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“Why do arrays start at 0?"

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  “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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#86137

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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#86131

Op 27.aug..2022 om 22:01 schreef Lynn McGuire:
> 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).  Or, the x is 
> subtracted from first: x--; *(x+i);.
> 
> Lynn
> 

I don't understand the idea of x--. Why modifying x? What happens if x 
is indexed later again?

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#86132

Fred. Zwarts <F.Zwarts@KVI.nl> schrieb:
> Op 27.aug..2022 om 22:01 schreef Lynn McGuire:
>> 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).  Or, the x is 
>> subtracted from first: x--; *(x+i);.
>> 
>> Lynn
>> 
>
> I don't understand the idea of x--. Why modifying x? What happens if x 
> is indexed later again?

The idea is to use this modified pointer for one-based array
accesses, so that it would be possible to translate Fortran's A(1)
into a[1] on the C side, or A(N) into a[n].

The correct way to do this according to the C standard would be to
translate A(N) into a[n-1] on the C side.  There are several reasons
why this might not have been done:  Readability of the generated
code (although f2c code is already hard to read), because it made
the code slower with compilers of the day, and probably because it
"just worked" with the compilers.

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#86133

Am 26.08.2022 um 22:49 schrieb 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

On the CPU-level you heave the least number of calculations to
determine an address of an indexed entity if the index starts
at zero.

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#86143 — Re: ???Why do arrays start at 0?"

In comp.lang.c++ Lynn McGuire <lynnmcguire5@gmail.com> wrote:
> ???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.

I don't know if it's the *original* reason, but I would assume that at least
in C one of the main reasons is the principle of maximum efficiency.

In many processor architectures the concept of "array" exists, at least
when it comes to values of the register sizes (ie. usually 1-byte,
2-byte, 4-byte and 8-byte elements, the last one at least on 64-bit
architectures). Prominently the concept of an indexable array exists
in the x86 architecture. (I don't remember now if it also exists in
the ARM architecture, but I would guess so.)

Generally when a processor architecture supports the concept of an "array",
it does so by having instructions that take (at least) two registers as
the input or the output parameter: A base address, and an offset. The
memory location of the element is calculated by adding those two. (The
number of bytes that an offset of 1 jumps depends on the instruction,
and thus multi-byte elements are supported.)

Thus zero-indexing is extraordinarily natural in processor architectures:
The "index" is actually an offset. It's a value you add to the base
address in order to get to the location you want. Thus, the first element
is at index/offset 0.

Since that's the case, the most optimal way to handle low-level arrays is
to have 0-based indexing in the programming language as well. That way
you don't need to be subtracting 1 from the index every time an array is
accessed (or you don't need an extraneous unused element at the beginning
of the array, consuming memory for no reason).

Also, since C supports pointer arithmetic, many operations become simpler.
Such as getting the index of an element when what you have is a pointer to
it (and the pointer to the start of the array).

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