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Re: “Why do arrays start at 0?"

Cross-posted to 2 groups.

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Lynn McGuire <lynnmcguire5@gmail.com> schrieb:
> 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. 
> Variables can point to anywhere, they are not illegal until referenced. 

Unfortunately not.

Looking at n2596.pdf, one finds under J.2, "Undefined behavior",

— Addition or subtraction of a pointer into, or just beyond,
an array object and an integer type produces a result that does
not point into, or just beyond, the same array object (6.5.6).

> Otherwise, code would be crashing all over the place.

Undefined behavior does not mean that the code will reliably crash.
It just says that the C standard gives no guarantee about what will
happen, and, even if it works right now, such code is at the mercy
of future compiler revisions, cosmic rays, and other forseen and
unforseen circumstances.

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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?" 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: 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?" Louis Krupp <lkrupp@invalid.pssw.com.invalid> - 2022-08-26 16:08 -0600
  Re: “Why do arrays start at 0?" Gawr Gura <gawrgura@mail.hololive.com> - 2022-08-26 15:18 -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?" 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?" 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?" 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?" 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?" Juha Nieminen <nospam@thanks.invalid> - 2022-08-29 11:15 +0000

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