avery@netcom.COM (Avery Colter) (02/25/91)
With all this about the #define X1 -10, I find it interesting that
indexing a constant yields the sum of the constant and the number inside
the brackets.
I could RTFM till the cows come home and never would have gleaned this.
Silly me, I would have thought that 10[25] would yield the value
residing in the position 25 positions away from address 10.
But, I guess there must be an exception made here, because it is
effectively a void pointer being indexed, with no way of telling
how many bytes are offset for each number in the brackets?
Very strange stuff indeed.
--
Avery Ray Colter {apple|claris}!netcom!avery {decwrl|mips|sgi}!btr!elfcat
(415) 839-4567 "I feel love has got to come on and I want it:
Something big and lovely!" - The B-52s, "Channel Z"dave@cs.arizona.edu (Dave P. Schaumann) (02/25/91)
In article <25694@netcom.COM> avery@netcom.COM (Avery Colter) writes: >With all this about the #define X1 -10, I find it interesting that >indexing a constant yields the sum of the constant and the number inside >the brackets. > >I could RTFM till the cows come home and never would have gleaned this. Well, if you would RTF code, you would've seen an initialization loop that looked something like this: for( i = 0 ; i < N ; i++ ) p[i] = i ; (That's probably not exactly what was in the code, but you get the idea.) Just another tricky day for you... -- Dave Schaumann | Is this question undecidable? dave@cs.arizona.edu |
brandis@inf.ethz.ch (Marc Brandis) (02/25/91)
In article <25694@netcom.COM> avery@netcom.COM (Avery Colter) writes: >With all this about the #define X1 -10, I find it interesting that >indexing a constant yields the sum of the constant and the number inside >the brackets. > I could not find a compiler that did it like that. All of them complained that they expected a variable to be indexed. Some of the compilers were ANSI (IBM S/6000), some were K&R (Sun 3, Sequent Symetry). Marc-Michael Brandis Computer Systems Laboratory, ETH-Zentrum (Swiss Federal Institute of Technology) CH-8092 Zurich, Switzerland email: brandis@inf.ethz.ch
henry@zoo.toronto.edu (Henry Spencer) (02/26/91)
In article <25694@netcom.COM> avery@netcom.COM (Avery Colter) writes: >With all this about the #define X1 -10, I find it interesting that >indexing a constant yields the sum of the constant and the number inside >the brackets. It shouldn't. The definition of a[b] is that it acts like *(a+b) [ignoring issues of parentheses and operator precedence]. Note the "*" on the front. -- "But this *is* the simplified version | Henry Spencer @ U of Toronto Zoology for the general public." -S. Harris | henry@zoo.toronto.edu utzoo!henry
wgh@ubbpc.UUCP (William G. Hutchison) (02/27/91)
In article <25694@netcom.COM>, avery@netcom.COM (Avery Colter) writes: > With all this about the #define X1 -10, I find it interesting that > indexing a constant yields the sum of the constant and the number inside > the brackets. > > I could RTFM till the cows come home and never would have gleaned this. > > Silly me, I would have thought that 10[25] would yield the value > residing in the position 25 positions away from address 10. > # cat ca.c main() { printf("%d\n", 10[25]); return 0; } # make ca cc -O ca.c -o ca "ca.c", line 2: illegal indirection *** Error code 1 Stop. I don't see how RTFMing can show you something that is not true??? -- Bill Hutchison, DP Consultant psuvax1!burdvax!ubbpc!wgh (work) Unisys UNIX Portation Center uunet!eidolon!wgh (home) P.O. Box 500, M.S. B121 "At the moment I feel more like arguing than Blue Bell, PA 19424 being good" Raymond Smullyan _The Tao is Silent_
ttobler@unislc.uucp (Trent Tobler) (02/28/91)
From article <25694@netcom.COM>, by avery@netcom.COM (Avery Colter): > With all this about the #define X1 -10, I find it interesting that > indexing a constant yields the sum of the constant and the number inside > the brackets. > > I could RTFM till the cows come home and never would have gleaned this. No, RTFM again. The definition of x[y] is not (x+y); it is *(x+y). So, 10[25] is *(10 + 25) which is illegal, since only pointers can be dereferenced. > > Silly me, I would have thought that 10[25] would yield the value > residing in the position 25 positions away from address 10. Not unless you do something like ((char *) 10)[25], which, depending on how the compiler treats this, may do what you thought it would. BTW, don't do this. --- Trent Tobler - ttobler@csulx.weber.edu
john@IASTATE.EDU (Hascall John Paul) (03/02/91)
In article <1991Feb27.223628.13900@unislc.uucp>, ttobler@unislc.uucp (Trent Tobler) writes: > From article <25694@netcom.COM>, by avery@netcom.COM (Avery Colter): : : > No, RTFM again. The definition of x[y] is not (x+y); it is *(x+y). > So, 10[25] is *(10 + 25) which is illegal, since only pointers can be > dereferenced. > > Silly me, I would have thought that 10[25] would yield the value > > residing in the position 25 positions away from address 10. > Not unless you do something like ((char *) 10)[25], which, depending > on how the compiler treats this, may do what you thought it would. BTW, > don't do this. ((char *)10)[25] and 10[(char *)25] are both perfectly legal, if somewhat unorthodox. I wouldn't go quite as far as "don't do this", but you certainly ought to think hard about it first. One possible example (of course, we all avoid magic numbers in our code ;-) ... #define CSRBASE ((char *)10) /* Starting address of registers */ #define FOOREG 25 /* Foo control is register #25 */ CSRBASE[ FOOREG ] = foo_act_cmd; /* activate foo-o-matic */ -- John Hascall An ill-chosen word is the fool's messenger. Project Vincent Iowa State University Computation Center john@iastate.edu Ames, IA 50011 (515) 294-9551