Hi all,

I am just learning DMA, and am supposed to write 2 functions utilizing the offset technique, such that a 2-D array starts and ends the index of each dimension as the user specifies.

matrix(): creates a 2D array of float type numbers, indexing from -n to n for each dimension. A proper pointer is to be returned so that after invocation of this function, a 2D array of float type numbers and the specified starting and ending indices can be used.

free_matrix(): releases the memory allocated for the 2D array created by matrix().

I am using C, and use a UNIX-based compiler to run my program. Below is what I have:

```
#include <stdio.h>
#include <stdlib.h>
float** matrix(float **x, int n);
void free_matrix(float **x, int n);
int main(void)
{
float **x;
int i, j, n;
scanf("%d", &n);
x = matrix(x, n);
for (i = -n ; i <= n ; i++)
{
for (j = -n ; j <= n ; j++)
x[i][j] = i + j;
}
for (i = -n ; i <= n ; i++)
{
for (j = -n ; j <= n ; j++)
printf("%f ", x[i][j]);
printf("\n");
}
free_matrix(x, n);
return 0;
}
float** matrix(float **x, int n)
{
int i;
x = (float**) malloc(2 * n * sizeof(float*));
for (i = 0 ; i < n ; i++)
{
x[i] = (float*) malloc(2 * n * sizeof(float));
x[i] -= n;
}
x -= n;
return x;
}
void free_matrix(float **x, int n)
{
int i;
x += n;
for (i = 0 ; i < n ; i++)
free(x[i] + n);
free(x);
}
```

I keep getting the error "Segmentation fault". What is wrong with my code?

All those in blue were given in the question, so I can't change any of that. I followed the code to create the array closely with that given in my notes, which showed how to create a 2D array.

On a similar matter, I was able to create a 1D array (with starting and ending indices as -n and n respectively), but had problems freeing the memory. My function to free the memory was

void free_vector(float *x, int n)

{

free(x + n);

}

but I got the same "Segmentation Fault" error. What is wrong?

Thank you.

Regards,

Rayne