Initialize a vector in C++ (7 different ways)

The following are different ways to construct or initialize a vector in C++ STL

1. Initializing by pushing values one by one:

C++

// C++ program to create an empty
// vector and push values one
// by one.
#include <iostream>
#include <vector>
using namespace std;

int main()
{
    // Create an empty vector
    vector<int> vect;

    vect.push_back(10);
    vect.push_back(20);
    vect.push_back(30);

    for (int x : vect)
        cout << x << " ";

    return 0;
}

Output

10 20 30 

2. Specifying size and initializing all values:

C++

// C++ program to create an empty
// vector and push values one
// by one.
#include <iostream>
#include <vector>
using namespace std;

int main()
{
    int n = 3;

    // Create a vector of size n with
    // all values as 10.
    vector<int> vect(n, 10);

    for (int x : vect)
        cout << x << " ";

    return 0;
}

Output

10 10 10 

3. Initializing like arrays:

C++

// C++ program to initialize
// a vector like an array.
#include <iostream>
#include <vector>
using namespace std;

int main()
{
    vector<int> vect{ 10, 20, 30 };

    for (int x : vect)
        cout << x << " ";

    return 0;
}

Output

10 20 30 

4. Initializing from an array:

C++

// C++ program to initialize
// a vector from an array.
#include <iostream>
#include <vector>
using namespace std;

int main()
{
    int arr[] = { 10, 20, 30 };
    int n = sizeof(arr) / sizeof(arr[0]);

    vector<int> vect(arr, arr + n);

    for (int x : vect)
        cout << x << " ";

    return 0;
}

Output

10 20 30 

5. Initializing from another vector:

C++

// C++ program to initialize a vector from
// another vector.
#include <iostream>
#include <vector>
using namespace std;

int main()
{
    vector<int> vect1{ 10, 20, 30 };

    vector<int> vect2(vect1.begin(), vect1.end());

    for (int x : vect2)
        cout << x << " ";

    return 0;
}

Output

10 20 30 

6. Initializing all elements with a particular value:

C++

// C++ Program to initialize vector using fill()
#include <iostream>
#include <vector>
using namespace std;

int main()
{
    // creating array with size 10
    vector<int> vect1(10);

    // initializing using fill() function
    int value = 5;
    fill(vect1.begin(), vect1.end(), value);

    // printing vector
    for (int x : vect1)
        cout << x << " ";

    return 0;
}

Output

5 5 5 5 5 5 5 5 5 5 

7. Initialize an array with consecutive numbers using std::iota:

C++

// C++ program to initialize a
// vector with consecutive
// numbers
#include <iostream>
#include <numeric>
#include <vector>
using namespace std;

int main()
{
    // declaring a vector with size 5
    vector<int> vec(5);

    // initializing using iota()
    iota(vec.begin(), vec.end(), 1);

    // printing the vector
    for (int i = 0; i < 5; i++) {
        cout << vec[i] << " ";
    }
    return 0;
}

Output

1 2 3 4 5 

Time complexity: O(N), where N is the size of the vector.

Auxiliary space: O(N).

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