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| From | Keith Thompson <kst-u@mib.org> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Why C has Pointers. |
| Date | 2011-12-17 11:45 -0800 |
| Organization | None to speak of |
| Message-ID | <lnty4z3rud.fsf@nuthaus.mib.org> (permalink) |
| References | <curixinfotech.54dl3z@no-mx.forums.yourdomain.com.au> <10353483.597.1324140693693.JavaMail.geo-discussion-forums@prez5> |
88888 Dihedral <dihedral88888@googlemail.com> writes:
> On Wednesday, December 7, 2011 8:22:56 PM UTC+8, curixinfotech wrote:
>> my Q is on the c language tell me now why c has pointers.
>> parshant
>
> 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
Why should the * be adjacent to "int"? My own preferred style (and, I
think, that of most C programmers) is:
int *ptr1;
The syntax of declarations in C follows (more or less) the syntax of
expressions; "declaration follows use". So "int *ptr;" says that *ptr
is an int -- from which it follows that ptr is a pointer to an int,
i.e., an int*.
This layout follows the grammar more closely, and makes an (admittedly
unwise) declaration like this:
int *pointer, not_a_pointer;
easier to understand.
But there is a school of thought that prefers to put the * next to the
type name. That seems to be the usual convention in C++. That's ok if
you avoid declaring multiple variables in one declaration (which you
should probably do anyway).
Note that the same principle might suggest:
int[3] arr;
which is of course a syntax error.
> int* ptr2; // not initialized
>
> ptr1=a; // Read this as to make ptr1 to be an alias of a.
That's not necessarily a good way to think of it. Since "a" is an array
object the expression consisting of its name decays to a pointer to its
first element. That pointer value is then stored in ptr1. ptr1 points
to the first element of a. To show that it's not a true alias, consider
sizeof a vs. sizeof ptr1.
> for(i=0;i<3;i++) printf("The value of a[%d]=%d.\n", i, a[i]);
> for(i=0;i<3;i++) printf("The value of ptr1[%d]=%d.\n", i, ptr1[i]);
> //This is a shadow copy.
> ptr1[0]=6; ptr2[1]=6;
> for(i=0;i<3;i++) printf("The value of a[%d]=%d.\n", i, a[i]);
> // a[0]=? a[1]=?
>
> //Deep copy is different.
> ptr2=malloc(3*sizeof(int)); // get another array from the heap
> // if (ptr2==NULL) then raise an error!!!
> memcpy(ptr2,ptr1,3*sizeof(int)); // make a deep copy
> ptr2[0]=-6; *(ptr+2)=-6;
>
> for(i=0;i<3;i++) printf("The value of a[%d]=%d.\n", i, a[i]);
> for(i=0;i<3;i++) printf("The value of ptr1[%d]=%d.\n", i, ptr1[i]);
> for(i=0;i<3;i++) printf("The value of ptr2[%d]=%d.\n", i, ptr2[i]);
Read section 6 of the comp.lang.c FAQ, <http://c-faq.com>.
--
Keith Thompson (The_Other_Keith) kst-u@mib.org <http://www.ghoti.net/~kst>
Will write code for food.
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
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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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