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Date: Wed, 12 Jun 2024 19:07:29 -0400
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Subject: Re: "undefined behavior"?
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On 6/12/2024 6:30 PM, Keith Thompson wrote:
> DFS <nospam@dfs.com> writes:
>> On 6/12/2024 5:30 PM, Barry Schwarz wrote:
>>> On Wed, 12 Jun 2024 16:47:23 -0400, DFS <nospam@dfs.com> wrote:
>>>
>>>> Wrote a C program to mimic the stats shown on:
>>>>
>>>> https://www.calculatorsoup.com/calculators/statistics/descriptivestatistics.php
>>>>
>>>> My code compiles and works fine - every stat matches - except for one
>>>> anomaly: when using a dataset of consecutive numbers 1 to N, all values
>>>>> 40 are flagged as outliers.  Up to 40, no problem.  Random numbers
>>>> dataset of any size: no problem.
>>>>
>>>> And values 41+ definitely don't meet the conditions for outliers (using
>>>> the IQR * 1.5 rule).
>>>>
>>>> Very strange.
>>>>
>>>> Edit: I just noticed I didn't initialize a char:
>>>> before: char outliers[100];
>>>> after : char outliers[100] = "";
>>>>
>>>> And the problem went away.  Reset it to before and problem came back.
>>>>
>>>> Makes no sense.  What could cause the program to go FUBAR at data point
>>>> 41+ only when the dataset is consecutive numbers?
> 
>>>> Also, why doesn't gcc just do you a solid and initialize to "" for you?
>>> Makes perfect sense.  The first rule of undefined behavior is
>>> "Whatever happens is exactly correct."  You are not entitled to any
>>> expectations and none of the behavior (or perhaps all of the behavior)
>>> can be called unexpected.
>>
>> I HATE bogus answers like this.
>>
>> Aren't you embarrassed to say things like that?
> 
> He has nothing to be embarrassed about.  What he wrote is correct.

No it's not.

"Whatever happens is exactly correct." is nonsense.

"You are not entitled to any expectations" is nonsense.




> The C standard's definition of "undefined behavior" is "behavior, upon
> use of a nonportable or erroneous program construct or of erroneous
> data, for which this International Standard imposes no requirements".
> 
> If you don't like the way C deals with undefined behavior, that's
> perfectly valid, and a lot of people are likely to agree with you.

Thanks for feeling my pain!

It's frustrating.  By now I spent a half-hour dealing with it.  gcc 
could've just filled the char[] variable with 0s by default.  I bet that 
would save a LOT of people time and headaches.


> But I advise against lashing out at people who are correctly explaining
> what the C standard says.

The C standard really says "Whatever happens is exactly correct."?



> DFS, since you've been posting in comp.lang.c for at least ten years,

Time flies.

How do you know I've posted here that long?



> I'm surprised you're having difficulties with this.

I'm surprised at some of the wonkiness of gcc and C.

* warns relentlessly when the printf specifier doesn't match the var
   type, but gives no warning when you use an int with memset (instead of
   the size_t specified in the function prototype).

* a missing bracket } throws 50 nonsensical compiler errors.

* warns of unused vars but not uninitialized ones

* one uninitialized var makes your program do crazy things.  Worse than
   crazy is it's identically crazy each time.

../prog 40 -c
outliers: none

../prog 41 -c
outliers: 41

../prog 42 -c
outliers: 41 42

../prog 43 -c
outliers: 41 42 43

../prog 44 -c
outliers: 41 42 43 44

etc.  And none were outliers - not even close.

At least if it showed nonsense data it would be easier to track down. 
Maybe.

The thing is, none of those values (40+) were ever in that char[] prior 
to running the code for a set of 50 consecutive values.

And I edited/compiled the code many times, but still got the identical 
error.

I doubt my environment (gcc 11.4 on Windows Subsys for Linux on Ubuntu) 
has anything to do with it.