Groups > comp.lang.c > #71966

Re: Learning functions; roots of the quadratic; two return values

From	Richard Heathfield <rjh@cpax.org.uk>
Newsgroups	comp.lang.c
Subject	Re: Learning functions; roots of the quadratic; two return values
Date	2015-10-05 12:57 +0100
Organization	A noiseless patient Spider
Message-ID	<mutof4$bh0$1@dont-email.me> (permalink)
References	(2 earlier) <pnr21b1287fff5arfericujjih1vk7to5b@4ax.com> <5611ccfd$0$3060$426a34cc@news.free.fr> <jhr31b58dqnsrblak6mmiqjcbs98hvevop@4ax.com> <5612537a$0$3299$426a74cc@news.free.fr> <mutmap$37d$1@dont-email.me>

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On 05/10/15 12:20, Bartc wrote:
> On 05/10/2015 11:39, regis wrote:
>> On 10/05/15 05:31, Rosario19 wrote:
>>
>>> do you want the correct name for variables?
>>
>> If you write: int myWeightInKg= 65; ...; myWeightInKg += 5;
>>
>> the value of myWeightInKg changes, but not its meaning:
>> I'm heavier by 5 kilograms, but it's still my weight in kilograms.
>
> OK, here myWeightInKg clearly refers to a physical quantity with a
> certain 'dimension'. So you couldn't do myWeightInKg *= myWeightInKg as
> the dimension then becomes kg-squared (with no meaning afaik).

kg^2 occurs in force calculations. The gravitational constant G has 
units Nm^2/kg^2. This doesn't affect your point, of course.

>
>> But when you write: double delta= b*b-4*a*c; ...; delta= sqrt(delta);
>> Things are different: You don't just change the value here,
>> you also change the meaning of delta.
>
> But what's delta? Isn't it just a number? (Until now I'd never even
> heard of delta in connection with solving quadratic equations).

"delta" is generally used to mean "small difference", although "small" 
is kind of optional. Personally, I'd write it:

   delta = sqrt(b * b - 4 * a * c);

thus sidestepping the issue completely.

>> It once meant the discriminant as it should
>> (as delta is universally used to denote the discriminant),
>> and then you make its name a liar by changing its meaning.
>>
>> So you should use 2 variables here: sqrtDelta= sqrt (delta);
>
>> Never use meaningless names for identifiers.
>
> So you'd never use x, y, z?

It depends on whether they are meaningful. I generally use x for the 
horizontal component of a vector, and y for the vertical component. If I 
need a depth component too, I use z. Spatial axes are typically referred 
to by these letters, so they do have meaning.

Similarly, something like 'i' is often a perfectly adequate name for a 
loop control. Within the context of:

for(i = 0; i < len; i++)
{
   c[i] = p[i] ^ key[i % klen];
}

it is perfectly obvious what i means.

> My math(s) textbooks used to be full of
> them! What about the a, b, c used for the coefficients of the equation?

Again, a, b, and c are the usual names given to those coefficients, and 
that is what gives the names meaning.

The point regis is making is a good one: meaningful names make for 
readable code, and it's easier to make a readable program correct than 
it is to make a correct program readable (and thus make it easier to 
maintain, and thus less likely to break in the future).

When the name of a 'variable' (if I may use the term) has no real 
connection with the concept it is modelling, or is overloaded with more 
than one meaning, it makes the code harder to read.

>> Never use a meaning for another in identifier names.

Right.

>> Never change the meaning of a variable during its lifetime.

Right.

> Sometimes that makes sense; if DataArray points to an int array for
> example, you don't to step DataArray along itself, because at the end if
> won't refer to the original array at all.

That's easy to deal with, though, because one simply chooses a better name:

void cpystr(char *target, const char *source)
{
   while(*target++ = *source++)
   {
     continue;
   }
   return;
}

> But otherwise I think you're worrying too much!

