
February 8th, 2011, 10:51 AM
#1
Dynamic Matrix
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

February 8th, 2011, 10:55 AM
#2
Re: Dynamic Matrix
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>.

February 8th, 2011, 11:19 AM
#3
Re: Dynamic Matrix
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 applyloadrules all
(gdb) "

February 8th, 2011, 11:30 AM
#4
Re: Dynamic Matrix
1. You have:
Code:
for (int i=0;i<n;i++)
matrix[n]=new int [n]; // should be "i" not "n"
2. your delete code is not correct
Code:
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.

February 8th, 2011, 11:30 AM
#5
Re: Dynamic Matrix
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;
}
I have highlighted the lines which are wrong.

February 8th, 2011, 11:49 AM
#6
Re: Dynamic Matrix
Originally Posted by Falkotux
yes, but in my code: ...
Please take a look at this alternative to your code:
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

February 9th, 2011, 04:38 AM
#7
Re: Dynamic Matrix
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
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