Is the number of elements necessary?

Question

In the C lessons, arrays are always defined like this:
int x[3] = {0, 1, 2}

Is it possible to define them like this so that length doesn't matter:
int x[] = {0, 1, 2}

Answer 1

The more important question is: Why would you want this functionality (except when initialising a string, which you're not doing)? It is _terrible_ practice because it makes code less readable and will cause compiler warnings (maybe even errors on some older ones). Rather use #define to define the array size if you may change it later.
Or, _if you want a dynamic array_, use malloc/calloc, memcpy, and free along with pointers.

Answer 2

The arrays like this x[] are allowed as these get their limits on runtime, after checking the biggest index that is used in code. Hence allowed, but better not have a hole in them
i.e int x[];
x[0]=1;
x[1]=2;
x[99]=3;