C - Read matrix of unknown size from file - c

Question

I have file that has 30 matrices and each matrix has unknown size of rows and columns(with a max size of 1000). For instance:
0 5 2
5 0 2
1 6 0
0 9 7 4
3 0 9 1
9 1 0 4
9 4 1 0
I need to read each matrix into a 2d array. What would be the most efficient way of doing this?
This is what I have so far:
int** mat=malloc(1000000*sizeof(int*));
for(i=0;i<1000000;++i)
mat[i]=malloc(4*sizeof(int));
while(!feof(file))
{
for(i=0;i<1000;i++)
{
for(j=0;j<1000;j++){
fscanf(file,"%d%*[^\n]%*c",&mat[i][j]);
printf("%d\n", mat[i][j]);
}
}
}

Answer 1

Well the most efficient way is definitely not that. First figure out how big an array you need, then allocate it.

Answer 2

Apparently some matrices are small, so there is no need to allocate the maximum size 1000x1000. One way is to put the matrix in a structure to make it easier to keep track of size:
struct s_matrix
{
int **matrix;
int N; //one side of the square matrix N x N
};
typedef struct s_matrix Matrix;
Then allocate and free the matrix
void allocate_matrix(Matrix *m, int N)
{
m->N = N;
m->matrix = (int**)malloc(N * sizeof(int*));
*m->matrix = (int*)malloc(N * N * sizeof(int));
for (int i = 0; i < N; i++)
m->matrix[i] = *m->matrix + i * N;
for (int i = 0; i < N; i++)
for (int j = 0; j < N; j++)
m->matrix[i][j] = 0;
}
void free_matrix(Matrix *m)
{
if (!m) return;
if (!m->matrix) return;
free(*m->matrix);
free(m->matrix);
}
Now we can declare how many matrices we need. It looks like this number is fixed at 30, so I don't think we need dynamic allocation.
int main()
{
const int max_count = 30;
Matrix list[max_count];
for (int i = 0; i < max_count; i++)
list[i].matrix = NULL;
allocate_matrix(&list[0], 3);//allocate 3x3 matrix
allocate_matrix(&list[1], 1000);//1000x1000
allocate_matrix(&list[2], 4000);//4000x4000
int **m;
m = list[0].matrix;
m[0][0] = 0;
m[0][1] = 1;
m[0][2] = 2;
for (int i = 0; i < 3; i++)
for (int j = 0; j < 3; j++)
printf("%02d %s", m[i][j], (j == 2) ? "\n" : "");
//...
for (int i = 0; i < max_count; i++)
free_matrix(&list[i]);
printf("hey it worked, or maybe it didn't, or it did but there is memory leak\n");
return 0;
}