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Re: Arrays Vs Pointers

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Keith Thompson wrote:
> 
> Keith Thompson <kst-u@mib.org> writes:
> > pete <pfiland@mindspring.com> writes:
> >> Keith Thompson wrote:
> >>> Willem <willem@toad.stack.nl> writes:
> >>> > peter wrote:
> >>> > ) I am currently exploring the world of pointers and have encounter some
> >>> > ) inconsistent information regarding the best way to reference an
> >>> > ) array: array[1]   OR    *(array +1).
> >>> > )
> >>> > ) One book says that you should not use the syntax array[1] because
> >>> > ) of performance reason.
> >>> >
> >>> > That book is completely and utterly wrong.
> >>> > In C, "array[index]" is *exactly the same* as "*(array + index)"
> >>> > I would go so far as to call it syntactic sugar.
> >>> > Hell, it's even commutative.  "array[index]" means the same as "index[array]" !
> >>> [...]
> >>>
> >>> Yes, but I doubt that that's what the book actually said.
> >>
> >> Teach Yourself C,
> >> by Herbert Schildt, copyright 1990,
> >> Chapter 6.3, Use Pointers With Arrays,
> >> page 197:
> >>
> >>     For somewhat complex reasons,
> >>     a C compiler will generally create faster executable code
> >>     for an expression such as
> >>
> >>     *(p+3)
> >>
> >>     than it will for the comparable array index
> >>
> >>     p[3]
> >
> > Had I known it was one of Schildt's books, I wouldn't have assumed
> > that it said something sensible.  (Do we know that that's the book to
> > which peter was referring?)


I don't know, 
but for some reason I remembered reading it a long time ago.


> > Willem is right: the book is completely and utterly wrong.
> > There's probably a valid point somewhere in the vicinity of what
> > Schildt wrote (as I discussed upthread), but Schildt missed it.
> 
> Let me amend that slightly.  It's possible that some compilers might
> generate faster code for *(p+3) than for p[3].  There's no good reason
> for that, since the two expressions are equivalent by definition, but
> it's possible.  It's also possible Schildt encountered such a compiler
> and reached an overly general conclusion.

I recall a thread about a year or so ago,
where the controling expression in a loop 
was shown to have been rendered
with a register, when {!= 0} was omitted.
That is to say that there was a compiler for which
    while (x)
was faster than
    while ((x) != 0)
I think it was a gcc compiler
and I remember being disappointed 
that the compiler had translated 
two semantically equal expressions differently.


-- 
pete

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Arrays Vs Pointers peter <pc722@nospam.mail.com.br> - 2012-02-07 21:28 +0000
  Re: Arrays Vs Pointers William Ahern <william@wilbur.25thandClement.com> - 2012-02-07 13:50 -0800
  Re: Arrays Vs Pointers James Kuyper <jameskuyper@verizon.net> - 2012-02-07 17:13 -0500
  Re: Arrays Vs Pointers Stephen Sprunk <stephen@sprunk.org> - 2012-02-07 16:38 -0600
    Re: Arrays Vs Pointers Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-07 23:09 +0000
  Re: Arrays Vs Pointers Keith Thompson <kst-u@mib.org> - 2012-02-07 14:41 -0800
    Re: Arrays Vs Pointers Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-07 23:07 +0000
      Re: Arrays Vs Pointers James Kuyper <jameskuyper@verizon.net> - 2012-02-07 19:14 -0500
    Re: Arrays Vs Pointers Shao Miller <sha0.miller@gmail.com> - 2012-02-07 18:21 -0500
      Re: Arrays Vs Pointers Keith Thompson <kst-u@mib.org> - 2012-02-07 16:25 -0800
        Re: Arrays Vs Pointers Shao Miller <sha0.miller@gmail.com> - 2012-02-07 20:31 -0500
    Re: Arrays Vs Pointers Ike Naar <ike@sverige.freeshell.org> - 2012-02-07 23:36 +0000
      Re: Arrays Vs Pointers Keith Thompson <kst-u@mib.org> - 2012-02-07 16:26 -0800
    Re: Arrays Vs Pointers Keith Thompson <kst-u@mib.org> - 2012-02-07 16:24 -0800
  Re: Arrays Vs Pointers Paul N <gw7rib@aol.com> - 2012-02-07 14:55 -0800
  Re: Arrays Vs Pointers Shao Miller <sha0.miller@gmail.com> - 2012-02-07 18:11 -0500
  Re: Arrays Vs Pointers John Bode <jfbode1029@gmail.com> - 2012-02-08 16:06 -0800
    Re: Arrays Vs Pointers Philip Lantz <prl@canterey.us> - 2012-02-08 23:25 -0800
      Re: Arrays Vs Pointers John Bode <jfbode1029@gmail.com> - 2012-02-09 05:34 -0800
        Re: Arrays Vs Pointers Philip Lantz <prl@canterey.us> - 2012-02-10 01:04 -0800
  Re: Arrays Vs Pointers Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-02-09 00:20 -0800
    Re: Arrays Vs Pointers 88888 Dihedral <dihedral88888@googlemail.com> - 2012-02-09 23:42 -0800
    Re: Arrays Vs Pointers gwowen <gwowen@gmail.com> - 2012-02-10 01:20 -0800
    Re: Arrays Vs Pointers David Thompson <dave.thompson2@verizon.net> - 2012-02-18 23:12 -0500
      Re: Arrays Vs Pointers Kaz Kylheku <kaz@kylheku.com> - 2012-02-19 05:53 +0000
        Re: Arrays Vs Pointers Markus Wichmann <nullplan@gmx.net> - 2012-02-19 18:54 +0100
          Re: Arrays Vs Pointers Willem <willem@toad.stack.nl> - 2012-02-19 18:51 +0000
          Re: Arrays Vs Pointers "io_x" <a@b.c.invalid> - 2012-02-19 21:29 +0100
          Re: Arrays Vs Pointers Markus Wichmann <nullplan@gmx.net> - 2012-02-20 11:24 +0100
          Re: Arrays Vs Pointers gwowen <gwowen@gmail.com> - 2012-02-21 01:51 -0800
  Re: Arrays Vs Pointers Willem <willem@toad.stack.nl> - 2012-02-19 12:32 +0000
    Re: Arrays Vs Pointers Keith Thompson <kst-u@mib.org> - 2012-02-19 12:21 -0800
      Re: Arrays Vs Pointers Willem <willem@toad.stack.nl> - 2012-02-19 21:00 +0000
        Re: Arrays Vs Pointers Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-02-19 21:42 +0000
      Re: Arrays Vs Pointers pete <pfiland@mindspring.com> - 2012-02-20 17:32 -0500
        Re: Arrays Vs Pointers James Kuyper <jameskuyper@verizon.net> - 2012-02-20 17:56 -0500
        Re: Arrays Vs Pointers Keith Thompson <kst-u@mib.org> - 2012-02-20 15:19 -0800
          Re: Arrays Vs Pointers Keith Thompson <kst-u@mib.org> - 2012-02-20 16:35 -0800
            Re: Arrays Vs Pointers pete <pfiland@mindspring.com> - 2012-02-20 22:53 -0500
              Re: Arrays Vs Pointers Keith Thompson <kst-u@mib.org> - 2012-02-20 21:16 -0800
            Re: Arrays Vs Pointers Richard Sanders <lnr729@gmail.com> - 2012-02-21 00:26 -0800
              Re: Arrays Vs Pointers pete <pfiland@mindspring.com> - 2012-02-21 11:46 -0500

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