I think that probably quite a lot of programmers would agree with you. 
Personally, however, I think regis is (a) not worrying at all, and (b) 
exactly right in his advice to choose good, meaningful names and to 
maintain the correlation between the name of an object and its purpose.

-- 
Richard Heathfield
Email: rjh at cpax dot org dot uk
"Usenet is a strange place" - dmr 29 July 1999
Sig line 4 vacant - apply within

Thread

Learning functions; roots of the quadratic; two return values Alla _ <modelling.data@gmail.com> - 2015-10-03 10:43 -0700
  Re: Learning functions; roots of the quadratic; two return values Andrew Cooper <amc96@cam.ac.uk> - 2015-10-03 19:00 +0100
    Re: Learning functions; roots of the quadratic; two return values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-03 19:34 +0100
      Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-03 21:29 +0200
        Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-03 21:36 +0200
        Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-03 21:43 +0200
        Re: Learning functions; roots of the quadratic; two return values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-03 20:46 +0100
          Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-03 22:46 +0200
            Re: Learning functions; roots of the quadratic; two return values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-03 23:18 +0100
              Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-04 01:08 +0200
              Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-04 01:36 +0200
                Re: Learning functions; roots of the quadratic; two return values Ian Collins <ian-news@hotmail.com> - 2015-10-04 12:52 +1300
                Re: Learning functions; roots of the quadratic; two return values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-04 15:42 +0100
          Re: Learning functions; roots of the quadratic; two return values jacobnavia <jacob@jacob.remcomp.fr> - 2015-10-03 23:05 +0200
            Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-03 23:41 +0200
              Re: Learning functions; roots of the quadratic; two return values jacobnavia <jacob@jacob.remcomp.fr> - 2015-10-04 00:14 +0200
            Re: Learning functions; roots of the quadratic; two return values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-03 23:07 +0100
              Re: Learning functions; roots of the quadratic; two return values jacobnavia <jacob@jacob.remcomp.fr> - 2015-10-04 00:32 +0200
              Re: Learning functions; roots of the quadratic; two return values Keith Thompson <kst-u@mib.org> - 2015-10-04 12:56 -0700
                Re: Learning functions; roots of the quadratic; two return values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-04 23:41 +0100
                Re: Learning functions; roots of the quadratic; two return values Keith Thompson <kst-u@mib.org> - 2015-10-04 18:24 -0700
              Re: Learning functions; roots of the quadratic; two return values James Kuyper <jameskuyper@verizon.net> - 2015-10-04 16:36 -0400
                Re: Learning functions; roots of the quadratic; two return values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-05 00:01 +0100
                Re: Learning functions; roots of the quadratic; two return values James Kuyper <jameskuyper@verizon.net> - 2015-10-04 19:24 -0400
          Re: Learning functions; roots of the quadratic; two return values Ian Collins <ian-news@hotmail.com> - 2015-10-04 10:18 +1300
            Re: Learning functions; roots of the quadratic; two return values jacobnavia <jacob@jacob.remcomp.fr> - 2015-10-03 23:27 +0200
              Re: Learning functions; roots of the quadratic; two return values Ian Collins <ian-news@hotmail.com> - 2015-10-04 11:08 +1300
                Re: Learning functions; roots of the quadratic; two return values Richard Damon <Richard@Damon-Family.org> - 2015-10-03 18:38 -0400
                Re: Learning functions; roots of the quadratic; two return values Ian Collins <ian-news@hotmail.com> - 2015-10-04 11:47 +1300
          Re: Learning functions; roots of the quadratic; two return values raltbos@xs4all.nl (Richard Bos) - 2015-10-04 14:07 +0000
            Re: Learning functions; roots of the quadratic; two return values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-04 15:48 +0100
              Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-04 18:59 +0200
                Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-04 20:06 +0200
