C - Read matrix of unknown size from file

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:

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]);
               }
        }
    }

score 0 · Answer 1 · answered Apr 05 '15 at 19:18

0

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

answered Apr 05 '15 at 19:18

Blindy

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score 0 · Answer 2 · answered Apr 06 '15 at 01:16

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;
}

C - Read matrix of unknown size from file

2 Answers2