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| From | John Bode <jfbode1029@gmail.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Why C has Pointers. |
| Date | 2011-12-19 05:14 -0800 |
| Organization | http://groups.google.com |
| Message-ID | <4743356.1584.1324300471824.JavaMail.geo-discussion-forums@yqba2> (permalink) |
| References | <curixinfotech.54dl3z@no-mx.forums.yourdomain.com.au> <10353483.597.1324140693693.JavaMail.geo-discussion-forums@prez5> <golpe7lj73rnopq19ndti2hq1k02v02cat@4ax.com> <vqrpe7pkb15dek159fddlo2kfepn9dtcvl@4ax.com> |
On Saturday, December 17, 2011 2:28:14 PM UTC-6, ralph wrote:
> On Sat, 17 Dec 2011 10:10:09 -0800, Barry Schwarz <schw...@dqel.com>
> wrote:
>
> >On Sat, 17 Dec 2011 08:51:33 -0800 (PST), 88888 Dihedral
> ><dihedr...@googlemail.com> wrote:
> >
> >>On Wednesday, December 7, 2011 8:22:56 PM UTC+8, curixinfotech wrote:
> >>> hi
> >>> my Q is on the c language tell me now why c has pointers.
> >>> parshant
> >>>
> >>>
> >>> --
> >>> curixinfotech
> >>> ------------------------------------------------------------------------
> >>> curixinfotech's Profile: http://forums.yourdomain.com.au/member.php?userid=134
> >>> View this thread: http://forums.yourdomain.com.au/showthread.php?t=3319
> >>
> >>OK, I'll explain how to use a pointer in C without any knowledge in the
> >>hardware or assembly here.
> >>//
> >>int i;
> >>int a[3]={ 1,2,3} ; // an array of 3 integers
> >>int* ptr1; // Note the star should be right after int, not initialize
> >>int* ptr2; // not initialized
> >>
> >>ptr1=a; // Read this as to make ptr1 to be an alias of a.
> >
> >If that were true, the sizeof(a) and sizeof(ptr1) would be equal
> >which, in the general case, they are not. ptr1 is simply a pointer
> >that points to the start of a. Even though they share some
> >similarities in the language syntax, a pointer is a completely
> >different type than an array. Calling one an alias of the other just
> >breeds unnecessary confusion.
> >
>
> I agree.
>
> I have trouble understanding the constant confusion between arrays and
> pointers. Way back when (and we are talking many moons ago <g>) when I
> started out in C my mentor explained it very simply in these terms in
> the first couple of days.
>
> 1) There is a difference between declarations and definitions.
> Remember which you are dealing with and when.
> 2) A pointer is a "variable", an array is a "constant"
A pointer is a pointer, an array is an array. They are completely different animals with different behavior.
Array *expressions* may not be the target of an assignment, because the language definition says so. No need to invoke anything about "constants", which is annoyingly inaccurate.
> 3) The array name evaluates to the address of the first element in the
> array.
In most circumstances, an expression with type "N-element array of T" will be *replaced* with a pointer expression of type "pointer to T" that evaluates to the location of the first element in the array. The exceptions to this rule are when the array expression is an operand of either the sizeof or unary & operators, or when the array expression is a string literal that is being used to initialize another array in a declaration.
> 4) A pointer is a variable that holds the address of another variable
> or an array.
> 5) All pointers must be intialized before you use them.
> 6) Since an array name is an address it can be used as a pointer that
> is initialized to point to the first element in the array.
In most circumstances, an expression with type "N-element array of T" will be *replaced* with a pointer expression of type "pointer to T" that evaluates to the location of the first element in the array. The exceptions to this rule are when the array expression is an operand of either the sizeof or unary & operators, or when the array expression is a string literal that is being used to initialize another array in a declaration.
> 7) *variable means the contents of the variable.
> 8) &variable means the address of the variable.
> 9) The array operator evaluates to a storage map equation, eg.
> type ar[i] := *(a + (i * sizeof(type))
The subscript operation a[i] is *defined* as *(a + i); pointer arithmetic takes the type of the pointer into account, so you're always referring to the i'th *element* of the array.
> 10) In all "pointer" operations you are either chewing on an address
> or the contents (value) at that address. Simple deconstrution of the
> even the most complex expression will quickly reveal which it is.
>
> I've since learned how to restate the above in fancier terminology,
> but those simple rules have stuck and served me well from my second
> week in C to today - thirty plus years later.
