ourList2.push_back(14);
    ourList2.push_back(22);

    // Declaring a map where key is a list
    // and value is integer itself
    map<list<int>, int> mapOfList1;

    mapOfList1[ourList1] = 5;
    mapOfList1[ourList2] = 10;

    // Printing the contents of the map
    printMapContent1(mapOfList1);

    // Declaring a map where key is integer
    // and value is a list of integers
    map<int, list<int> > mapOfList2;

    cout << "\n\n";
    mapOfList2[3] = ourList1;
    mapOfList2[7] = ourList2;

    printMapContent2(mapOfList2);

    return 0;
}

Output

   Key         Value

[ 2 10 13 ]     5
[ 7 14 22 ]     10


   Key         Value

   3          [ 2 10 13 ]
   7          [ 7 14 22 ]

Forward List in STL

Forward list in STL implements singly linked list. Introduced from C++11, forward lists are more useful than other containers in insertion, removal, and moving operations (like sort) and allow time constant insertion and removal of elements. forward_list can also be used with the map container.

Syntax:

map<datatype, forward_list<datatype>> map_of_list
This stores a forward list corresponding to a datatype

or

map<forward_list<datatype>, datatype> map_of_list
This stores a datatype corresponding to a forward list

Below is the implementation of forward_list in C++- 

C++

// C++ program to implement
// the above approach
#include <bits/stdc++.h>
using namespace std;

void printMapContent1(map<forward_list<int>,
                          int>& mapOfList)
{
    cout << "   Key         Value"
         << "\n\n";
    for (auto p : mapOfList) {
        // Key is a list of integers
        forward_list<int> ourList = p.first;

        // Value is an integer
        int val = p.second;

        // Printing list elements
        cout << "[ ";
        for (auto it = ourList.begin();
             it != ourList.end(); it++) {
            // Dereferencing value pointed by
            // iterator
            cout << (*it) << ' ';
        }

        cout << ']';
        cout << "     ";

        // Printing value
        cout << mapOfList[ourList] << '\n';
    }
}

void printMapContent2(map<int,
                          forward_list<int> >& mapOfList)
{

    cout << "   Key         Value"
         << "\n\n";
    for (auto p : mapOfList) {
        // Key is an integer
        int key = p.first;

        // Value is a list of integers
        forward_list<int> ourList = p.second;

        cout << "   ";
        cout << key << "          ";

        // Printing list elements
        cout << "[ ";
        for (auto it = ourList.begin();
             it != ourList.end(); it++) {
            // Dereferencing value pointed by
            // iterator
            cout << (*it) << ' ';
        }

        cout << ']';
        cout << "\n";
    }
}

// Driver code
int main()
{
    // Declaring forward list
    forward_list<int> forwardList1;

    // Declaring another forward list
    forward_list<int> forwardList2;

    // Assigning values using assign()
    forwardList1.assign({ 5, 3, 13 });
    forwardList2.assign({ 8, 9, 13 });

    map<forward_list<int>,
        int>
        mapOfList1;

    mapOfList1[forwardList1] = 3;
    mapOfList1[forwardList2] = 7;

    printMapContent1(mapOfList1);
    cout << "\n\n";

    map<int,
        forward_list<int> >
        mapOfList2;

    mapOfList2[3] = forwardList1;
    mapOfList2[7] = forwardList2;

    printMapContent2(mapOfList2);
    return 0;
}

Output

   Key         Value

[ 5 3 13 ]     3
[ 8 9 13 ]     7


   Key         Value

   3          [ 5 3 13 ]
   7          [ 8 9 13 ]