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; } 

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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; 

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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.