In practice, the difference is in the location where the preprocessor searches for the included file.
For #include <filename> the preprocessor searches in an implementation dependent manner, normally in search directories pre-designated by the compiler/IDE. This method is normally used to include standard library header files.
For #include "filename" the preprocessor searches first in the same directory as the file containing the directive, and then follows the search path used for the #include <filename> form. This method is normally used to include programmer-defined header files.
A more complete description is available in the GCC documentation on search paths.
Executive summary:
int a[17];
size_t n = sizeof(a)/sizeof(a[0]);
Full answer:
To determine the size of your array in bytes, you can use the sizeof
operator:
int a[17];
size_t n = sizeof(a);
On my computer, ints are 4 bytes long, so n is 68.
To determine the number of elements in the array, we can divide
the total size of the array by the size of the array element.
You could do this with the type, like this:
int a[17];
size_t n = sizeof(a) / sizeof(int);
and get the proper answer (68 / 4 = 17), but if the type of
a changed you would have a nasty bug if you forgot to change
the sizeof(int) as well.
So the preferred divisor is sizeof(a[0]) or the equivalent sizeof(*a), the size of the first element of the array.
int a[17];
size_t n = sizeof(a) / sizeof(a[0]);
Another advantage is that you can now easily parameterize
the array name in a macro and get:
#define NELEMS(x) (sizeof(x) / sizeof((x)[0]))
int a[17];
size_t n = NELEMS(a);
Best Solution
C doesn't allow access to memory beyond the end of the array. It does, however, allow a pointer to point at one element beyond the end of the array. The distinction is important.
Thus, this is OK:
(Doing
*endwould be an error.)And that shows the reason why this feature is useful: a pointer pointing at the (non-existent) element after the end of the array is useful for comparisons, such as in loops.
Technically speaking, that is everything the C standard allows. However, in practice, the C implementation (compiler and runtime) does not check whether you access memory beyond the end of the array, whether it is one element or more. There would have to be bounds checking and that would slow down program execution. The kinds of programs C is best suited for (systems programming, general purpose libraries) tend to benefit more from the speed than the security and safety bounds checking would give.
That means C is perhaps not a good tool for general purpose application programming.