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Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ?

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On 1/13/2013 22:08, Keith Thompson wrote:
> Shao Miller <sha0.miller@gmail.com> writes:
>> On 1/13/2013 19:57, Keith Thompson wrote:
>>> Shao Miller <sha0.miller@gmail.com> writes:
>>>> On 1/13/2013 18:39, Keith Thompson wrote:
>>> [...]
>>>>> So you'd rather iterate over every element of the array, and then
>>>>> test i%10 to decide whether to process the current one?
>>>>>
>>>>>        for (i = 0; i < sizeof arr / sizeof arr[0]; i ++) {
>>>>>            if (i % 10 == 0) {
>>>>>                /* ... */
>>>>>            }
>>>>>        }
>>>>>
>>>>> I find that more verbose, less clear, and almost certainly less
>>>>> efficient (unless the compiler is clever enough to transform it
>>>>> into the first form).  All to solve a problem that is not a problem
>>>>> at all.
>>>>>
>>>>> If there's some concern about i overflowing the upper limit of its
>>>>> type (INT_MAX or SIZE_MAX), I'd probably precompute the last index
>>>>> I want to look at and use that to control the loop.  But I see no
>>>>> point in doing so just to avoid letting i exceed the upper bound
>>>>> of the array, when the code explicitly avoids accessing anything
>>>>> outside the array bounds.
>>>>
>>>> No, I was talking about if the loop was for initialization.  In such a
>>>> case, I'd initialize every element (advancing by 1), but if something
>>>> special was needed for every tenth, I could determine that with your 'if'.
>>>
>>> Who says the loop is for initialization?  It might not even be writing
>>> to the array.
>>
>> I did.  "The loop body for this could very well initialize ten elements
>> at a time, but I'd probably advance 'i' by 1 instead of by 10, and use
>> the '% 10' operator in the loop's body, if need be."  (If you'll forgive
>> the 10 with "operator")  I'm saying "if, then", not making a claim about
>> the loop's purpose.
>
> My point is that you were making an unwarranted assumption.  The example
> illustrates iterating over every 10 (or N) elements of an array.
>

For the second time, I'm saying "if, then". :)  I'm handling a 
possibility with a response.

Why didn't you suggest that the 'for' loop has nothing to do with 
iterating over the array?  Maybe the programmer simply wants to load a 
value into 'i' and knows something about the length of the array that 
can help them to get the right value in there, when combined with the 
'for' loop's expression-3?

>>>> For just _working_ with an already-initialized array, I'd probably use
>>>> precisely this loop (though I might possibly be more inclined to put it
>>>> in a function and pass the count).  BUT, as was snipped, if the count of
>>>> 'arr' is 4, then the programmer _really_ ought to have their head
>>>> checked. :)
>>>
>>> Absurd.
>>>
>>> A function that, for whatever reason, needs to access every 10th
>>> (or every Nth) element, can perfectly sensibly work on an array of
>>> any size.  If the array has 10 or fewer elements, the function just
>>> accesses element 0.  I see no reason to treat that as a special case.
>>
>> If you are talking about:
>>
>>     void some_func1(void) {
>>         int arr[4];
>>         int i;
>>
>>         for (i = 0; i < sizeof arr / sizeof arr[0]; i += 10)
>>           /* Anything */
>>       }
>>
>> then I'd certainly say it's silly!  We can see right away that the
>> advancement of 'i' is silly!  Consider what kind of code review you'd
>> offer someone about this.
>
> It's common to use "magic numbers" (4 and 10 in this case) for code
> snippets meant to illustrate a point.  In real life, clearly the
> value would be either a named constant or a value computed at run
> time (in the latter case allocating the array is more complicated).
>
> Is that the trivial point you've been trying to make all this time?
>
> Ok, suppose we replace 4 by ARRAY_SIZE, a macro whose value
> is determined by some build-time configuration process.  Maybe
> ARRAY_SIZE happens to be 4.
>
> Or suppose it's a constant value 95, which will result in the final
> value of i being 100, past the end of the array.
>

Yes.  Using 'ARRAY_SIZE' with that loop strikes me as waiting for an 
accident.

