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From: David Brown <david.brown@hesbynett.no>
Newsgroups: comp.lang.c
Subject: Re: "undefined behavior"?
Date: Fri, 14 Jun 2024 19:18:35 +0200
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On 13/06/2024 16:38, DFS wrote:
> On 6/13/2024 9:21 AM, David Brown wrote:
>> On 13/06/2024 00:34, DFS wrote:
>>> On 6/12/2024 6:22 PM, Keith Thompson wrote:
>>>> Janis Papanagnou <janis_papanagnou+ng@hotmail.com> writes:
>>>>> On 12.06.2024 22:47, DFS wrote:
>>>> [...]
>>>>>> before: char outliers[100];
>>>>>> after : char outliers[100] = "";
>>>> [...]
>>>>> 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?
>>>>
>>>> This:
>>>>      char outliers[100] = "";
>>>> initializes all 100 elements to zero.  So does this:
>>>>      char outliers[100] = { '\0' };
>>>> Any elements or members not specified in an initializer are set to 
>>>> zero.
>>
>> Yes.  It's good to point that out, since people might assume that 
>> using a string literal here only initialises the bit covered by that 
>> string literal.
>>
>> (In C23 you can also write "char outliers[100] = {};" to get all zeros.)
>>
>>>>
>>>> If you want to set an array's 0th element to 0 and not waste time
>>>> initializing the rest, you can assign it separately:
>>>>      char outliers[100];
>>>>      outliers[0] = '\0';
>>>> or
>>>>      char outliers[100];
>>>>      strcpy(outliers, "");
>>>> though the overhead of the function call is likely to outweigh the
>>>> cost of initializing the array.
>>
>> A good compiler will generate the same code for both cases - strcpy() 
>> is often inlined for such uses.
>>
>>>
>>> Thanks.  I'll have to remember these things.  I like to use char arrays.
>>>
>>> The problem is I don't use C very often, so I don't develop muscle 
>>> memory.
>>>
>>
>> What programming language do you usually use?  And why are you writing 
>> in C instead?  (Or do you simply not do much programming?)
> 
> I write a little code every few days.  Mostly python.

Certainly if I wanted to calculate some statistics from small data sets, 
I'd go for Python - it would not consider C unless it was for an 
embedded system.

> 
> I like C for it's blazing speed.  Very addicting.  And it's much more 
> challenging/frustrating than python.

With small data sets, Python has blazing speed - /every/ language has 
blazing speed.  And for large data sets, use numpy on Python and you 
/still/ have blazing speeds - a lot faster than anything you would write 
in C (because numpy's underlying code is written in C by people who are 
much better at writing fast numeric code than you or I).

The only reason to use C for something like is is for the challenge and 
fun, which is fair enough.

> 
> I coded a subset (8 stat measures) of this C program 3.5 years ago, and 
> recently decided to finish duplicating all 23 stats shown at:
> 
> https://www.calculatorsoup.com/calculators/statistics/descriptivestatistics.php
> 
> Working on the outliers code, I decided to add an option to generate 
> data with consecutive numbers.  That's when I ran $./dfs 50 -c and 
> noticed every value above 40 was considered an outlier.  And this didn't 
> change over a bunch of code edits/file saves/compiles.
> 
> Understanding how an uninitialized variable caused that persistent issue 
> is beyond my pay grade.

Understanding that you should not read from a variable that has never 
been given a value is well within the pay grade of every programmer. 
And it's something that every C programmer should understand.  (And now 
you understand it too!)

> 
> That's when I whined to clc.  Before I even posted, though, I spotted 
> the uninitialized var (outliers).  Later I spotted another one (mode).
> 
> One led to 'undefined behavior', the other to 'stack smashing'.  Both 
> only occurred when using consecutive numbers.
> 
> But with y'all's help I believe I found and fixed ALL issues.  I can 
> dream anyway.
>