Best way to represent a 2-D array in C++ with size determined at run time

Question

In C++ I'd like to do something like:

int n = get_int_from_user();

char* matrix = new char[n][n];

matrix[0][0] = 'c';
//...
matrix[n][n] = 'a';

delete [][] matrix;

but of course this doesn't work. What is the best way to do something similar? I've seen some solutions to this but they seem pretty messy.

Answer 1

The manual dynamic way:

Let's say you want an array of width*height, the most efficient way is to just use a single dimensional array:

char *matrix = new char[width*height];

To delete it:

delete[] matrix;

To access it:

char getArrayValue(char *matrix, int row, int col)
{
  return matrix[row + col*width];
}

To modify it:

void setArrayValue(char *matrix, int row, int col, char val)
{
  matrix[row + col*width] = val;
}

---

Boost Matrix:

Consider using boost::matrix if you can have the dependency.

You could then tie into the boost linear algebra libraries.

Here is some sample code of boost::matrix:

#include <boost/numeric/ublas/matrix.hpp>
using namespace boost::numeric::ublas;
matrix<char> m (3, 3);
for (unsigned i = 0; i < m.size1 (); ++ i)
    for (unsigned j = 0; j < m.size2 (); ++ j)
        m (i, j) = 3 * i + j;

---

On the stack for some compilers:

Some compilers actually allow you to create arrays on the stack with runtime determined sizes. g++ is an example of such a compiler. You cannot do this by default VC++ though.

So in g++ this is valid code:

int width = 10;
int height = 10; 
int matrix[width][height];

Drew Hall mentioned that this C99 feature is called Variable Length Arrays (VLAs) and it can probably be turned on in any modern compiler.