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Re: “Why do arrays start at 0?"

From Paavo Helde <eesnimi@osa.pri.ee>
Newsgroups comp.lang.fortran, comp.lang.c++
Subject Re: “Why do arrays start at 0?"
Date 2022-08-28 19:43 +0300
Organization A noiseless patient Spider
Message-ID <teg5v0$lrls$1@dont-email.me> (permalink)
References <tebblf$2gpm$1@dont-email.me> <tedr4l$1cc0$1@gioia.aioe.org> <tedt6n$757$1@gioia.aioe.org> <tee5i3$cqm$4@newsreader4.netcologne.de> <teeo2r$8tl$1@gioia.aioe.org>

Cross-posted to 2 groups.

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28.08.2022 06:40 Lynn McGuire kirjutas:
> On 8/27/2022 5:24 PM, Thomas Koenig wrote:
>> Lynn McGuire <lynnmcguire5@gmail.com> schrieb:
>>> On 8/27/2022 2:26 PM, Fred. Zwarts wrote:
>>>> Op 26.aug..2022 om 22:49 schreef Lynn McGuire:
>>>>> “Why do arrays start at 0?"
>>>>> https://buttondown.email/hillelwayne/archive/why-do-arrays-start-at-0/
>>>>>
>>>>> "It's not the reason you think. No, it's not that reason either.”
>>>>>
>>>>> My Fortran starts at one.  My C++ starts at zero.  This has made my
>>>>> life hell.
>>>>>
>>>>> Lynn
>>>>>
>>>>
>>>> I assumed that it was done because in C x[i] is equivalent to *(x+i).
>>>
>>> Yup.  So Fortran x(i) is equivalent to *(x+i-1).
>>
>> To be more precise, x(i) is equivalent to *(x+i-lbound(x,1))
>>
>>> Or, the x is
>>> subtracted from first: x--; *(x+i);.
>>
>> That's an f2c idiom, which is not valid C, AFAIK, because
>> the pointer would point before the actual array.
> 
> While the second pointer is a bad pointer, it is not a illegal pointer. 

The C++ standard (n4861) calls this "an invalid pointer value".

For pointers which continue to point to freed objects, it says "A 
pointer value becomes invalid when the storage it denotes reaches the 
end of its storage duration".

About invalid pointers it says:
"Indirection through an invalid pointer value and passing an invalid 
pointer value to a deallocation function have undefined behavior. Any 
other use of an invalid pointer value has implementation-defined behavior."

There is also a footnote:
"Some implementations might define that copying an invalid pointer value 
causes a system-generated runtime fault."

So, while technically one might get away with the "x--" trick most of 
the time with the linear memory addressing used by mainstream 
implementations nowadays, still various diagnostic tools would mark 
these as invalid pointers, causing an avalanche of errors whenever you 
want to solve your actual memory access problems.

I guess it might also subvert automatic garbage collection which is 
sometimes used with C++. There is a special case of "safely-derived" 
pointer values which I suspect is made exactly for making GC possible, 
and changing the pointer value to x-1 would apparently ruin this.

And with segmented memory, like with 16-bit x86, it might cause all kind 
of surprises.

