Vector resize() in C++ STL

In C++, the vector resize() is a built-in method used to change the size of vector container after it is declared. It can be used to increase or decrease the size of vector.

Let’s take a look at an example that illustrates the vector resize() method:

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

int main() {
    
    // Creating vector of size 5
    vector<int> v(5);

    // Vetor resized to 9
    v.resize(9);

    cout << v.size();
    return 0;
}

9

In the above example, we have first created a vector of size 5. But in the next statement, we changed the size of the vector to 9 elements using vector resize() function. This can be verified by checking the final size of the vector.

Syntax of Vector resize()

The vector resize() is the member method of std::vector defined inside <vector> header file.

v.resize(n, val);

Parameters:

• n: New size of vector. It can be both greater or smaller than the current size but can only be positive.
• val: Value by which extra elements of vector will be initialized. It is optional and by default is set to 0.

Return Value:

• This function does not return any value.

Examples of vector resize()

The below examples demonstrate the use of vector resize() function in different cases:

Decrease the Size of Vector

Resizing the vector to a smaller size than its current one will truncate elements from the end.

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

int main() {
    vector<int> v = {1, 2, 3, 4, 5};

    // Vector resized to 3
    v.resize(3);

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

1 2 3 

Explanation: Initially, the size of the vector is 5, as it is initialized with 5 elements. After that, we resize the vector to 3, reducing its size and removing the last two elements.

Increase Size and Initialize New Elements

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

int main() {
    vector<int> v = {1, 4, 6};

    // Vector resized to 7 and added 
    // elements initialized to 9
    v.resize(7, 9);

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

1 4 6 9 9 9 9 

Explanation: Initially the size of vector is 3, after that we resize the vector to 7 and initialized the added 4 elements with value 9.