Hi talents,
I have noticed that atof() function approximates the string I pass to
it.
when I use atof() , as atof(" 184.64")
and it returns 184.63999999999
But I would like to have the exact value that is passed.
Could anyone help me?
Thanks
Suresh Ooty
Apr 12 '06
21  8369 
> I don't think it's irrelevant. Debugging is not defined in C++ language Standard. Yes, I know, the world is a cruel place. But can't it be that
your debugger is wrong?
Maybe, but I was building my point view on float representation of
numbers and the debugger seemed to prove it...
The point I was trying to make was simple: you cannot compare two different
representations . You need to compare two representations of the same kind.
External with external, for example. If you read five significant digits,
you need to compare it with the output that has only five significant digits
otherwise it's bogus.
I couldn't agree more... But as you can see this
makes no difference as a whole as the calculation would be affected
by a very small amount, if any, and all the printing function would
consider this...
I don't understand that statement, sorry.
For the first I meant: It's a fact that float can't represent all
numbers accuratley (that's why we need that Epsilon test), however the
Inaccuracy is negligible and won't affect the calculation.
For the second statement I meant : if atoi() would return 184.64 as
184.63999999 then printf() would print 184.63999999 as 184.64
Abdo Haji-Ali
Programmer
In|Framez
"Victor Bazarov" writes: osmium wrote: That *is* the exact value that was passed. It is the best
approximation for that number that the compiler could come up with
and it doesn't look like adding bits to the representation would
help.
Actually, it's the debugger's best approximation of the best approximation
that the run-time could come up with. The compiler doesn't get its hands
on the actual double value. It's a sequence of chars in the source.
If there is information there, as opposed to words, it eludes me.
osmium <r1********@com cast.net> wrote: I would guess there are some helpful links or information on this problem in
the FAQ, or try Wikipedia.
I would recommend reading "What Every Computer Scientist Should Know
About Floating-Point Arithmetic" by David Goldberg: http://docs.sun.com/source/806-3568/ncg_goldberg.html
--
Marcus Kwok
Replace 'invalid' with 'net' to reply
osmium wrote: "Victor Bazarov" writes:
osmium wrote:
That *is* the exact value that was passed. It is the best
approximatio n for that number that the compiler could come up with
and it doesn't look like adding bits to the representation would
help.
Actually, it's the debugger's best approximation of the best approximation
that the run-time could come up with. The compiler doesn't get its hands
on the actual double value. It's a sequence of chars in the source.
If there is information there, as opposed to words, it eludes me.
The debugger translates the internal representation into a text
representation, just like printf does. It typically shows higher
precision, but you can do the same with printf or with iostreams.
--
Pete Becker
Roundhouse Consulting, Ltd.
osmium wrote: "Victor Bazarov" writes:
osmium wrote:
That *is* the exact value that was passed. It is the best
approximation for that number that the compiler could come up with
and it doesn't look like adding bits to the representation would
help.
Actually, it's the debugger's best approximation of the best
approximation that the run-time could come up with. The compiler
doesn't get its hands on the actual double value. It's a sequence
of chars in the source.
If there is information there, as opposed to words, it eludes me.
Too bad.
V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask
Victor Bazarov wrote: By the way, even the same number, can be represented in two different
representation, that it: if you have: double a = 10.5, b = 10.5; it's
very probable that a != b;
Really? How "very probable" is that? 0.8? 0.9?
AFAIK the mapping of "very probable" to a numeric function is up to the
individual application: http://www.doc.ic.ac.uk/~nd/surprise.../article2.html
(C++ being a useful language in which fuzzy applications can be
written in usefully ... an' all.)
regards
Andy Little
Victor Bazarov wrote: If I try using "EPSILON" in my program, it says "undefined identifier".
Do I need to include some header for that?
There isn't any, EPSILON depends on your accuracy demands, it could be
0.001, 1 or even 100 so you should define it yourself.
Abdo Haji-Ali
Programmer
In|Framez
asterisc wrote: By the way, even the same number, can be represented in two different
representation, that it: if you have: double a = 10.5, b = 10.5; it's
very probable that a != b;
Actually, it's extremely improbable that two objects of exactly the
same basic type, initialized using exactly the same expression, within
the same region of program text (same declaration even, sharing the
same set of declaration specifiers), will compare unequal.
> Actually, it's extremely improbable that two objects of exactly the same basic type, initialized using exactly the same expression, within
the same region of program text (same declaration even, sharing the
same set of declaration specifiers), will compare unequal.
Indeed, very improbable, but not impossible. Anyway, i just wanted to
express that if you need to check if two floating numbers are equal,
then don't rely on the '==';
If I try using "EPSILON" in my program, it says "undefined identifier".
Do I need to include some header for that?
There is no *standard* EPSILON. You have to define your own, depending
on your needs.
For example, i work in CAD industry, so i have my own EPSILON like
this:
#define EPSILON 0.00001
It's better to use a function that return that (instead of a macro),
or, better a singleton object.
asterisc wrote: #define EPSILON 0.00001
It's better to use a function that return that (instead of a macro),
or, better a singleton object.
A macro is certainly not the best choice, but, if the epsilon approach
with that value is appropriate for your application, why is a function
or a singleton better than
const double epsilon = 0.00001;
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