2D arrays (dynamcally alocated, with clases, operator overload)

[catalin14]

Hi there. I'm strugling to learn C++, and some concepts seem to rub a bit harder on me than others.

At the moment I'm making a program which adds 2 dynamically created 2D arrays, and I'm taking it step by step.

 #include <iostream>
#include <new>
using namespace std;

class matrix {
public:	int **matrice;
	int n, i, j, rand, col;

	matrix(int, int);	
};

matrix::matrix(int rand, int col) {
	matrice = new int*[rand];
	for (i = 0; i < rand; i++)
		matrice[rand] = new int[col]; 

	for (i = 0; i < rand; i++)
		for (j = 0; j < col; j++)
		{
			cout << "randul " << i + 1 << " coloana " << j + 1 << " = ";
			cin >> *(*(matrice + i) + j);
		}
}

int main()
{ 
	matrix a(2, 2);
        return 0;
}

When I run it, I get some exceptions after I read the first value of the array. An "Access violation writing location 0xCDCDCDCD". And I have no ideea why...

[Kevin C]

Use std::vector to create dynamic arrays.

It's simple:

Define a new empty matrix:

vector< vector<int> > matrix;

Make a matrix of given size:

matrix = vector< vector<int> >(rows,vector<int>(columns));

[catalin14]

Would it still be using dynamic alocated memory?

[Kevin C]

Yes, std::vector uses dynamically allocated memory. It will automatically 'resize' it if you change the size of a vector.

[catalin14]

Thank you, I will try this method also.

And I also found the mistake in my code... a silly one.

for (i = 0; i < rand; i++)
		matrice[i] = new int[col]; 

instead of

for (i = 0; i < rand; i++)
		matrice[rand] = new int[col]; 

I'll contiune my program and post my progress.

[catalin14]

I just added a simple function to my class to print the arrays I read. But it seems that the arrays disappear after the constructor done its job.
This is the function:

void matrix::print_m() {
	for (i = 0; i < rand; i++)
	{
		for (j = 0; j < col; j++)
			cout << matrice[i][j] << ' ';
		cout << endl;

And if I do this:

matrix a(2,2);
a.print_m();

after i fill the array, nothing happens; it doesnt get printed.Any ideeas why?

[catalin14]

Managed to fix that too. It was because rand and col were not actually initialized anywhere. Onward to the operator overloading now!!!

[catalin14]

Alas, my program is done. It has a minor inconvenience but it works.

#include <iostream>
#include <new>
using namespace std;

class matrix {
public:	int **matrice;
	int  i, j, rand, col;

	matrix();
	matrix(int, int);
	void print_m();
	matrix  operator + (const matrix&);	
};

matrix::matrix() {
	cout << "how many rand/cols : "; cin >> rand >> col;
	matrice = new int*[rand];
	for (i = 0; i < rand; i++)
		matrice[i] = new int[col];
}

matrix::matrix(int rand, int col) {
	this->rand=rand; this->col=col;
	matrice = new int*[rand];
	for (i = 0; i < rand; i++)
		matrice[i] = new int[col]; 

	for (i = 0; i < rand; i++)
	{
		for (j = 0; j < col; j++)
		{
			cout << "randul " << i + 1 << " coloana " << j + 1 << " = ";
			cin >> *(*(matrice + i) + j);
		}
		cout << endl;
	}
}

matrix matrix::operator + (const matrix& other)
{
	matrix result;
	for (i = 0; i < rand; i++)
		for (j = 0; j < col;j++)
		result.matrice[i][j] = matrice[i][j] + other.matrice[i][j];
		return result;
}

void matrix::print_m() {
	for (i = 0; i < rand; i++)
	{
		for (j = 0; j < col; j++)
			cout << matrice[i][j] << ' ';
		cout << endl;
	}
}

int main()
{
	matrix a(3,3),b(3,3),c;
	c = a + b;
	c.print_m();
}

The inconvenience is that the default constructor

matrix::matrix() {
	cout << "how many rand/cols : "; cin >> rand >> col;
	matrice = new int*[rand];
	for (i = 0; i < rand; i++)
		matrice[i] = new int[col];
}

is ran twice and you must write rand and col twice. But it does what its supposed to do.