>
> -ralph
Simple but imprecise. I've discovered over the last 20+ years that it's better to be precise at the expense of brevity if you want people to really understand how the C language behaves.
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Why C has Pointers. curixinfotech <curixinfotech.54dl3z@no-mx.forums.yourdomain.com.au> - 2011-12-07 07:22 -0500
Re: Why C has Pointers. Malcolm McLean <malcolm.mclean5@btinternet.com> - 2011-12-07 04:35 -0800
Re: Why C has Pointers. Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-12-07 07:58 -0500
Re: Why C has Pointers. Mike Manilone <zhtx10@gmail.com> - 2011-12-07 13:15 +0000
Re: Why C has Pointers. Bill Reid <hormelfree@gmail.com> - 2011-12-08 06:29 -0800
Re: Why C has Pointers. Vincenzo Mercuri <vincenzo.mercuri@hotmail.it> - 2011-12-07 14:50 +0100
Re: Why C has Pointers. Keith Thompson <kst-u@mib.org> - 2011-12-07 11:01 -0800
Re: Why C has Pointers. Vincenzo Mercuri <vincenzo.mercuri@yahoo.it> - 2011-12-07 20:49 +0100
Re: Why C has Pointers. Brian <nospam@nospam.com> - 2011-12-07 20:00 +0000
Re: Why C has Pointers. James Kuyper <jameskuyper@verizon.net> - 2011-12-07 15:30 -0500
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-07 12:43 -0800
Re: Why C has Pointers. Keith Thompson <kst-u@mib.org> - 2011-12-07 13:09 -0800
Re: Why C has Pointers. Vincenzo Mercuri <vincenzo.mercuri@hotmail.it> - 2011-12-08 19:39 +0100
Re: Why C has Pointers. James Kuyper <jameskuyper@verizon.net> - 2011-12-08 14:16 -0500
Re: Why C has Pointers. Vincenzo Mercuri <vincenzo.mercuri@hotmail.it> - 2011-12-08 20:23 +0100
Re: Why C has Pointers. Kaz Kylheku <kaz@kylheku.com> - 2011-12-07 20:48 +0000
Re: Why C has Pointers. Vincenzo Mercuri <vincenzo.mercuri@yahoo.it> - 2011-12-07 22:07 +0100
Re: Why C has Pointers. Jorgen Grahn <grahn+nntp@snipabacken.se> - 2011-12-10 07:34 +0000
Re: Why C has Pointers. August Karlstrom <fusionfile@gmail.com> - 2011-12-07 16:16 +0100
Re: Why C has Pointers. Borax Man <rotflol2@hotmail.com> - 2011-12-08 18:32 +1100
Re: Why C has Pointers. Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-12-08 00:20 -0800
Re: Why C has Pointers. André Gillibert <MetaEntropy.removeThis@gmail.com> - 2011-12-08 13:44 +0100
Re: Why C has Pointers. Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-12-09 05:04 -0800
Re: Why C has Pointers. gazelle@shell.xmission.com (Kenny McCormack) - 2011-12-09 14:59 +0000
Re: Why C has Pointers. Malcolm McLean <malcolm.mclean5@btinternet.com> - 2011-12-09 20:19 -0800
Re: Why C has Pointers. Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-12-10 07:36 -0800
Re: Why C has Pointers. Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-12-08 00:12 -0800
Re: Why C has Pointers. Cong Wang <xiyou.wangcong@gmail.com> - 2011-12-15 07:26 +0000
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-17 08:51 -0800
Re: Why C has Pointers. Barry Schwarz <schwarzb@dqel.com> - 2011-12-17 10:10 -0800
Re: Why C has Pointers. ralph <nt_consulting64@yahoo.net> - 2011-12-17 14:28 -0600
Re: Why C has Pointers. ralph <nt_consulting64@yahoo.net> - 2011-12-17 15:00 -0600
Re: Why C has Pointers. Keith Thompson <kst-u@mib.org> - 2011-12-17 13:55 -0800
Re: Why C has Pointers. ralph <nt_consulting64@yahoo.net> - 2011-12-17 18:27 -0600
Re: Why C has Pointers. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-12-18 01:26 +0000
Re: Why C has Pointers. ralph <nt_consulting64@yahoo.net> - 2011-12-17 22:34 -0600
Re: Why C has Pointers. "Philip Lantz" <prl@canterey.us> - 2011-12-17 23:48 -0800
Re: Why C has Pointers. superpollo <superpollo@tznvy.pbz> - 2011-12-18 11:06 +0100
Re: Why C has Pointers. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-12-18 13:44 +0000
Re: Why C has Pointers. pete <pfiland@mindspring.com> - 2011-12-18 11:05 -0500
