Array of unknown length

You need to allocate memory dynamically using the malloc function:

 int** arr = (int**)malloc(sizeof(int*) * y); for (int i = 0; i < y; ++i) { arr[i] = (int*)malloc(sizeof(int) * x); } 

Remember to free up memory when it is not needed:

 for (int i = 0; i < y; ++i) { free(arr[i]); } free(arr); 

Well, if your compiler supports variable-length arrays, then you can use it exactly as you wrote: https://ideone.com/lQfCcb

But if not - then you need to use a dynamic array. Two options -

 int **array = malloc(sizeof(int*)*y); for(int i = 0; i < y; ++i) array[y] = malloc(sizeof(int)*x); 

If this is really C, not C ++, a cast of malloc not required.

The second option -

 int * array = malloc(sizeof(int)*x*y); 

But the call to array[i][j] will look like array[i*x+j] .