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From: Janis Papanagnou <janis_papanagnou+ng@hotmail.com>
Newsgroups: comp.lang.c
Subject: Re: "undefined behavior"?
Date: Wed, 12 Jun 2024 23:38:55 +0200
Organization: A noiseless patient Spider
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On 12.06.2024 22:47, DFS 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?

Yeah, I had a similar problem like you; I had a declaration

  char answer[100];

and was surprised that it wasn't initialized with "42".

Seriously; why do you expect [in C] a declaration to initialize that
stack object? (There are other languages that do initializations as
the language defines it, but C doesn't; it may help to learn before
programming in any language?) And why do you think that "" would be
an appropriate initialization (i.e. a single '\0' character) and not
all 100 elements set to '\0'? (Someone else might want to access the
element 'answer[99]'.) And should we pay for initializing 1000000000
characters in case one declares an appropriate huge array?

Janis