Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > comp.lang.c > #14674

Re: Why C has Pointers.

Show all headers | View raw

On Sunday, December 18, 2011 9:26:59 AM UTC+8, Ben Bacarisse wrote:
> ralph <nt_cons...@yahoo.net> writes:
> 
> > On Sat, 17 Dec 2011 13:55:44 -0800, Keith Thompson <ks...@mib.org>
> > wrote:
> >
> >>ralph <nt_cons...@yahoo.net> writes:
> >>[...]
> >>> 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"
> >>
> >>That doesn't really make sense.  An array variable is not a constant:
> >>
> >>    int arr[10];
> >>    arr[5] = 42;
> >>
> >>I suppose what it means is that an array name, when it appears in an
> >>expression (in most but not all context) decays to a pointer to the
> >>array's first element, and that pointer value cannot be modified.  But I
> >>think that saying an array is a constant just reinforces the confusion
> >>between arrays and pointers.
> >>
> >
> > I disagree. 
> >
> > Once the beginner gets it through his head that you can never do an
> > assignment to 'arr' you resolve a ton of the questions we get around
> > here. That as close to the definition of a "constant" as you can get.
> 
> I'd say that "unchanging" or "immutable" were closer definitions, and
> arrays can be changed.
> 
> What you call a constant, I'd call a value, and I'd explain that
> array-valued expressions are (usually) converted automatically to
> pointer values.
> 
> Most people learning programming quickly get the idea of variables and
> values.  Variables can be assigned to, but values can't.  This avoids
> calling arrays "constant".  It's also a start towards some more
> technical matters such as why
> 
>   int x;
>   x = 0;
> 
> is ok, but
> 
>  (int)x = 0;
> 
> is not.
> 
> >>> 3) The array name evaluates to the address of the first element in the
> >>> array.
> >>
> >>This applies to any expression of array type, not just the name of a
> >>declared array object.  But it doesn't apply in all contexts.
> >>Specifically, an expression of array type is converted to a pointer to
> >>the array object's first element *unless* it's:
> >>
> >>    The operand of a unary "sizeof" operator (sizeof arr gives you the
> >>    same of the array, not of a pointer);
> >>
> >>    The operand of a unary "&" operator (&arr gives you the address of
> >>    the array object; it's the same memory location as the address of
> >>    its first element, but it's of a different type);
> >>
> >>    A string literal in an initializer used to initialize an array
> >>    object.  In
> >>        char arr[6] = "hello";
> >>    the *value* of "hello" is copied into arr.
> >>
> >
> > Yep. But what's the big deal. 
> >
> > And of course I disagree that it actually matters to the beginner that
> > the address is of a "different type". Semantically and syntactically
> > it comes out the same. Plenty of time to get into vague language
> > later.
> 
> I agree about putting this off, but 'arr' and '&arr' are not the same
> semantically.  If they were,
> 
>   arr + 1
> 
> and
> 
>   &arr + 1
> 
> would have the same value but they don't.  (I'm using arr as defined
> above.)
> 
> <snip>
> >>In most contexts; see above.  It would be clearer to say that an array
> >>name *decays to* an address (or pointer value).  And I wouldn't use the
> >>word "initialized".
> >
> > I would. And again what does 'decay' meant to the beginner?
> > In code the name "arr" is changed to, or evaluates to, the address of
> > the first element.
> 
> I'd just say "converted".  C has lots of automatic conversions and,
> sooner or later, these need to be explains.  The array to pointer
> conversion fits into this general pattern.
> 
> <snip>
> >>> 9) The array operator evaluates to a storage map equation, eg.
> >>>     type ar[i] := *(a + (i * sizeof(type))
> >>
> >>I'm not sure what syntax you're using.  What does "type" mean here?  And
> >>C pointer arithmetic is already scaled by the size of the pointed-to
> >>type.
> >>
> >>arr[i] is by definition equivalent to *(arr + i).
> >
> > Think about it. 
> >
> > You say that now, but you come back later in other response to point
> > out "sizes". That "already scaled" is because the compiler knows the
> > type, and he knows the type because you declared it.
> 
> It's not easy to explain the lower-level arithmetic without reference to
> some particular machine.  For example, on a machine with, say, byte and
> word addresses, you can't describe it as you have done.  By all means,
> take a simple architecture and explain what a[0] means on one particular
> system using addresses and size calculations, but you will also have to
> explain, eventually, that arr[i] is, by definition, *(arr + i).
> 
> <snip>
> -- 
> Ben.


In C there is no boundary checking  in the index of an array in a read 
or write operation to the array.






 




   

Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

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

csiph-web