Re: "undefined behavior"?

From	Keith Thompson <Keith.S.Thompson+u@gmail.com>
Newsgroups	comp.lang.c
Subject	Re: "undefined behavior"?
Date	2024-06-12 14:57 -0700
Organization	None to speak of
Message-ID	<87bk457rk0.fsf@nosuchdomain.example.com> (permalink)
References	<666a095a$0$952$882e4bbb@reader.netnews.com>

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DFS <nospam@dfs.com> writes:
> Wrote a C program to mimic the stats shown on:
>
> https://www.calculatorsoup.com/calculators/statistics/descriptivestatistics.php

And where is that C program?  Do you expect us to help debug it without
seeing your code?

> 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.

Great, you've found the problem and solved it.

See question 1.30 of the comp.lang.c FAQ, <https://www.c-faq.com/>.

> Makes no sense.  What could cause the program to go FUBAR at data
> point 41+ only when the dataset is consecutive numbers?

Without seeing your code, we can't really tell what's going on, but
assuming that your `outliers` array has automatic storage duration
(i.e., is defined inside a function definition without `static`), its
initial value is indeterminate.  It's not random; it's garbage.  It
might be consistent from one run of your program to the next, just
because it gets its initial value from whatever is in memory when the
object is allocated.  It might even happen to be all zeros, though
that's apparently not what happened in your case.

> Also, why doesn't gcc just do you a solid and initialize to "" for you?

Because you didn't ask it to.  The language says that static objects
(ones defined at file scope or with the `static` keyword) are
initialized to zero (that's a bit of an oversimplification; I'm skipping
over what "zero" means in this context).  Automatic (local) objects
without initializers start with garbage values.

Initializing a character array with a string literal initializes the
entire array.  Characters beyond the length of the string literal are
initialized to 0 ('\0').

Compilers can sometimes warn you when your code depends on indeterminate
values.  I'd expect gcc to do so with the right options (try "-Wall").

Initializing automatic objects to all-bits-zero might be useful,
and I think some compilers might offer that as an option.  But it
could substantially hurt performance.  If you're careful enough to
write code that never depends on the values of uninitialized objects,
zero-initialization is a waste of time.  Initializing static objects
to zero is cheap; it's done at program load time.

There are languages that make it difficult, or even impossible, to
read uninitialized objects.  C is not such a language.  It places a
greater burden on the programmer for the sake of runtime performance.
And yes, C's behavior here is a source of bugs that are often
difficult to diagnose, though compiler warnings can be very helpful
if you enable them.

