negative array index

[David Osborn]

I have a pointer "a" that points to an element in an array that is not
the first element. I want to use a negative index to derefence the
pointer, like so: a[-1]. Is this valid C99? If not, I can just say
*(a-1), right?

[Artie Gold]

David Osborn wrote:

I have a pointer "a" that points to an element in an array that is not
the first element. I want to use a negative index to derefence the
pointer, like so: a[-1]. Is this valid C99? If not, I can just say
*(a-1), right?

Yes, as long as your pointing to a valid location in an `array'.
[BTW a[-1] and *(a - 1) are completely equivalent; (-1)[a], though ugly,
would also mean the same thing.]

HTH,
--ag

--
Artie Gold -- Austin, Texas
http://it-matters.blogspot.com (new post 12/5)
http://www.cafepress.com/goldsays

[Jens.Toerring]

David Osborn <sp**@habitualhiatus.com> wrote:

I have a pointer "a" that points to an element in an array that is not
the first element. I want to use a negative index to derefence the
pointer, like so: a[-1]. Is this valid C99? If not, I can just say
*(a-1), right?

a[ -1 ] and *(a-1) are identical, the second being what the compiler
converts the first to (already with C89). And yes, you can access
elements of the array also with negative indexes for 'a' as long as
you stay within the limits of the array 'a' is pointing into.

Regards, Jens
--
\ Jens Thoms Toerring ___ Je***********@physik.fu-berlin.de
\__________________________ http://www.toerring.de

[Jack Klein]

On Mon, 23 May 2005 18:40:58 GMT, David Osborn
<sp**@habitualhiatus.com> wrote in comp.lang.c:

I have a pointer "a" that points to an element in an array that is not
the first element. I want to use a negative index to derefence the
pointer, like so: a[-1]. Is this valid C99? If not, I can just say
*(a-1), right?

This is valid in any version of C, not just C99, as long as the
pointer itself is either within the bounds of the array or points to
one past the last element, and the combination of the pointer and the
subscript does point into the array, not past the end and not before
the beginning.

Examples:

int a[10] = { /* whatever */ };
int i;
b = a + 10; /* one past end */

for (i = -10; i < 0; i++)
{
int x = b[i]; /* all valid */
}

....but b[-11] is invalid.

--
Jack Klein
Home: http://JK-Technology.Com
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[Malcolm]

"David Osborn" <sp**@habitualhiatus.com> wrote

I have a pointer "a" that points to an element in an array that is not the
first element. I want to use a negative index to derefence the pointer,
like so: a[-1]. Is this valid C99? If not, I can just say *(a-1), right?

The form a[-1] is allowed.
In my opinion this is compileable gibberish rather than C, and it has to be
*(a-1), but other people may disagree.

[Barry Schwarz]

On Tue, 24 May 2005 06:27:18 +0000 (UTC), "Malcolm"
<re*******@btinternet.com> wrote:

"David Osborn" <sp**@habitualhiatus.com> wrote

I have a pointer "a" that points to an element in an array that is not the
first element. I want to use a negative index to derefence the pointer,
like so: a[-1]. Is this valid C99? If not, I can just say *(a-1), right?

The form a[-1] is allowed.
In my opinion this is compileable gibberish rather than C, and it has to be
*(a-1), but other people may disagree.

Since the two are guaranteed to have the same meaning, along with
(-1)[a], other than aesthetics what do you use to determine gibberish?
<<Remove the del for email>>

[Malcolm]

"Barry Schwarz" <sc******@deloz.net> wrote

Since the two are guaranteed to have the same meaning, along with
(-1)[a], other than aesthetics what do you use to determine gibberish?

(-1)[a] is very clearly compileable gibberish.
A programmer who knew only a little C wouldn't have a clue what that
construct meant, whilst an experienced programmer would find such code hard
to read and to maintain.
It is a question of aethetics, but that doesn't mean "trivial" or
"unimportant".