Dynamic Matrix

[Falko-tux]

Hello,
i've a question: how can I get the access to all the elements of a dynamic matrix?
I create a dynamic matrix in this way:
Cout<<"insert matrix dimensions";
cin>>n;
int ** matrix= new int * [n];
for (int i=0;i<n;i++)
matrix [n]= new int [n];

//but when I try to initialize the element in this way:
for (int i=0;i<n;i++)
for (int J=0;j<n;j++)
matrix [i]*[j]=0;

the program gives me an error, how could I insert and modify all the location of the dynamic matrix?

Thanks a lot for attention

[Lindley]

matrix[i][j] = 0;

Note, you'll need to clean up that memory. Consider defining your matrix as a std::vector<std::vector<int>> instead, or with a boost::multi_array<int, 2>.

[Falko-tux]

yes, but in my code:

#include <iostream>
using namespace std;

int main()
{
int n;
cout<<"Inserisci le dimensioni della matrice NxN\n";
cin>>n;
int **matrix = new int * [n];

for (int i=0;i<n;i++)
matrix[n]=new int [n];

for (int i=0;i<n;i++)
for(int j=0;j<n;j++)
------> matrix [i][j]=0;

for (int i=0;i<n;i++)
{
for(int j=0;j<n;j++)
cout<<matrix [i][j];
cout<<endl;
}
delete []matrix[n];
return 0;
}

the program returns:"Program received signal: “EXC_BAD_ACCESS”.
sharedlibrary apply-load-rules all
(gdb) "

[Philip Nicoletti]

1. You have:

for (int i=0;i<n;i++)
  matrix[n]=new int [n];  // should be "i" not "n"

2. your delete code is not correct

    for (int i=0; i<n; ++i)
    {
       delete [] matrix[i];
    }
    
    delete [] matrix;

3. There are more efficient ways to allocate the matrix. A search on codeguru
should yield a better method.

[Lindley]

#include <iostream>
using namespace std;

int main()
{
        int n;
        cout<<"Inserisci le dimensioni della matrice NxN\n";
        cin>>n;
        int **matrix = new int * [n];
	
        for (int i=0;i<n;i++)
                matrix[n]=new int [n];
        
        for (int i=0;i<n;i++)
                for(int j=0;j<n;j++)
        ------>	matrix [i][j]=0;
        
        for (int i=0;i<n;i++)
        {
                for(int j=0;j<n;j++)
 	                cout<<matrix [i][j];
                cout<<endl;
        }
        delete []matrix[n];
        return 0;
}

I have highlighted the lines which are wrong.

[Paul McKenzie]

yes, but in my code: ...

Please take a look at this alternative to your code:

int main()
{
   int n;
   cout<<"Inserisci le dimensioni della matrice NxN\n";
   cin>>n;

   // Create row pointers
   int **matrix = new int * [n];

   // Create the data for the matrix in one call.
   int *matrix_data = new int[n*n];

   // Create n*n matrix
   for (int i=0; i < n; i++, matrix_data+=n)
      matrix[i] = matrix_data;  // point row pointers to matrix data

   // delete n*n matrix
    delete []matrix[0];  // delete the beginning of the matrix data
    delete [] matrix;     // delete the row pointers.
}

Compare this with your code. Hopefully you see the major difference in how the matrix is created.

In your version, you are calling new[]/delete[] n+1 times, while in the code above, there are only 2 calls, regardless of the size of the matrix. If n were 1000, you would be calling the allocator 1001 times, while the version above only calls it 2 times.

Basically this method:

1) saves time (since the allocator is only being called twice for new[] and delete[]),
2) Memory fragmentation is minimized
3) The resulting matrix has the same layout as a regular 2 dimensional array, in other words, the data is contiguous.

Regards,

Paul McKenzie

[Falko-tux]

It works! Thank you, and thanks for the new way of creating dynamic matrix (more efficient). This is the best programming forum i've ever seen. Thank you again