isinf()

[Richard Tobin]

The GNU Autoconf manual provides a work-around for missing isinf()
and isnan() functions, which exist in C99 and many, but not all,
pre-C99 implementations :

http://www.gnu.org/software/libtool/...rtability.html

The given definition of isinf(x) boils down to return isnan (x - x),
which will return true for NaNs. Surely this is wrong?

-- Richard
--
Please remember to mention me / in tapes you leave behind.

[James Kuyper]

Richard Tobin wrote:

The GNU Autoconf manual provides a work-around for missing isinf()
and isnan() functions, which exist in C99 and many, but not all,
pre-C99 implementations :

http://www.gnu.org/software/libtool/...rtability.html

The given definition of isinf(x) boils down to return isnan (x - x),
which will return true for NaNs. Surely this is wrong?

It seems so to me.

[Malcolm McLean]

"Richard Tobin" <ri*****@cogsci .ed.ac.ukwrote in message

int isinf(double x)
{
if(x DBL_MAX)
return 1;
else
return 0;
}

[Keith Thompson]

"Malcolm McLean" <re*******@btin ternet.comwrite s:

"Richard Tobin" <ri*****@cogsci .ed.ac.ukwrote in message

int isinf(double x)
{
if(x DBL_MAX)
return 1;
else
return 0;
}

That fails for negative infinity.

--
Keith Thompson (The_Other_Keit h) ks***@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"

[CBFalconer]

Richard Tobin wrote:

>
The GNU Autoconf manual provides a work-around for missing isinf()
and isnan() functions, which exist in C99 and many, but not all,
pre-C99 implementations :

http://www.gnu.org/software/libtool/...rtability.html

The given definition of isinf(x) boils down to return isnan (x - x),
which will return true for NaNs. Surely this is wrong?

Think about it. If x is not a number, what can you say about x-x?
I fail to see any way that expression could become a number.

--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home .att.net>
Try the download section.

[Martien Verbruggen]

On 8 Oct 2008 14:11:29 GMT,
Richard Tobin <ri*****@cogsci .ed.ac.ukwrote:

The GNU Autoconf manual provides a work-around for missing isinf()
and isnan() functions, which exist in C99 and many, but not all,
pre-C99 implementations :

http://www.gnu.org/software/libtool/...rtability.html

The given definition of isinf(x) boils down to return isnan (x - x),
which will return true for NaNs. Surely this is wrong?

If isnan(x) is true, then x - x is also NaN, and isnan(x - x) would also
be true.

Check Annex F to the C99 standard. Specifically, in F8.2 it is made
clear that the equivalence x - x == 0.0 does not hold when x can be NaN
or Inf.

Martien
--
|
Martien Verbruggen | I took an IQ test and the results were
| negative.
|

[Nate Eldredge]

CBFalconer <cb********@yah oo.comwrites:

Richard Tobin wrote:

>>
The GNU Autoconf manual provides a work-around for missing isinf()
and isnan() functions, which exist in C99 and many, but not all,
pre-C99 implementations :

http://www.gnu.org/software/libtool/...rtability.html

The given definition of isinf(x) boils down to return isnan (x - x),
which will return true for NaNs. Surely this is wrong?

Think about it. If x is not a number, what can you say about x-x?
I fail to see any way that expression could become a number.

Indeed, if x is NaN then so is x-x, so isnan(x-x) would be true. But
7.12.3.3 says "The isinf macro returns a nonzero value if and only if
its argument has an infinite value." That makes it sound as if
isinf(x) should return zero if x is NaN, since NaN does not have an
infinite value.

[Richard Tobin]

In article <o9***********@ news.heliotrope .home>,
Martien Verbruggen <mg**@tradingpo st.com.auwrote:

>The given definition of isinf(x) boils down to return isnan (x - x),
which will return true for NaNs. Surely this is wrong?

>If isnan(x) is true, then x - x is also NaN, and isnan(x - x) would also
be true.

Exactly. And that would be wrong, because isinf(x) should be false
if x is a NaN.

Anyway, I eventually discovered that this mistake has already been
reported to the autoconf project.

-- Richard
--
Please remember to mention me / in tapes you leave behind.

[James Kuyper]

Martien Verbruggen wrote:

On 8 Oct 2008 14:11:29 GMT,
Richard Tobin <ri*****@cogsci .ed.ac.ukwrote:

>The GNU Autoconf manual provides a work-around for missing isinf()
and isnan() functions, which exist in C99 and many, but not all,
pre-C99 implementations :

http://www.gnu.org/software/libtool/...rtability.html

The given definition of isinf(x) boils down to return isnan (x - x),
which will return true for NaNs. Surely this is wrong?

If isnan(x) is true, then x - x is also NaN, and isnan(x - x) would also
be true.

Check Annex F to the C99 standard. Specifically, in F8.2 it is made
clear that the equivalence x - x == 0.0 does not hold when x can be NaN
or Inf.

Exactly. Which is why isnan(x-x) doesn't make sense as an implementation
of ifinf(x). isinf(x) isn't supposed to return true when x is a NaN;
it's only supposed to return true when x is infinite.

[Martien Verbruggen]

On Thu, 09 Oct 2008 12:57:20 GMT,
James Kuyper <ja*********@ve rizon.netwrote:

Martien Verbruggen wrote:

>On 8 Oct 2008 14:11:29 GMT,
Richard Tobin <ri*****@cogsci .ed.ac.ukwrote:

>>The GNU Autoconf manual provides a work-around for missing isinf()
and isnan() functions, which exist in C99 and many, but not all,
pre-C99 implementations :

http://www.gnu.org/software/libtool/...rtability.html

The given definition of isinf(x) boils down to return isnan (x - x),
which will return true for NaNs. Surely this is wrong?

If isnan(x) is true, then x - x is also NaN, and isnan(x - x) would also
be true.

Check Annex F to the C99 standard. Specifically, in F8.2 it is made
clear that the equivalence x - x == 0.0 does not hold when x can be NaN
or Inf.

Exactly. Which is why isnan(x-x) doesn't make sense as an implementation
of ifinf(x). isinf(x) isn't supposed to return true when x is a NaN;
it's only supposed to return true when x is infinite.

Sorry all. I misread the isinf() as isnan(). I thought the OP was
confused about why isnan(x-x) returned true.

Martien
--
| Yes; Windows is great for running &
Martien Verbruggen | developing viruses, for instance. It's also
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| common cold. -- Dave Hinz