Path: utzoo!attcan!uunet!auspex!guy
From: guy@auspex.auspex.com (Guy Harris)
Newsgroups: comp.lang.c
Subject: Re: Pointer incrementation and assignment
Message-ID: <1704@auspex.auspex.com>
Date: 26 May 89 19:15:17 GMT
References: <2015@csuna.csun.edu>
Reply-To: guy@auspex.auspex.com (Guy Harris)
Organization: Auspex Systems, Santa Clara
Lines: 44

>But would doing this:
>
>      a += 1;
>
>increase a by 1 or by 2?

It would increment the value of the variable "a" by 1, just as would
"a++".  That is, it would cause "a" to point to an element one further
along in the (putative) array into which it points.  In other words, on
your example machine, it would increment the address stored in the
variable "a" by 2.

>Or how about
>
>      a = a + 1;

Same thing.

"a++", "a += 1", and "a = a + 1" all add 1 to the value of "a",
regardless of the type of "a"; it's just that the meaning of "add 1 to
the value of 'a'" differs depending on the type of "a".

If "a" is of integral type, its value plus 1 is the successor to the
integer that is its value (modulo - no pun intended - overflow or
modular-arithmetic wraparound for unsigned types). 

If "a" is of floating-point type, its value plus 1 is the sum of its
current value and 1.0, given the implementation of floating-point
arithmetic on your machine (taking into account overflow, loss of
significance of the "1.0" - i.e., 1.0e36 + 1.0 probably isn't
1.000......01e36 unless you have lots of bits of precision).

If "a" is of pointer type, its value plus 1 is a pointer to the element
following the one pointed to by "a" in the (putative) array to which the
element pointed to by "a" belongs.  (If the value of "a" is represented
by the number N - i.e., "a" points to something starting at the Nth
byte, 0-origin of course, in your address space - the value of "a+1" is
represented by the number N+sizeof (*a).  Be careful *not* to just think
of this as "adding 'sizeof(*a)' to 'a'.")

Think of it in those terms, rather than, say, thinking of "a++" as
shorthand for "part of how you get at your machine's autoincrement
address mode", and it becomes clearer (*and* more machine-independent -
not all machines *have* an autoincrement addressing mode, for instance).