allocate v[][] dynamically?

Hey.
To dynamically allocate memory for a 2d array(x[][]), youd use this:



[edited by - erjo on December 11, 2002 12:03:06 PM]

A couple of things here, firstly which language are you using? C or C++?

For C use malloc, (I''ve forgotten the implementation (haven''t used C for years!)

For C++:

float* v = new float[100*100];

will give you the memory, but don''t forget to free it!

delete[] v;

will accomplish this when you''ve finished with it. Don''t get it confused with:

delete v;

which won''t delete the array properly.

I guess you want to do it this way so that you have the convenience of the [4][9] subscript notation.

Probably a safer, cleaner way to do that is to make a class like this:

template<class Type>
class Matrix {
public:
    Matrix(int rows, int columns);
    Type& operator()(int row, int column);
private:
    favouriteContainer<Type> data_;
};  

then you can access it like this

Matrix<double> board(8, 8);
board(3, 7) = 16.3;

The container member data_ can hold rows*columns amount of data like MrScruff said
The operator() can translate the row and column into an index into your data.
Ask if you need anything claifying.

Peace

[edited by - petewood on December 11, 2002 12:32:48 PM]

thank guys, i appreciate the quick replies and i''ll see what i can do.

thanks again,
Scott


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Probably the quickest way to do it is this:

template<class T>
class matrix : public std::vector<std::vector< T > >
{
public:
	matrix(){}
	matrix(size_t x,size_t y)
	{
		resize(x);
		for(size_t i = 0; i < x; ++i)
			(*this)[i].resize(y);
	}
};
  

[edited by - daerid on December 11, 2002 1:46:23 PM]

Hey,
In our AP comp sci class we use apmatrix and apvector. I have included links to the source, and you might be able to get an idea of what is going on by looking through the code. Our matrix class is a vector of vectors, which is what daerid has shown. I personally have just used a line of memory and accessed it by pointer arithmitic, seems a bit easier with fewer dependant classes. Here are the links:

http://cs.columbusacademy.org/reference/code/apvectorh.html
http://cs.columbusacademy.org/reference/code/apvector.html
http://cs.columbusacademy.org/reference/code/apmatrixh.html
http://cs.columbusacademy.org/reference/code/apmatrix.html

the first two are the apvector code, with the .h and .cpp respectively, the second is apmatrix with the .h and .cpp respectively.


Brendan