Re: Why C has Pointers. James Kuyper <jameskuyper@verizon.net> - 2011-12-18 11:50 -0500
Re: Why C has Pointers. Keith Thompson <kst-u@mib.org> - 2011-12-18 13:10 -0800
Re: Why C has Pointers. ralph <nt_consulting64@yahoo.net> - 2011-12-27 16:03 -0600
Re: Why C has Pointers. Barry Schwarz <schwarzb@dqel.com> - 2011-12-27 14:40 -0800
Re: Why C has Pointers. Keith Thompson <kst-u@mib.org> - 2011-12-27 15:08 -0800
Re: Why C has Pointers. ralph <nt_consulting64@yahoo.net> - 2011-12-27 20:09 -0600
Re: Why C has Pointers. Keith Thompson <kst-u@mib.org> - 2011-12-27 23:00 -0800
Re: Why C has Pointers. Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-12-28 02:53 -0800
Re: Why C has Pointers. James Kuyper <jameskuyper@verizon.net> - 2011-12-28 08:39 -0500
Re: Why C has Pointers. James Kuyper <jameskuyper@verizon.net> - 2011-12-27 18:14 -0500
Re: Why C has Pointers. Keith Thompson <kst-u@mib.org> - 2011-12-27 15:36 -0800
Re: Why C has Pointers. James Kuyper <jameskuyper@verizon.net> - 2011-12-27 18:53 -0500
Re: Why C has Pointers. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2011-12-28 13:20 +0000
Re: Why C has Pointers. John Bode <jfbode1029@gmail.com> - 2011-12-28 08:42 -0800
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-17 20:36 -0800
Re: Why C has Pointers. Seebs <usenet-nospam@seebs.net> - 2011-12-18 04:56 +0000
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-17 21:37 -0800
Re: Why C has Pointers. Seebs <usenet-nospam@seebs.net> - 2011-12-18 05:42 +0000
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-18 08:08 -0800
Re: Why C has Pointers. Robert Wessel <robertwessel2@yahoo.com> - 2011-12-18 10:36 -0600
Re: Why C has Pointers. Seebs <usenet-nospam@seebs.net> - 2011-12-18 18:50 +0000
Re: Why C has Pointers. Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-12-19 02:39 -0800
Re: Why C has Pointers. gordonb.1i27q@burditt.org (Gordon Burditt) - 2011-12-21 22:34 -0600
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-19 07:56 -0800
Re: Why C has Pointers. John Tsiombikas <nuclear@member.fsf.org> - 2011-12-19 18:38 +0000
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-19 13:47 -0800
Re: Why C has Pointers. Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-12-28 02:58 -0800
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-28 03:21 -0800
Re: Why C has Pointers. Keith Thompson <kst-u@mib.org> - 2011-12-18 13:07 -0800
Re: Why C has Pointers. "io_x" <a@b.c.invalid> - 2011-12-19 04:02 +0100
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-17 15:35 -0800
Re: Why C has Pointers. ralph <nt_consulting64@yahoo.net> - 2011-12-17 18:41 -0600
Re: Why C has Pointers. John Bode <jfbode1029@gmail.com> - 2011-12-19 05:14 -0800
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-18 09:50 -0800
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2012-01-09 15:48 -0800
Re: Why C has Pointers. Nick Keighley <nick_keighley_nospam@hotmail.com> - 2012-01-10 00:21 -0800
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2012-01-10 01:03 -0800
Re: Why C has Pointers. Keith Thompson <kst-u@mib.org> - 2011-12-17 11:45 -0800
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-17 15:18 -0800
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-19 14:10 -0800
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-20 10:46 -0800
Re: Why C has Pointers. "Skybuck Flying" <Windows7IsOK@DreamPC2006.com> - 2011-12-17 19:15 +0100
Re: Why C has Pointers. "Skybuck Flying" <Windows7IsOK@DreamPC2006.com> - 2011-12-17 19:19 +0100
Re: Why C has Pointers. Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-12-19 02:48 -0800
Re: Why C has Pointers. 88888 Dihedral <dihedral88888@googlemail.com> - 2011-12-18 11:05 -0800
Re: Why C has Pointers. Nick Keighley <nick_keighley_nospam@hotmail.com> - 2011-12-19 02:48 -0800
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