Multidimentional Array allocation with new

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Hello,

I have a little issue with new and multidimentional allocation :

my code is as below :

C++

void reset_board(int **a,int num_row)
{

	a = new int*[num_row];
	for(int i = 0; i < num_row; i++)
		a[i] = new int[num_row];

	for (int i=0; i< num_row ; i++)
		for (int j=0; j< num_row ; j++)
			a[i][j] = i+j;
}

int main()
{
	int **a = new int *[];
	
	reset_board(a,4);

	for (int i=0; i< 4 ; i++)
		for (int j=0; j< 4 ; j++)
			cout << a[i][j]<< endl ;
	return 0;
}

the prob is : in the fucntion the matrix is well full and i could write its content on the screen but in the main when i try to show the content it s crashes, it's like the temporary var is not implicitly allocated to the matrix.

How could i resolve that ? Thank you

Posted 6-Mar-12 6:56am

Schehaider_Aymen

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lewax00 · Accepted Answer · 2012-03-06T07:25:00

Solution 1

Change the signature of reset_board to

C++

void reset_board(int **&a,int num_row)

You want a reference or pointer to 'a' if you're going to change 'a' itself, otherwise the local variable 'a' in reset_board is getting a new value, and the 'a' from main is not.

Posted 6-Mar-12 7:25am

lewax00

Updated 6-Mar-12 7:26am

v3

Comments

Schehaider_Aymen 6-Mar-12 14:47pm

Yup, that was the trick ^^.
Thank You Lewax00

Maximilien · Accepted Answer · 2012-03-06T07:25:00

Since you are programming in C++, couldn't you simply use vectors ?

It's safer, cleaner, and more modern.

something like that:

C++

void reset_board(std::vector < std::vector <int> >& a,int num_row)
{
    a.resize(num_row);
    for(int i = 0; i < num_row; i++)
    {
        a[i].resize(num_row);
    }

    for (int i=0; i< num_row ; i++)
    {
        for (int j=0; j< num_row ; j++)
        {
            a[i][j] = i+j;
        }
    }
}

int _tmain(int argc, _TCHAR* argv[])
{
    std::vector < std::vector <int> > a;

    reset_board(a,4);

    for (int i=0; i< 4 ; i++)
    {
        for (int j=0; j< 4 ; j++)
        {
            std::cout << a[i][j]<< " ";
        }

        std::cout << std::endl;
    }

}