Question

Just trying to really get my head round Arrays and Pointers in C and the differences between them and am having some trouble with 2d arrays.

For the normal 1D array this is what I have learned:

char arr[] = "String constant";

creates an array of chars and the variable arr will always represent the memory created when it was initialized.

char *arr = "String constant";

creates a pointer to char which is currently pointing at the first index of the char array “String constant”. The pointer could point somewhere else later.

char *point_arr[] = {
    "one", "two","three", "four"
};

creates an array of pointers which then point to the char arrays “one, “two” etc.

My Question

If we can use both:

char *arr = "constant";

and

char arr[] = "constant";

then why can’t I use:

char **pointer_arr = {
    "one", "two", "three", "four"
};

instead of

char *pointer_arr[] = {
    "one", "two", "three", "four"
};

If I try the char ** thing then I get an error like “excess elements in scalar initializer”. I can make the char** example work by specifically allocating memory using calloc, but as I didn’t have to do this with char *arr = "blah";. I don’t see why it is necessary and so I don’t really understand the difference between:

char **arr_pointer;

and

char *arr_pointer[];

Many thanks in advance for your advice.

Answer 1

In short, you cannot use { ... } as an initialiser for a scalar.

char **arr_pointer declares a scalar, not an array. In contrast, the reason you can do char *arr = "constant"; is because you’re still declaring a scalar, it just happens to point at a string literal.