Different Ways to Initialize an Set in C++

Initializing a set means assigning some initial values to the elements of the set container. In this article, we will learn different methods to initialize an std::set in C++.

Using Initializer List

The most common method to initialize a std::set container is by using initializer list at the time of its declaration. An initializer list is a list of values enclosed inside braces {} and separated by a comma. As set is ordered container, it will arrange the elements of the initializer list according to the specified order.

Syntax

set<type> s = {v1, v2, v3, ...}

where v1, v2, v3, ... are values inside for initialization.

Example

// C++ Program to initialize std::set using 
// intializer list
#include <bits/stdc++.h>
using namespace std;

int main() {

    // Initialize a set
    set<int> s = {11, 9, 7, 45};

    for (auto i : s)
        cout << i << " ";
    return 0;
}

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If we want to initialize the set using initializer list after its declaration, we can pass this list to set::insert() method.

One by One Initialization

We can also initialize the set by inserting elements one by one using the set::insert() method. This method allows the insertion of the given value and maintains the order of the set.

Example

// C++ Program to initialize std::set by
// inserting elements one by one
#include <bits/stdc++.h>
using namespace std;

int main() {
    set<int> s;

    // Iniitalize set by inserting elements
  	// one by one
    s.insert(11);
    s.insert(9);
    s.insert(7);
    s.insert(45);

    for (auto i : s)
        cout << i << " ";
    return 0;
}

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From Another std::set

In C++, we can initialize a set by copying all elements from an already existing set with the help of copy constructor.

Syntax

set<int> s2(s1);

where s1 and s2 are the two set containers.

Example

// C++ Program to initialize std::set
// from another std::set
#include <bits/stdc++.h>
using namespace std;

int main() {
    set<int> s1 = {11, 9, 7, 45};

    // Initialize a set using another set
    set<int> s2 = (s1);

    for (auto i : s2)
        cout << i << " ";
    return 0;
}

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From Another STL Container or Array

We can use the range constructor of set to initialize it from any STL container or even an array.

Syntax

set<int> s(first, last);

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

Example

// C++ program to initialize a set
// from an array
#include <bits/stdc++.h>
using namespace std;

int main() {
    int arr[]= {11, 9, 7, 45};
  	int n = sizeof(arr)/sizeof(arr[0]);

    // Initializing a set from array
    set<int> s(arr, arr + n);

    for (int i : s)
        cout << i << " ";
    return 0;
}

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