How to Resize a 2D Vector in C++?

In C++, resizing the 2D vector means increasing or decreasing the row and column size of 2D vector. In this article, we will learn different methods to resize a 2D vector in C++.

The most efficient way to resize the 2D vector is by using vector resize() function. Let’s take a look at a simple example:

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

int main() {
    vector<vector<int>> v = {{1, 2, 3},
                             {7, 8, 9},
                             {4, 5, 6}};

    // Change the column size
    v.resize(2);

    // Change the row size
    for (auto &i : v)
        i.resize(3);

    cout << "Rows: " << v.size() << endl;
    cout << "Cols: " << v[0].size();
    return 0;
}

Rows: 2
Cols: 3

Explanation: The column size (number of rows) can be changed using vector resize() on the 2D vector, but the row size (number of columns) has to be changed one by one for individual member vector.

C++ also provides other methods to resize the 2D vector. Some of them are as follows:

Resize Empty 2D Vector

If the 2D vector is empty, then the vector resize() function can directly resize the rows and columns simultaneously in a single line.

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

int main() {
    vector<vector<int>> v;

    // Resize the rows and cols of 2d vector
    v.resize(2, vector<int>(3));

    cout << "Rows: " << v.size() << endl;
    cout << "Cols: " << v[0].size();
    return 0;
}

Rows: 2
Cols: 3

This method cannot be used on the vectors that contains elements.

Using assign() Method

The vector assign() method works by assigning a newly created 2d vector of desired size to our current vector.

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

int main() {
    vector<vector<int>> v(4, vector<int>(5));

    // Resize the 2d vector
    v.assign(2, vector<int>(3));

    cout << "Rows: " << v.size() << endl;
  	cout << "Cols: " << v[0].size();
    return 0;
}

Rows: 2
Cols: 3

Vector assign() creates a new vector, removing all previously stored elements.