Dynamic allocation of matrix?

[andmej]

Hello,

I know I can allocate an array dynamically like this:

Code: Select all

#include <iostream>
int * array;

int main(){
  int n;
  cin >> n;
  
  array = new int[n];
  //~ Use the array here...
  delete [] array;
  return 0;
}

Do you know how can I allocate a matrix of size n*n in a similar fashion? That is, I don't know how big the matrix will be until runtime and need to read the size.

I'd appreciate your help. Thanks in advance.

[mf]

You can allocate an array of size n*n, and do extra arithmetic (which you can encapsulate in macros or classes) to access its elements:

Code: Select all

int *matrix = new int[n*n];
#define MATRIX(i, j) matrix[(i)*n + (j)]

MATRIX(2, 3) = 42;
cout << MATRIX(2, 3) << endl;

delete[] matrix;

Or allocate an array of int*, and initialize each member with a pointer to another allocated array:

Code: Select all

int *matrix = new int*[n];
for (int i = 0; i < n; i++) matrix[i] = new int[n];
...
matrix[2][3] = 42;
...

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

And with STL this solution is even shorter:

Code: Select all

vector<vector<int> > matrix(n, vector<int>(n, 0));
matrix[2][3] = 42;
cout << matrix[2][3] << endl;
...

[andmej]

Thanks mf for your good answer

Is the STL solution slower than the other solutions?

[mf]

Yes, unfortunately it's quite a bit slower, maybe about 1.5-3 times slower than other codes, but that heavily depends on the compiler and optimization flags.

[Megov]

By the way there is a mistake in second example of code : instead of "int *matrix" you need to write "int **matrix"

[shiggy]

You could write a function like:

Code: Select all

int** create_2d_array(int rows, int cols)
{
	int** array = new int*[rows];

	for (int i = 0; i < rows; i++)
		array[i] = new int[cols];

	return array;
}

//...

int** array = create_2d_array(5,5);
array[0][0] = 1;
array[3][4] = 1;
//etc

Replacing int with whatever type you need