Array indexing vs. pointers...

[Scott Wilson]

In article <8809191521.AA17824 at ucbvax.Berkeley.EDU> U23405 at UICVM (Michael J. Steiner) writes:
>First, I have a question. How and why is array indexing slower than
>pointer arithmetic? They are very similar. Also, I think that compilers
>should automatically translate array indexing into pointer arithmetic when
>dealing with arrays in C. Any comments, opinions?

In K&R pg. 186 it says: "E1[E2] is identical (by definition) to
*((E1)+(E2))."  So array indexing is exactly the same as pointer
arithmetic.  What you might be hearing is about the efficiency of using
array indexing in loops versus using pointers.  For example:

	int i, sum = 0, array[100];

	for (i = 0; i < 100; i++)
		sum += array[i];

is less efficient than:

	int i, sum = 0, array[100], *ip = array;

	for (i = 0; i < 100; i++, ip++)
		sum += *ip;

In the first case, using array[i] results in a shift and an add
to compute the address of the array element.  (If the array element
size was not a power of two then a multiplication would be needed.)
In the second we are just bumping a pointer up each time through
the loop.  There is more that can be done to the second example
as well (why increment two vars side-by-side).

--
Scott Wilson		arpa: swilson at sun.com
Sun Microsystems	uucp: ...!sun!swilson
Mt. View, CA