A matrix pointer into sub-matrices in C - c

Question

I have this matrix in C.
[1,2,3,4]
[5,6,7,8]
[9,10,11,12]
[13,14,15,16]
n x n, squared matrix.
I need to split this into four matrices:
[1,2] [3,4] [9,10] [11,12]
[5,6] [7,8] [13,14] [15,16]
This will be represented inside an array like this:
array[16] = [1,2,5,6,3,4,7,8,9,10,13,14,11,12,15,16]
So far, I've done this:
int i,j;
int k = 0;
for(i = 0; i < 2; i++ )
{
for(j = 0; j < 2; j++)
{
array[k] = matrix[i][j];
k++;
}
}
for(i = 0; i < 2; i++ )
{
for(j = 2; j < 4; j++)
{
array[k] = matrix[i][j];
k++;
}
}
for(i = 2; i < 4; i++ )
{
for(j = 0; j < 2; j++)
{
array[k] = matrix[i][j];
k++;
}
}
for(i=2;i<4;i++)
{
for(j=2;j<4;j++)
{
array[k] = matrix[i][j];
k++;
}
}
As you can see, I've done 4 double for this, but, is there a dynamic way to do this? If I got a bigger matrix, like 8 x 8, how to do this? The split if bigger is the same as the example.

Answer 1

You should see the pattern of the routines you are duplicating.
Basically you do the same thing, except that the start and end of i and j are different. So create a sub routine and pass them as parameter.
For example:
void get_sub_matrix(int input[][N], int start_row, int end_row, int start_col, int end_col, int[] result, int* result_offset)
{
int offset = *result_offset;
for (int i = start_row; i < end_row; i++)
{
for (int j = start_col; j < end_col; j++)
{
result[offset++] = input[i][j];
}
}
*result_offset = offset;
}
Notice how the result-offset is increased inside the routine as more elements are added to the result array.
Now you can do:
int matrix[N][N];
int array[N*N];
int k = 0;
get_sub_matrix(matrix, 0, 2, 0, 2, array, &k);
get_sub_matrix(matrix, 0, 2, 2, 4, array, &k);
get_sub_matrix(matrix, 2, 4, 0, 2, array, &k);
get_sub_matrix(matrix, 2, 4, 2, 4, array, &k);
P.S: Haven't compiled it.