Some day, someone else could look at the loop and think, "Oh, it seems 
that this array is always going to be a multiple of ten, in length."  I 
don't think a quick glance would make that an unfair assumption.

Also bad: Someone _worries_ about it being out-of-bounds and wastes time 
looking at it, because it's suspicious and they recently had a crash.

>> If you are talking about:
>>
>>     void some_func1(int * arr, size_t n) {
>>         int i;
>>
>>         for (i = 0; i < n; i += 10)
>>           /* Anything */
>>       }
>>
>> then I'd certainly say it's not silly at all!
>
> What I *thought* you were objecting to is having i end up with a
> value past the last valid index of arr, to the point where you'd
> want a compiler to warn you about it.  Both code snippets quoted
> above have the same non-problem.
>

There is another C-devoted forum that I spend some time in.  I see a lot 
of code flow by.  When I see weird loops, I usually mention them to the 
programmer.  Some day, maybe a compiler could take care of that bit, as 
a quick pass. :)

-- 
- Shao Miller
--
"Thank you for the kind words; those are the kind of words I like to hear.

Cheerily,"  -- Richard Harter

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[MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? m.labanowicz@gmail.com - 2013-01-09 07:26 -0800
  Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? James Kuyper <jameskuyper@verizon.net> - 2013-01-09 11:30 -0500
  Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Eric Sosman <esosman@comcast-dot-net.invalid> - 2013-01-09 11:58 -0500
    Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Robert Wessel <robertwessel2@yahoo.com> - 2013-01-09 12:27 -0600
    Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Keith Thompson <kst-u@mib.org> - 2013-01-09 11:22 -0800
  Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-09 14:39 -0500
  Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? m.labanowicz@gmail.com - 2013-01-10 02:40 -0800
    Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-10 08:31 -0500
      Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-10 08:49 -0500
        Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? m.labanowicz@gmail.com - 2013-01-10 06:48 -0800
          Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-10 10:33 -0500
      Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Keith Thompson <kst-u@mib.org> - 2013-01-11 08:19 -0800
        Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-11 12:46 -0500
          Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Keith Thompson <kst-u@mib.org> - 2013-01-11 14:04 -0800
            Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-11 22:42 +0000
              Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-12 01:44 +0000
            Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-11 19:30 -0500
              Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-11 20:11 -0500
              Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-12 01:17 +0000
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-11 20:46 -0500
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-12 03:14 +0000
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Keith Thompson <kst-u@mib.org> - 2013-01-12 14:17 -0800
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-12 18:02 -0500
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Keith Thompson <kst-u@mib.org> - 2013-01-12 15:13 -0800
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-13 16:36 -0500
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Keith Thompson <kst-u@mib.org> - 2013-01-13 15:39 -0800
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-13 19:17 -0500
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Keith Thompson <kst-u@mib.org> - 2013-01-13 16:57 -0800
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-13 20:50 -0500
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Keith Thompson <kst-u@mib.org> - 2013-01-13 19:08 -0800
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-14 00:00 -0500
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-14 21:21 +0000
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-14 17:50 -0500
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-14 01:23 +0000
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-13 21:04 -0500
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Keith Thompson <kst-u@mib.org> - 2013-01-13 19:13 -0800
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2013-01-14 21:37 +0000
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Keith Thompson <kst-u@mib.org> - 2013-01-11 18:28 -0800
              Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Keith Thompson <kst-u@mib.org> - 2013-01-11 18:40 -0800
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-12 04:30 -0500
    Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-10 14:37 +0000
      Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? m.labanowicz@gmail.com - 2013-01-10 06:51 -0800
        Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? m.labanowicz@gmail.com - 2013-01-10 07:41 -0800
          Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2013-01-10 16:55 +0000
          Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? Shao Miller <sha0.miller@gmail.com> - 2013-01-10 17:41 -0500
            Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? m.labanowicz@gmail.com - 2013-01-11 03:03 -0800
              Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? m.labanowicz@gmail.com - 2013-01-11 04:14 -0800
                Re: [MUDFLAP] Is sizeof(ARRAY[0]) equivalent to sizeof(*ARRAY) ? James Kuyper <jameskuyper@verizon.net> - 2013-01-11 08:32 -0500

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