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| From | Keith Thompson <kst-u@mib.org> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Converting Floats to Strings yields erratic results |
| Date | 2011-06-07 14:25 -0700 |
| Organization | None to speak of |
| Message-ID | <lnpqmpe3lu.fsf@nuthaus.mib.org> (permalink) |
| References | <ism43f$cg1$1@speranza.aioe.org> |
"Dirk T. Shelley" <nospam@nospam.com> writes:
> I'm writing, for my own personal enlightenment, a little program
> that will convert a floating-point number, passed as an argument on
> the command line, into a character string. I'm now aware of
> sprintf(), but I wasn't when I started writing this program, and I
> feel it is my duty to finish it ;-)
Typically you *can't* pass floating-point numbers on the command line.
You can only pass strings. Presumably you convert those strings to
floating-point numbers somehow, but you don't tell us how.
> Everything, in general, works fine, except that some values are
> converted to one less than their actual value. For example, 15
> returns 15, 14.123 returns 14, but 14 returns 13. I am _completely_
> clueless, although I suspect it has something to do with rounding and
> truncating happening behind my back. The important part is as
> follows...
If you don't underrstand what's happening, how do you know that this is
the important part?
> while (foo >= 10) {
> foo = foo/10.0;
This will yield an inexact result (and for all we know the original
value of foo was inexact). Floating-point numbers are represented in
binary, not decimal.
Suggested reading: section 14 of the comp.lang.c FAQ,
<http://www.c-faq.com/>. For more advanced reading, search for
Goldberg's "What Every Computer Scientist Should Know About
Floating-Point Arithmetic".
> tenDivs++;
> }
>
> i = 0;
> while (tenDivs >= 0) {
> while (!(foo < 1)) {
> foo--;
> oneDivs++;
> }
> str[i] = 48 + oneDivs;
Where did the value 48 come from?
Oh, I see, it's the ASCII value of '0'. Just write '0' (yes, character
constants are of type int); it makes your code clearer and potentially
more portable. It assumes that the representations of '0' through '9'
are contiguous; as it happens, the language guarantees that.
> foo = foo * 10;
> oneDivs = 0;
> tenDivs--;
> i++;
> }
>
> Where "foo" is the value to be converted. Can anybody help, or point
> me to a source that can?
I haven't taken the time to analyze what your program is doing, but
consider this. As I mentioned above, dividing a floating-point
number x by 10.0 is likely to give you an inexact result.
Multiplying that result by 10.0 could give you something other than
the original number, and truncating that result could give you x - 1.
> In an unrelated question: Does anyone know of any link which describes
> the (relative) performance of all kinds of C operations? e.g: how fast is
> "add" comparing with "multiplication" on a typical machine.
I don't know. You can probably assume that addition is faster
than multiplication, and division is slower than either, but there
are no consistent guarantees. The relative performance of integer
and floating-point arithmetic can vary a great deal depending on
the hardware.
--
Keith Thompson (The_Other_Keith) kst-u@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"
Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread
Converting Floats to Strings yields erratic results "Dirk T. Shelley" <nospam@nospam.com> - 2011-06-07 21:10 +0000
Re: Converting Floats to Strings yields erratic results Keith Thompson <kst-u@mib.org> - 2011-06-07 14:25 -0700
Re: Converting Floats to Strings yields erratic results Peter Nilsson <airia@acay.com.au> - 2011-06-07 20:13 -0700
Re: Converting Floats to Strings yields erratic results Keith Thompson <kst-u@mib.org> - 2011-06-07 20:27 -0700
Re: Converting Floats to Strings yields erratic results "Dirk T. Shelley" <nospam@nospam.com> - 2011-06-08 16:34 +0000
Re: Converting Floats to Strings yields erratic results Ian Collins <ian-news@hotmail.com> - 2011-06-09 07:53 +1200
Re: Converting Floats to Strings yields erratic results "Dirk T. Shelley" <nospam@nospam.com> - 2011-06-08 20:46 +0000
Re: Converting Floats to Strings yields erratic results "BartC" <bc@freeuk.com> - 2011-06-08 22:40 +0100
Re: Converting Floats to Strings yields erratic results "BartC" <bc@freeuk.com> - 2011-06-09 11:03 +0100
Re: Converting Floats to Strings yields erratic results James Kuyper <jameskuyper@verizon.net> - 2011-06-09 08:06 -0400
Re: Converting Floats to Strings yields erratic results "BartC" <bc@freeuk.com> - 2011-06-09 16:57 +0100
Re: Converting Floats to Strings yields erratic results Noob <root@127.0.0.1> - 2011-06-09 14:13 +0200
Re: Converting Floats to Strings yields erratic results Keith Thompson <kst-u@mib.org> - 2011-06-09 09:26 -0700
Re: Converting Floats to Strings yields erratic results "BartC" <bc@freeuk.com> - 2011-06-09 19:07 +0100
Re: Converting Floats to Strings yields erratic results Keith Thompson <kst-u@mib.org> - 2011-06-09 11:52 -0700
Re: Converting Floats to Strings yields erratic results "BartC" <bc@freeuk.com> - 2011-06-09 20:30 +0100
Re: Converting Floats to Strings yields erratic results "io_x" <a@b.c.invalid> - 2011-06-13 12:21 +0200
Re: Converting Floats to Strings yields erratic results "io_x" <a@b.c.invalid> - 2011-06-13 22:31 +0200
Re: Converting Floats to Strings yields erratic results Noob <root@127.0.0.1> - 2011-06-10 10:58 +0200
Re: Converting Floats to Strings yields erratic results Seebs <usenet-nospam@seebs.net> - 2011-06-10 22:49 +0000
Re: Converting Floats to Strings yields erratic results Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-08 21:51 -0400
Re: Converting Floats to Strings yields erratic results Mark Bluemel <mark_bluemel@pobox.com> - 2011-06-09 08:50 +0100
Re: Converting Floats to Strings yields erratic results gordonb.9ycz5@burditt.org (Gordon Burditt) - 2011-06-07 18:13 -0500
Re: Converting Floats to Strings yields erratic results Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-07 20:47 -0400
Re: Converting Floats to Strings yields erratic results Keith Thompson <kst-u@mib.org> - 2011-06-07 20:08 -0700
Re: Converting Floats to Strings yields erratic results Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-07 23:38 -0400
Re: Converting Floats to Strings yields erratic results Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-07 23:49 -0400
Re: Converting Floats to Strings yields erratic results Shao Miller <sha0.miller@gmail.com> - 2011-06-08 00:54 -0500
Re: Converting Floats to Strings yields erratic results Thad Smith <ThadSmith@acm.org> - 2011-06-07 20:38 -0700
Re: Converting Floats to Strings yields erratic results Keith Thompson <kst-u@mib.org> - 2011-06-07 20:30 -0700
Re: Converting Floats to Strings yields erratic results falk@rahul.net (Edward A. Falk) - 2011-06-08 20:54 +0000
Re: Converting Floats to Strings yields erratic results Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-08 21:56 -0400
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