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

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"undefined behavior"? DFS <nospam@dfs.com> - 2024-06-12 16:47 -0400
  Re: "undefined behavior"? Barry Schwarz <schwarzb@delq.com> - 2024-06-12 14:30 -0700
    Re: "undefined behavior"? DFS <nospam@dfs.com> - 2024-06-12 17:53 -0400
      Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-12 15:30 -0700
        Re: "undefined behavior"? DFS <nospam@dfs.com> - 2024-06-12 19:07 -0400
          Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-12 17:33 -0700
            Re: "undefined behavior"? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-06-13 05:47 +0100
              Re: "undefined behavior"? scott@slp53.sl.home (Scott Lurndal) - 2024-06-13 15:39 +0000
                Re: "undefined behavior"? Ben Bacarisse <ben@bsb.me.uk> - 2024-06-13 18:08 +0100
                Re: "undefined behavior"? bart <bc@freeuk.com> - 2024-06-13 19:01 +0100
                Re: "undefined behavior"? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-06-13 19:54 +0100
                Re: "undefined behavior"? "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-06-13 12:34 -0700
                Re: "undefined behavior"? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-06-14 00:32 +0100
                Re: "undefined behavior"? Ben Bacarisse <ben@bsb.me.uk> - 2024-06-14 00:55 +0100
                Re: "undefined behavior"? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-06-14 02:48 +0100
                Re: "undefined behavior"? Ben Bacarisse <ben@bsb.me.uk> - 2024-06-14 12:44 +0100
                Re: "undefined behavior"? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-06-14 15:30 +0100
                Re: "undefined behavior"? Richard Harnden <richard.nospam@gmail.invalid> - 2024-06-14 16:32 +0100
                Re: "undefined behavior"? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-06-14 19:06 +0100
                Re: "undefined behavior"? bart <bc@freeuk.com> - 2024-06-14 19:31 +0100
                Re: "undefined behavior"? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-06-14 20:13 +0100
                Re: "undefined behavior"? Ben Bacarisse <ben@bsb.me.uk> - 2024-06-14 22:29 +0100
                Re: "undefined behavior"? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-06-14 23:35 +0100
                Re: "undefined behavior"? Ben Bacarisse <ben@bsb.me.uk> - 2024-06-15 00:14 +0100
                Re: "undefined behavior"? David Brown <david.brown@hesbynett.no> - 2024-06-15 20:57 +0200
                Re: "undefined behavior"? Richard Harnden <richard.nospam@gmail.invalid> - 2024-06-15 20:27 +0100
                Re: "undefined behavior"? Ben Bacarisse <ben@bsb.me.uk> - 2024-06-15 23:13 +0100
                Re: "undefined behavior"? David Brown <david.brown@hesbynett.no> - 2024-06-16 12:53 +0200
                Re: "undefined behavior"? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-06-16 14:44 +0100
                Re: "undefined behavior"? "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-06-14 11:49 -0700
                Re: "undefined behavior"? Ben Bacarisse <ben@bsb.me.uk> - 2024-06-14 22:32 +0100
                Re: "undefined behavior"? "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-06-15 00:56 -0700
                Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-13 15:58 -0700
                Re: "undefined behavior"? bart <bc@freeuk.com> - 2024-06-14 02:18 +0100
                Re: "undefined behavior"? David Brown <david.brown@hesbynett.no> - 2024-06-14 19:08 +0200
                Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-14 12:34 -0700
                Re: "undefined behavior"? David Brown <david.brown@hesbynett.no> - 2024-06-15 22:13 +0200
                Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-14 13:43 -0700
              Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-13 14:47 -0700
                Re: "undefined behavior"? Malcolm McLean <malcolm.arthur.mclean@gmail.com> - 2024-06-14 00:41 +0100
                Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-13 17:09 -0700
  Re: "undefined behavior"? David Brown <david.brown@hesbynett.no> - 2024-06-12 23:38 +0200
    Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-12 15:18 -0700
      Re: "undefined behavior"? David Brown <david.brown@hesbynett.no> - 2024-06-13 14:42 +0200
        Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-13 16:39 -0700
      Re: "undefined behavior"? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-06-18 17:23 -0700
        Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-18 17:42 -0700
          Re: "undefined behavior"? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2024-06-22 09:28 -0700
    Re: "undefined behavior"? DFS <nospam@dfs.com> - 2024-06-12 18:29 -0400
      Re: "undefined behavior"? Ike Naar <ike@sdf.org> - 2024-06-13 07:25 +0000
        Re: "undefined behavior"? DFS <nospam@dfs.com> - 2024-06-13 11:13 -0400
          Re: "undefined behavior"? scott@slp53.sl.home (Scott Lurndal) - 2024-06-13 15:40 +0000
          Re: "undefined behavior"? Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2024-06-13 15:49 +0000
            Re: "undefined behavior"? DFS <nospam@dfs.com> - 2024-06-13 13:05 -0400
      Re: "undefined behavior"? David Brown <david.brown@hesbynett.no> - 2024-06-13 15:15 +0200
        Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-13 16:47 -0700
          Re: "undefined behavior"? David Brown <david.brown@hesbynett.no> - 2024-06-14 19:13 +0200
  Re: "undefined behavior"? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-06-12 23:38 +0200
    Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-12 15:22 -0700
      Re: "undefined behavior"? DFS <nospam@dfs.com> - 2024-06-12 18:34 -0400
        Re: "undefined behavior"? David Brown <david.brown@hesbynett.no> - 2024-06-13 15:21 +0200
          Re: "undefined behavior"? DFS <nospam@dfs.com> - 2024-06-13 10:38 -0400
            Re: "undefined behavior"? David Brown <david.brown@hesbynett.no> - 2024-06-14 19:18 +0200
              Re: "undefined behavior"? scott@slp53.sl.home (Scott Lurndal) - 2024-06-14 17:36 +0000
                Re: "undefined behavior"? David Brown <david.brown@hesbynett.no> - 2024-06-15 22:15 +0200
              Re: "undefined behavior"? DFS <nospam@dfs.com> - 2024-06-14 19:05 -0400
                Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-14 18:39 -0700
                Re: "undefined behavior"? DFS <nospam@dfs.com> - 2024-06-14 23:49 -0400
                Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-14 20:56 -0700
                Re: "undefined behavior"? DFS <nospam@dfs.com> - 2024-06-15 00:45 -0400
                Re: "undefined behavior"? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-06-15 07:03 +0200
                Re: "undefined behavior"? DFS <nospam@dfs.com> - 2024-06-15 07:39 -0400
                Re: "undefined behavior"? James Kuyper <jameskuyper@alumni.caltech.edu> - 2024-06-15 01:05 -0400
                Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-14 22:20 -0700
                Re: "undefined behavior"? bart <bc@freeuk.com> - 2024-06-15 09:37 +0100
                Re: "undefined behavior"? David Brown <david.brown@hesbynett.no> - 2024-06-15 22:22 +0200
      Re: "undefined behavior"? Janis Papanagnou <janis_papanagnou+ng@hotmail.com> - 2024-06-13 02:19 +0200
        Re: "undefined behavior"? David Brown <david.brown@hesbynett.no> - 2024-06-13 15:28 +0200
  Re: "undefined behavior"? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2024-06-12 14:57 -0700
  Re: "undefined behavior"? bart <bc@freeuk.com> - 2024-06-13 10:43 +0100
  Re: "undefined behavior"? Bonita Montero <Bonita.Montero@gmail.com> - 2024-06-13 11:45 +0200

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