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Thread

“Why do arrays start at 0?" Lynn McGuire <lynnmcguire5@gmail.com> - 2022-08-26 15:49 -0500
  Re: “Why do arrays start at 0?" Gary Scott <garylscott@sbcglobal.net> - 2022-08-26 16:56 -0500
    Re: “Why do arrays start at 0?" Mr Flibble <flibble@reddwarf.jmc.corp> - 2022-08-27 00:46 +0100
      Re: “Why do arrays start at 0?" d thiebaud <thiebauddick2@aol.com> - 2022-08-26 22:29 -0400
        Re: “Why do arrays start at 0?" gah4 <gah4@u.washington.edu> - 2022-08-26 19:34 -0700
        Re: “Why do arrays start at 0?" Robin Vowels <robin.vowels@gmail.com> - 2022-08-26 20:34 -0700
        Re: “Why do arrays start at 0?" Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-08-26 22:05 -0700
          Re: “Why do arrays start at 0?" Mr Flibble <flibble@reddwarf.jmc.corp> - 2022-08-27 12:06 +0100
    Re: “Why do arrays start at 0?" gah4 <gah4@u.washington.edu> - 2022-08-26 18:24 -0700
    Re: Re: “Why do arrays start at 0?" scott@slp53.sl.home (Scott Lurndal) - 2022-08-27 14:28 +0000
    Re: �Why do arrays start at 0?" Charlie Roberts <croberts@gmail.com> - 2022-08-28 10:25 -0400
  Re: “Why do arrays start at 0?" Louis Krupp <lkrupp@invalid.pssw.com.invalid> - 2022-08-26 16:08 -0600
    Re: �Why do arrays start at 0?" Charlie Roberts <croberts@gmail.com> - 2022-08-28 10:28 -0400
      Re: “Why do arrays start at 0?" Thomas Koenig <tkoenig@netcologne.de> - 2022-08-28 16:30 +0000
      Re: “Why do arrays start at 0?" Ron Shepard <nospam@nowhere.org> - 2022-08-29 02:42 -0500
        Re: “Why do arrays start at 0?" Thomas Koenig <tkoenig@netcologne.de> - 2022-08-30 17:18 +0000
          Re: “Why do arrays start at 0?" gah4 <gah4@u.washington.edu> - 2022-08-30 12:15 -0700
  Re: “Why do arrays start at 0?" Mr Flibble <flibble@reddwarf.jmc.corp> - 2022-08-27 00:44 +0100
  Re: “Why do arrays start at 0?" Robin Vowels <robin.vowels@gmail.com> - 2022-08-26 20:31 -0700
    Re: “Why do arrays start at 0?" "fiz...@gmail.com" <fiziqs@gmail.com> - 2022-08-27 01:48 -0700
      Re: “Why do arrays start at 0?" gah4 <gah4@u.washington.edu> - 2022-08-27 02:17 -0700
        Re: “Why do arrays start at 0?" Thomas Koenig <tkoenig@netcologne.de> - 2022-08-27 09:30 +0000
          Re: “Why do arrays start at 0?" gah4 <gah4@u.washington.edu> - 2022-08-27 10:01 -0700
            Re: “Why do arrays start at 0?" Thomas Koenig <tkoenig@netcologne.de> - 2022-08-27 22:13 +0000
              Re: “Why do arrays start at 0?" gah4 <gah4@u.washington.edu> - 2022-08-27 22:26 -0700
                Re: “Why do arrays start at 0?" Thomas Koenig <tkoenig@netcologne.de> - 2022-08-28 06:59 +0000
                Re: “Why do arrays start at 0?" Lynn McGuire <lynnmcguire5@gmail.com> - 2022-08-28 13:42 -0500
                Re: “Why do arrays start at 0?" Thomas Koenig <tkoenig@netcologne.de> - 2022-08-28 20:17 +0000
                Re: “Why do arrays start at 0?" gah4 <gah4@u.washington.edu> - 2022-08-28 14:08 -0700
                Re: “Why do arrays start at 0?" gah4 <gah4@u.washington.edu> - 2022-08-28 14:05 -0700
                Re: “Why do arrays start at 0?" Robin Vowels <robin.vowels@gmail.com> - 2022-08-28 21:54 -0700
                Re: “Why do arrays start at 0?" Robin Vowels <robin.vowels@gmail.com> - 2022-08-28 21:53 -0700
              Re: “Why do arrays start at 0?" Ron Shepard <nospam@nowhere.org> - 2022-08-29 02:07 -0500
                Re: “Why do arrays start at 0?" Robin Vowels <robin.vowels@gmail.com> - 2022-08-29 01:30 -0700
                Re: “Why do arrays start at 0?" Ron Shepard <nospam@nowhere.org> - 2022-08-29 10:13 -0500
                Re: “Why do arrays start at 0?" Robin Vowels <robin.vowels@gmail.com> - 2022-08-29 12:30 -0700
                Re: “Why do arrays start at 0?" gah4 <gah4@u.washington.edu> - 2022-08-29 03:48 -0700
      Re: “Why do arrays start at 0?" Robin Vowels <robin.vowels@gmail.com> - 2022-08-27 05:16 -0700
  Re: “Why do arrays start at 0?" Thomas Koenig <tkoenig@netcologne.de> - 2022-08-27 05:47 +0000
    Re: “Why do arrays start at 0?" d thiebaud <thiebauddick2@aol.com> - 2022-08-27 15:19 -0400
      Re: “Why do arrays start at 0?" FortranFan <parekhvs@gmail.com> - 2022-08-27 12:45 -0700
      Re: “Why do arrays start at 0?" Thomas Koenig <tkoenig@netcologne.de> - 2022-08-27 22:22 +0000
  Re: “Why do arrays start at 0?" "Fred. Zwarts" <F.Zwarts@KVI.nl> - 2022-08-27 21:26 +0200
    Re: “Why do arrays start at 0?" Lynn McGuire <lynnmcguire5@gmail.com> - 2022-08-27 15:01 -0500
      Re: “Why do arrays start at 0?" Thomas Koenig <tkoenig@netcologne.de> - 2022-08-27 22:24 +0000
        Re: “Why do arrays start at 0?" John <urbanjost@comcast.net> - 2022-08-27 19:34 -0700
          Re: “Why do arrays start at 0?" Robin Vowels <robin.vowels@gmail.com> - 2022-08-28 01:50 -0700
        Re: “Why do arrays start at 0?" Lynn McGuire <lynnmcguire5@gmail.com> - 2022-08-27 22:40 -0500
          Re: “Why do arrays start at 0?" Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2022-08-27 22:07 -0700
            Re: “Why do arrays start at 0?" gah4 <gah4@u.washington.edu> - 2022-08-27 23:04 -0700
          Re: “Why do arrays start at 0?" Thomas Koenig <tkoenig@netcologne.de> - 2022-08-28 07:50 +0000
            Re: “Why do arrays start at 0?" David Brown <david.brown@hesbynett.no> - 2022-08-28 11:52 +0200
          Re: “Why do arrays start at 0?" Paavo Helde <eesnimi@osa.pri.ee> - 2022-08-28 19:43 +0300
      Re: “Why do arrays start at 0?" "Fred. Zwarts" <F.Zwarts@KVI.nl> - 2022-08-28 10:01 +0200
        Re: “Why do arrays start at 0?" Thomas Koenig <tkoenig@netcologne.de> - 2022-08-28 08:14 +0000
  Re: “Why do arrays start at 0?" Bonita Montero <Bonita.Montero@gmail.com> - 2022-08-28 11:07 +0200
    Re: “Why do arrays start at 0?" gah4 <gah4@u.washington.edu> - 2022-08-28 02:55 -0700
    Re: “Why do arrays start at 0?" Robin Vowels <robin.vowels@gmail.com> - 2022-08-28 05:47 -0700
  Re: ???Why do arrays start at 0?" Juha Nieminen <nospam@thanks.invalid> - 2022-08-29 11:15 +0000
    Re: ???Why do arrays start at 0?" gah4 <gah4@u.washington.edu> - 2022-08-29 22:59 -0700

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