              Re: Learning functions; roots of the quadratic; two return values Tim Rentsch <txr@alumni.caltech.edu> - 2015-10-04 15:21 -0700
                Re: Learning functions; roots of the quadratic; two return values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-05 05:25 +0000
        Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-03 21:48 +0200
          Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-04 00:31 +0200
    Re: Learning functions; roots of the quadratic; two return values alla.rashitova@gmail.com - 2015-10-05 08:40 -0700
      Re: Learning functions; roots of the quadratic; two return values Richard Heathfield <rjh@cpax.org.uk> - 2015-10-05 17:10 +0100
      Re: Learning functions; roots of the quadratic; two return values Barry Schwarz <schwarzb@dqel.com> - 2015-10-05 10:41 -0700
  Re: Learning functions; roots of the quadratic; two return values "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-10-03 12:04 -0700
    Re: Learning functions; roots of the quadratic; two return values Philip Lantz <prl@canterey.us> - 2015-10-03 12:48 -0700
    Re: Learning functions; roots of the quadratic; two return values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-03 20:55 +0100
      Re: Learning functions; roots of the quadratic; two return values "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-10-03 13:27 -0700
      Re: Learning functions; roots of the quadratic; two return values jacobnavia <jacob@jacob.remcomp.fr> - 2015-10-03 23:08 +0200
        Re: Learning functions; roots of the quadratic; two return values Richard Heathfield <rjh@cpax.org.uk> - 2015-10-03 22:52 +0100
    Re: Learning functions; roots of the quadratic; two return values Rosario19 <Ros@invalid.invalid> - 2015-10-04 20:46 +0200
      Re: Learning functions; roots of the quadratic; two return values Keith Thompson <kst-u@mib.org> - 2015-10-04 15:47 -0700
      Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-05 03:06 +0200
        Re: Learning functions; roots of the quadratic; two return values Rosario19 <Ros@invalid.invalid> - 2015-10-05 05:31 +0200
          Re: Learning functions; roots of the quadratic; two return values Rosario19 <Ros@invalid.invalid> - 2015-10-05 11:58 +0200
            Re: Learning functions; roots of the quadratic; two return values Rosario19 <Ros@invalid.invalid> - 2015-10-05 12:00 +0200
              Re: Learning functions; roots of the quadratic; two return values Rosario19 <Ros@invalid.invalid> - 2015-10-05 12:06 +0200
          Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-05 12:39 +0200
            Re: Learning functions; roots of the quadratic; two return values Bartc <bc@freeuk.com> - 2015-10-05 12:20 +0100
              Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-05 13:50 +0200
              Re: Learning functions; roots of the quadratic; two return values Richard Heathfield <rjh@cpax.org.uk> - 2015-10-05 12:57 +0100
        Re: Learning functions; roots of the quadratic; two return values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-05 05:40 +0000
          Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-05 08:30 +0200
          Re: Learning functions; roots of the quadratic; two return values Philip Lantz <prl@canterey.us> - 2015-10-05 10:03 -0700
            Re: Learning functions; roots of the quadratic; two return values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-05 22:25 +0000
              Re: Learning functions; roots of the quadratic; two return values James Kuyper <jameskuyper@verizon.net> - 2015-10-05 19:26 -0400
                Re: Learning functions; roots of the quadratic; two return values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-06 03:44 +0000
                Re: Learning functions; roots of the quadratic; two return values James Kuyper <jameskuyper@verizon.net> - 2015-10-06 10:55 -0400
                Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-06 23:07 +0200
                Re: Learning functions; roots of the quadratic; two return values James Kuyper <jameskuyper@verizon.net> - 2015-10-06 18:35 -0400
                Re: Learning functions; roots of the quadratic; two return values raltbos@xs4all.nl (Richard Bos) - 2015-10-14 11:08 +0000
                Re: Learning functions; roots of the quadratic; two return values James Kuyper <jameskuyper@verizon.net> - 2015-10-14 11:11 -0400
                Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-14 22:22 +0200
                Re: Learning functions; roots of the quadratic; two return values James Kuyper <jameskuyper@verizon.net> - 2015-10-14 17:17 -0400
                Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-14 23:45 +0200
                Re: Learning functions; roots of the quadratic; two return values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-15 00:11 +0000
                Re: Learning functions; roots of the quadratic; two return values Bartc <bc@freeuk.com> - 2015-10-15 01:37 +0100
                Re: Learning functions; roots of the quadratic; two return values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-15 05:52 +0000
                Re: Learning functions; roots of the quadratic; two return values James Kuyper <jameskuyper@verizon.net> - 2015-10-14 21:54 -0400
                Re: Learning functions; roots of the quadratic; two return values asetofsymbols@gmail.com - 2015-10-14 22:40 -0700
                Re: Learning functions; roots of the quadratic; two return values asetofsymbols@gmail.com - 2015-10-15 05:00 -0700
                Re: Learning functions; roots of the quadratic; two return values supercat@casperkitty.com - 2015-10-15 11:58 -0700
                Re: Learning functions; roots of the quadratic; two return values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-15 06:04 +0000
                Re: Learning functions; roots of the quadratic; two return values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-06 23:18 +0000
                Re: Learning functions; roots of the quadratic; two return values supercat@casperkitty.com - 2015-10-06 16:31 -0700
                Re: Learning functions; roots of the quadratic; two return values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-07 00:38 +0000
                Re: Learning functions; roots of the quadratic; two return values supercat@casperkitty.com - 2015-10-06 21:19 -0700
                Re: Learning functions; roots of the quadratic; two return values James Kuyper <jameskuyper@verizon.net> - 2015-10-06 22:15 -0400
                Re: Learning functions; roots of the quadratic; two return values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-07 08:45 +0000
                Re: Learning functions; roots of the quadratic; two return values Melzzzzz <mel@zzzzz.com> - 2015-10-07 05:19 +0200
                Re: Learning functions; roots of the quadratic; two return values Keith Thompson <kst-u@mib.org> - 2015-10-07 09:12 -0700
                Re: Learning functions; roots of the quadratic; two return values Melzzzzz <mel@zzzzz.com> - 2015-10-08 00:53 +0200
                Re: Learning functions; roots of the quadratic; two return values Keith Thompson <kst-u@mib.org> - 2015-10-07 16:23 -0700
              Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-06 02:16 +0200
                Re: Learning functions; roots of the quadratic; two return values supercat@casperkitty.com - 2015-10-06 15:43 -0700
                Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-07 02:48 +0200
                Re: Learning functions; roots of the quadratic; two return values Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-10-14 14:00 -0700
                Re: Learning functions; roots of the quadratic; two return values regis <regis.barbanchon@free.fr> - 2015-10-14 23:23 +0200
  Re: Learning functions; roots of the quadratic; two return values Geoff <geoff@invalid.invalid> - 2015-10-03 12:50 -0700
  Re: Learning functions; roots of the quadratic; two return values Bartc <bc@freeuk.com> - 2015-10-03 21:16 +0100
    Re: Learning functions; roots of the quadratic; two return values alla.rashitova@gmail.com - 2015-10-05 09:02 -0700
      Re: Learning functions; roots of the quadratic; two return values Richard Heathfield <rjh@cpax.org.uk> - 2015-10-05 17:13 +0100
  Re: Learning functions; roots of the quadratic; two return values Barry Schwarz <schwarzb@dqel.com> - 2015-10-03 15:37 -0700
  Re: Learning functions; roots of the quadratic; two return values Rosario19 <Ros@invalid.invalid> - 2015-10-05 23:44 +0200
    Re: Learning functions; roots of the quadratic; two return values Geoff <geoff@invalid.invalid> - 2015-10-05 18:37 -0700
    Re: Learning functions; roots of the quadratic; two return values Barry Schwarz <schwarzb@dqel.com> - 2015-10-05 19:24 -0700
      Re: Learning functions; roots of the quadratic; two return values Rosario19 <Ros@invalid.invalid> - 2015-10-06 07:36 +0200
    Re: Learning functions; roots of the quadratic; two return values Rosario19 <Ros@invalid.invalid> - 2015-10-06 07:34 +0200

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