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

The following are different ways to create and initialize a vector in C++ STL

1. Initializing by pushing values one by one :

CPP

// CPP 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 :

CPP

// CPP 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 :

CPP

// CPP 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 :

CPP

// CPP 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 :

CPP

// CPP 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 :

CPP

#include <iostream>
#include <vector>
using namespace std;

int main()
{
    vector<int> vect1(10);
    int value = 5;
    fill(vect1.begin(), vect1.end(), value);
    for (int x : vect1)
        cout << x << " ";
}

Output

5 5 5 5 5 5 5 5 5 5 

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

C++

// CPP program to initialize an array with consecutive
// numbers
#include <iostream>
#include <numeric>
using namespace std;

int main()
{
    int arr[5];
    iota(arr, arr + 5, 1);
    for (int i = 0; i < 5; i++) {
        cout << arr[i] << " ";
    }
    return 0;
}

Output

1 2 3 4 5 

Time complexity: O(N) // N is the size of vector. time complexity of insertion is O(1) but time complexity of traversal is O(N).

Auxiliary space: O(N). //because size of vector is not defined.

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