Question

When I declare a two-dimensional array like this:

char myArray[20][30] = {"ABC", "Is Easy As One Two Three"};

Can I assume that all other chars in this array are now set to \000?

Answer 1

Objects are never partially initialised in C – if you have used an initialiser, then the entire object is guaranteed to be initialised (in this case, the “object” is the whole array of 20 arrays of 30 chars each). Any members not explicitly initialised are recursively initialised to zero (for arithmetic types), or NULL (for pointer types).

So the answer, in this case, is yes – all the chars not explicitly given values by the initialiser are guaranteed to be 0.

This is described in the C99 standard in section 6.7.8, Initialisation. The relevant paragraphs are:

21 If there are fewer initializers in
a brace-enclosed list than there are
elements or members of an aggregate,
or fewer characters in a string
literal used to initialize an array of
known size than there are elements in
the array, the remainder of the
aggregate shall be initialized
implicitly the same as objects that
have static storage duration.

and

10 If an object that has automatic
storage duration is not initialized
explicitly, its value is
indeterminate. If an object that has
static storage duration is not
initialized explicitly, then:

— if it
has pointer type, it is initialized to
a null pointer;

— if it has arithmetic
type, it is initialized to (positive
or unsigned) zero;

— if it is an
aggregate, every member is initialized
(recursively) according to these
rules;

— if it is a union, the first
named member is initialized
(recursively) according to these
rules.