Different Ways to Initialize a Map in C++

Initializing map refers to the process of assigning the initial values to the elements of map container. In this article, we will learn different methods to initialize the map in C++. Let's start from the easiest method:

Using Initializer List

The simplest way to initialize an std::map container is by passing the key-value pairs inside an initializer list to its constructor.

Syntax

map<ktype, vtype> m = { {k1, v1}, {k2, v2}, …}

where, k1, k2,… are the keys and v1, v2,… are the associated values.

#include <bits/stdc++.h>
using namespace std;

int main() {

    // Inializing std::map using initializer list
    map<int, char> m = {{1, 'a'},
                        {3, 'b'},
                        {2, 'c'}};

    for (auto i : m)
        cout << i.first << ": " << i.second << endl;
    return 0;
}

1: a
2: c
3: b

Explanation: The key-value pairs specified in the initializer list are used as initial value of map elements. These elements are then organized on the basis of keys.

Apart from the above method, C++ also provides other different methods to initialize a map depending on the situation. Some of them are:

Using One by One Initialization

Map can be initialized by initializing all elements one by one using [] operator or by using the map insert() method.

#include <bits/stdc++.h>
using namespace std;

int main() {
    map<int, char> m;

    // Adding key-value pairs using [] or map::insert()
    m[1] = 'a';
    m[3] = 'b';
    m.insert({2, 'c'});

    for (auto i : m)
        cout << i.first << ": " << i.second << endl;
    return 0;
}

1: a
2: c
3: b

This method is generally used to initialize map after its declaration.

From Another Map

Maps can be initialized from an already existing map of the same type by copying all elements with the help of copy constructor.

Syntax

map<ktype, vtype> m2(m1);

where, m1 and m2 are the two map containers.

#include <bits/stdc++.h>
using namespace std;

int main() {
    map<int, char> m1 = {{1, 'a'},
                         {3, 'b'},
                         {2, 'c'}};

    //Create and initialize the map m2 with map m1
    map<int, char> m2(m1);

    for (auto i : m2)
        cout << i.first << ": " << i.second << endl;
    return 0;
}

1: a
2: c
3: b

We can also move the map using std::move() function if we don’t need the first map later in the program.

From Any STL Container or Array of Pairs

The map range constructor can initialize a map from any other STL container or an array of pairs.

Syntax

map<int, char> m(first, last);

where, first and last are the iterator to the first element and the element just after the last element of the range.

#include <bits/stdc++.h>
using namespace std;

int main() {
    vector<pair<int, char>> v = {{1, 'a'},
                                 {3, 'b'}, 
                                 {2, 'c'}};

    // Create and initialize the map with vector
    map<int, char> m(v.begin(), v.end());

    for (auto i : m)
        cout << i.first << ": " << i.second << endl;
    return 0;
}

1: a
2: c
3: b

Note: The other container or array of pairs must be of the same type as ours map.