Re: "undefined behavior"?

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.