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Re: Why C has Pointers.

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On 12/27/2011 05:03 PM, ralph wrote:
> On Sun, 18 Dec 2011 13:10:27 -0800, Keith Thompson <kst-u@mib.org>
> wrote:
> 
>> ralph <nt_consulting64@yahoo.net> writes:
>>> On Sun, 18 Dec 2011 01:26:59 +0000, Ben Bacarisse
>>> <ben.usenet@bsb.me.uk> wrote:
>> [...]
>>>> 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).
>>>>
>>>
>>> Sure it is because it ain't *(arr + i). It is ...
>>
>> Yes, it is.  C99 6.5.2.1p2:
>>
>>    The definition of the subscript operator [] is that E1[E2] is
>>    identical to (*((E1)+(E2))).
>>
>>> *(arr + ( i * sizeof(datatype)))
>>>
>>> So for int arr[10];
>>> arr[5] is *(arr + (5 * sizeof(int)))
>>> byte arr[10];
>>> arr[5] is *(arr + (5 * sizeof(byte)))
>>
>> You are using your own invented "+" operator that works in units of
>> byte, rather than the actual C "+" operator that works in units of the
>> size of the pointed-to type.  That's fine, but only if you explicitly
>> state that "+" doesn't mean what it means in C.
> 
> [So an 'actual' unit of "size of the pointed-to-type" doesn't have the
> same meaning as an 'invented' unit of "sizeof(<declared>type))".

I can't tell whether those things have the same meaning, because I don't
know what you mean by those terms. In C, when adding an integer (like
5*sizeof(int)) to a pointer to some type (like arr), the integer is
interpreted as counting units of the pointed-at type. Therefore on an
implementation where sizeof(int) is 2, then the expression arr+10 will
point at the 11th 'int' in arr, which in this case is one past the end
of the array.

> ... If
> you look closer you will will see there is not a single pointer in my
> statement nor in the Standard - there ain't NO "pointed-to-type" and
> that is the whole point. <bg>]

There's lots of pointers in the Standard - it's just plain nonsense to
claim otherwise. I'm going to assume that you meant something else. My
best guess is that you meant "there is not a single pointer in my
statement as interpreted in accordance with the Standard". Please let me
know if that guess is incorrect.

Item 9 in your response to Barry Schwartz on 2011-12-17 used := as if it
were an operator. Therefore, it was pretty clear that you weren't using
C syntax, even if it wasn't clear what the syntax you were using was
supposed to mean. If you say that your statement doesn't contain any
pointers, I'll have to defer to your superior knowledge of the syntax
you were using.

However, in later messages you used or talked about the expressions
arr[i], and *(arr+i), and *(arr+5*sizeof(int)). These look exactly like
normal C expressions. In a message posted to a newsgroup devoted to C,
we're entitled to assume that they carry their normal C meaning.

According to that standard, "an expression that has type ‘‘array of
type’’ is converted to an expression with type ‘‘pointer to type’’ that
points to the initial element of the array object" (6.3.2.1p3). There
are exceptions to that rule, but none of the exceptions apply in any of
these three expressions. In all three of them, "arr" is an expression
that has the type "array of int", so there most definitely is a pointer
to int in each of those expressions, and also a pointed-to type: "int".

> When I mentioned this phenomena as an aid for new programmers, I used
> the term "storage map equation" which I assumed everyone understood as
> an implementation context.

I've never heard the term "storage map equation" before. My personal
guess was that your statement that

	arr[5] is *(arr + (5 * sizeof(int)))

(which is false according to C syntax and semantics on any
implementation where sizeof(int)>1) corresponds the statement

	arr[5] is *(int*)((char*)arr + 5*sizeof(char))

(which is guaranteed to be true on all implementations according to C
syntax and semantics). In a newsgroup associated with the C programming
language, it would have saved a lot of trouble if you'd actually written
it that way.

> The concepts behind ...
> 
>  datatype arr[i]  ~==  *(arr + ( i * sizeof(datatype))) 

Your use of '~==' as if it were an operator again clarifies that you're
not using C syntax and semantics. If you insist on using your own
made-up syntax, you should provide an explanation of what it means. This
statement looks very similar to an earlier one, except that "~=="
replaces ":=". What is the significance of that change?

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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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