How do you dynamically allocate a 2D matrix in C++? I have tried based on what I already know:

```
#include <iostream>
int main(){
int rows;
int cols;
int * arr;
arr = new int[rows][cols];
}
```

It works for one parameter, but now for two. What should I do?

A matrix is actually an array of arrays.

```
int rows = ..., cols = ...;
int** matrix = new int*[rows];
for (int i = 0; i < rows; ++i)
matrix[i] = new int[cols];
```

Of course, to delete the matrix, you should do the following:

```
for (int i = 0; i < rows; ++i)
delete [] matrix[i];
delete [] matrix;
```

I have just figured out another possibility:

```
int rows = ..., cols = ...;
int** matrix = new int*[rows];
if (rows)
{
matrix[0] = new int[rows * cols];
for (int i = 1; i < rows; ++i)
matrix[i] = matrix[0] + i * cols;
}
```

Freeing this array is easier:

```
if (rows) delete [] matrix[0];
delete [] matrix;
```

This solution has the advantage of allocating a single big block of memory for all the elements, instead of several little chunks. The first solution I posted is a better example of the *arrays of arrays* concept, though.

You can also use `std::vectors`

for achieving this:

using `std::vector< std::vector<int> >`

Example:

```
std::vector< std::vector<int> > a;
//m * n is the size of the matrix
int m = 2, n = 4;
//Grow rows by m
a.resize(m);
for(int i = 0 ; i < m ; ++i)
{
//Grow Columns by n
a[i].resize(n);
}
//Now you have matrix m*n with default values
//you can use the Matrix, now
a[1][0]=1;
a[1][1]=2;
a[1][2]=3;
a[1][3]=4;
//OR
for(i = 0 ; i < m ; ++i)
{
for(int j = 0 ; j < n ; ++j)
{ //modify matrix
int x = a[i][j];
}
}
```

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