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From: Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: "undefined behavior"?
Date: Wed, 12 Jun 2024 15:30:52 -0700
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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.

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

DFS, since you've been posting in comp.lang.c for at least ten years,
I'm surprised you're having difficulties with this.

[...]

-- 
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
void Void(void) { Void(); } /* The recursive call of the void */