defining a 2D array with malloc and modifying it

Question

How do i define a 2D array using malloc ? (lets say 10X20).

second, can i increase number of rows or cols without creating a new increased array and copying all data

Answer 1

While malloc() does not directly support multi-dimensional arrays, there are workarounds, such as:

int rows = 10;
int cols = 30;
int *array = malloc(rows * cols * sizeof(int));

// Element (5,6)
int x = 5;
int y = 6;
int element = array [ x * cols + y ];

While this isn't directly a 2D array, it works, and in my opinion it's the simplest. But if you want to use the [][] syntax instead, you would have to make pointers to pointers, for instance:

int rows = 10;
int cols = 30;
// Rows
int **array = malloc(rows * sizeof(int*));
// Cols
int i;
for(i = 0; i < rows; i++)
  array[i] = malloc(cols * sizeof(int));

// Element (5,6)
int x = 5;
int y = 6;
int element = array[x][y];