Hi,
I'd like to compare 2 floating point numbers within a given error. I'd
rather not use a absolute error but one related to the number of
values that can be represented between the two floats. I've been
reading: http://www.cygnus-software.com/paper...ringfloats.htm
where the following function is provided to do this:
bool AlmostEqual2sCo mplement(float A, float B, int maxUlps) {
// Make sure maxUlps is non-negative and small enough that the
// default NAN won't compare as equal to anything.
assert(maxUlps 0 && maxUlps < 4 * 1024 * 1024);
int aInt = *(int*)&A;
// Make aInt lexicographical ly ordered as a twos-complement int
if (aInt < 0)
aInt = 0x80000000 - aInt;
// Make bInt lexicographical ly ordered as a twos-complement int
int bInt = *(int*)&B;
if (bInt < 0)
bInt = 0x80000000 - bInt;
int intDiff = abs(aInt - bInt);
if (intDiff <= maxUlps)
return true;
return false;
}
However, as the article states, this relies on a number of compiler
specific features, such as the size of int (and I guess float). It
also relies on the floats using IEEE representation (I guess all
compilers use this, but is it in the standard?).
So my question is this. Is there a good compiler independent method
for comparing floating point numbers with a relative error?
Apr 30 '07
14  2516 
P.J. Plauger wrote:
"nw" <ne*@soton.ac.u kwrote in message
news:11******** **************@ h2g2000hsg.goog legroups.com...
>>I think you need to read the cited article to find out what this is
about. This isn't really a "relative error", it's asking the
question "is the number of representable floating-point values
between given A and B less than N?"
Yes what he said. :)
Sorry, the question wasn't defined as clearly as it could have been.
So is there any compiler independent method for this? Or is it
perhaps not even a particularly good idea?
You need the C99 function nexttoward, which does almost exactly what
you want. It'll be a part of the next C++ Standard, but right now it's
relatively rare. See our Compleat Library, available at our web site.
There is also nextafter which is almost the same but maybe slightly more
appropriate since it takes parameters of the same type instead of
having the second parameter always as a long double for reasons
probably only numerical analysts know.
They are also included with recent GNU libc versions that are used on
most Linux systems.
--
Markus
On Apr 30, 3:15 pm, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:
nw wrote:
I'd like to compare 2 floating point numbers within a given error. [...]
However, as the article states, this relies on a number of compiler
specific features, such as the size of int (and I guess float). It
also relies on the floats using IEEE representation (I guess all
compilers use this, but is it in the standard?).
Right.
Which standard. IEEE representation is a standard, but it's not
required in C++, and is far from universal.
--
James Kanze (Gabi Software) email: ja*********@gma il.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
James Kanze wrote:
On Apr 30, 3:15 pm, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:
>nw wrote:
>>I'd like to compare 2 floating point numbers within a given error.
[...]
>>However, as the article states, this relies on a number of compiler
specific features, such as the size of int (and I guess float). It
also relies on the floats using IEEE representation (I guess all
compilers use this, but is it in the standard?).
>Right.
Which standard. IEEE representation is a standard, but it's not
required in C++, and is far from universal.
Yes. Are you agreeing or disagreeing with my saying "right"?
V
--
Please remove capital 'A's when replying by e-mail
I do not respond to top-posted replies, please don't ask
On May 1, 3:29 pm, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:
James Kanze wrote:
On Apr 30, 3:15 pm, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:
nw wrote:
I'd like to compare 2 floating point numbers within a given error.
[...]
>However, as the article states, this relies on a number of compiler
specific features, such as the size of int (and I guess float). It
also relies on the floats using IEEE representation (I guess all
compilers use this, but is it in the standard?).
Right.
Which standard. IEEE representation is a standard, but it's not
required in C++, and is far from universal.
Yes. Are you agreeing or disagreeing with my saying "right"?
It depends on what you meant by "right". The way I read it was:
>I guess all compilers use IEEE representation, but is it in
the standard?
Right [meaning yes, it is in the standard].
However, I don't find "right" very idiomatic for responding to
this kind of question, so maybe you meant for it to apply to
something else.
--
James Kanze (GABI Software) email:ja******* **@gmail.com
Conseils en informatique orientée objet/
Beratung in objektorientier ter Datenverarbeitu ng
9 place Sémard, 78210 St.-Cyr-l'École, France, +33 (0)1 30 23 00 34
James Kanze wrote:
On May 1, 3:29 pm, "Victor Bazarov" <v.Abaza...@com Acast.netwrote:
>James Kanze wrote:
>>On Apr 30, 3:15 pm, "Victor Bazarov" <v.Abaza...@com Acast.net>
wrote:
nw wrote:
I'd like to compare 2 floating point numbers within a given error.
[...]
>>>>However, as the article states, this relies on a number of
compiler specific features, such as the size of int (and I guess
float). It also relies on the floats using IEEE representation (I
guess all compilers use this, but is it in the standard?).
>>>Right.
>>Which standard. IEEE representation is a standard, but it's not
required in C++, and is far from universal.
>Yes. Are you agreeing or disagreeing with my saying "right"?
It depends on what you meant by "right".
It is meant as a confirmation response. What else could I mean?
The way I read it was:
>>I guess all compilers use IEEE representation, but is it in
the standard?
> Right [meaning yes, it is in the standard].
However, I don't find "right" very idiomatic for responding to
this kind of question, so maybe you meant for it to apply to
something else.
Good, you're doubting yourself. That's the first step to finding
the common ground.
I responded to the whole paragraph. Chiefly, it means I responded
to "this relies" and "It also relies". If I were to answer the
the "is it in the standard?" question, I'd use the word "standard"
in my reply somehow.
V
--
Please remove capital 'A's when replying by e-mail
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