Question

I have this part of code:

for(i = 0; i < matrix->y; i++)
{
  printf("3.%i - %i %i\n", i, matrix->y, matrix->x);//write correct "3.0 - 3 4"
  for(j = 0; j < matrix->x; j++)
  {
    printf("N - %i %i\n", matrix->y, matrix->x);//never write anything
    fscanf(input, "%i", &grid[i][j]);
  }
  printf("3.%i - %i %i\n", i, matrix->y, matrix->x);//write wrong "3.0 - 3 0"
}

first printf outputs y=3 and x=4, but it never goes inside loop, it never reach the second printf inside for-loop. When I write the same printf from the first line after for-loop, it tels me y=3 and x=0.
Where I have done mistake.

Thanks.

Edit

Code is writen as it is. No lines skipped.

Matrix definition

typedef struct matrix
{
  int x;
  int y;
  int ** grid;
} Matrix;

i and j is defined by

int i, j;

Whole function

Matrix * loadMatrix(char * filename)
{
  Matrix * matrix;
  FILE * input;
  input = fopen(filename, "r");
  if (input == NULL)
    printError (ERR_READFILE);
  else
  {
    fscanf(input, "%i", &(matrix->y));
    fscanf(input, "%i", &(matrix->x));
    int i, j;
    int grid[matrix->y][matrix->x];
    matrix->grid = grid;
    printf("2 - %i %i\n", matrix->y, matrix->x);
    for(i = 0; i < matrix->y; i++)
    {
      printf("3.%i - %i %i\n", i, matrix->y, matrix->x);
      for(j = 0; j < matrix->x; j++)
    {
    printf("N - %i %i\n", matrix->y, matrix->x);
    fscanf(input, "%i", &grid[i][j]);
    }
    printf("3.%i - %i %i\n", i, matrix->y, matrix->x);
    fclose(input);
  }
  return matrix;
}

Answer 1

Your program’s behavior is undefined because you’re not allocating storage for matrix. So anything could happen.

Add this somewhere after you have declared matrix (just before the fscanf lines seems optimal):

matrix = malloc(sizeof(Matrix));

and change the line where you declare matrix to:

Matrix *matrix = NULL;

(otherwise you’ll return garbage if the fopen fails.)

Second problem is that you’re then also failing to allocate proper storage for matrix->grid. You can’t keep a pointer to a local variable after the function has returned.

You’ll need to create that storage with malloc or calloc too. See this question: How do I work with dynamic mutli-dimensional arrays in C for an example of how to do that.

You’ll also need to take care to free both of those once you’re finished using them.

Don’t forget to #include <stdlib.h>, which is required for malloc and free.