Hello.
I want to be able to portably determine whether or not a given double
value is negative.
Under C99, I do:
if (signbit(d)) negative = 1;
Or, if the implementation doesn't provide signbit(), I do:
union real {
unsigned long long n;
double d;
};
if ((real.n >63) & 1) negative = 1;
However, what if I am to support an implementation that only provides
C89 (no long long, no signbit())?
Anybody got a reliable, portable method of handling this?
thanks,
mc
27  6080  ar**********@go oglemail.com wrote, On 27/04/07 21:48:
Hello.
I want to be able to portably determine whether or not a given double
value is negative.
Under C99, I do:
if (signbit(d)) negative = 1;
<snip>
However, what if I am to support an implementation that only provides
C89 (no long long, no signbit())?
Anybody got a reliable, portable method of handling this?
What is wrong with
if (d < 0)
?
It won't detect negative 0, but is that important to you?
--
Flash Gordon
On Apr 27, 9:53 pm, Flash Gordon <s...@flash-gordon.me.ukwro te:
>
Anybody got a reliable, portable method of handling this?
What is wrong with
if (d < 0)
?
It won't detect negative 0, but is that important to you?
Yes, unfortunately it is.
mc
In article <11************ **********@b40g 2000prd.googleg roups.com>,
<ar**********@g ooglemail.comwr ote:
>Hello.
I want to be able to portably determine whether or not a given double
value is negative.
[...]
>Anybody got a reliable, portable method of handling this?
Why isn't "double_val <0", possibly combined with tests for magic values
like NaN that will or won't pass this test when they shoudn't or should,
reliable or portable enough?
(I am not a numerical analyst; there may be a valid reason why not.
But if you're asking C programmers who aren't numerical analysts how to
do this, you should be able to tell us why the obvious solution doesn't
meet your standards.)
dave
--
Dave Vandervies dj******@csclub .uwaterloo.ca
Surprise your compiler. Write better code than it asks you to.
--Keith Thompson in comp.lang.c
<ar**********@g ooglemail.comha scritto nel messaggio
news:11******** **************@ b40g2000prd.goo glegroups.com.. .
Hello.
I want to be able to portably determine whether or not a given double
value is negative.
Under C99, I do:
if (signbit(d)) negative = 1;
Or, if the implementation doesn't provide signbit(), I do:
union real {
unsigned long long n;
double d;
};
if ((real.n >63) & 1) negative = 1;
However, what if I am to support an implementation that only provides
C89 (no long long, no signbit())?
Anybody got a reliable, portable method of handling this?
if (d < 0) negative = 1;
<ar**********@g ooglemail.comha scritto nel messaggio
news:11******** **************@ s33g2000prh.goo glegroups.com.. .
On Apr 27, 9:53 pm, Flash Gordon <s...@flash-gordon.me.ukwro te:
>>
Anybody got a reliable, portable method of handling this?
What is wrong with
if (d < 0)
?
It won't detect negative 0, but is that important to you?
Yes, unfortunately it is.
Then use if (d != fabs(d))
On Apr 27, 10:18 pm, "Army1987" <please....@for .itwrote:
>
Then use if (d != fabs(d))
Ah! Yes, that's the one. Very smart.
thanks,
MC
Army1987 wrote On 04/27/07 17:18,:
<ar**********@g ooglemail.comha scritto nel messaggio
news:11******** **************@ s33g2000prh.goo glegroups.com.. .
>>On Apr 27, 9:53 pm, Flash Gordon <s...@flash-gordon.me.ukwro te:
>>>>Anybody got a reliable, portable method of handling this?
What is wrong with
if (d < 0)
?
It won't detect negative 0, but is that important to you?
Yes, unfortunately it is.
Then use if (d != fabs(d))
Won't work; -0 is equal to +0, and there are problems
with NaNs and infinities, too, if they exist. That's
probably why signbit() exists, and I think it's the best
solution to artifact's problem.
Suggestion: Write the code to use signbit(), and use
the system-provided signbit() if one exists. For platforms
where it doesn't, provide your own platform-specific code
to implement it. 100% portability can't be guaranteed; in
particular, the kludge with a union will not always work.
(The sign bit may not be in the position where you expect
it, `long' and `double' may not be the same size, ...)
-- Er*********@sun .com
On Apr 27, 10:01 pm, dj3va...@caffei ne.csclub.uwate rloo.ca (Dave
Vandervies) wrote:
>
Anybody got a reliable, portable method of handling this?
Why isn't "double_val <0", possibly combined with tests for magic values
like NaN that will or won't pass this test when they shoudn't or should,
reliable or portable enough?
(I am not a numerical analyst; there may be a valid reason why not.
But if you're asking C programmers who aren't numerical analysts how to
do this, you should be able to tell us why the obvious solution doesn't
meet your standards.)
The simple answer is that I need to be able to detect negative zero
(after
doing all appropriate checks for NaN, inf, -inf etc.
MC ar**********@go oglemail.com wrote:
Hello.
I want to be able to portably determine whether or not a given double
value is negative.
inline int negativep(doubl e x) { return x < 0; }
Under C99, I do:
if (signbit(d)) negative = 1;
see above.
>
Or, if the implementation doesn't provide signbit(), I do:
union real {
unsigned long long n;
double d;
};
if ((real.n >63) & 1) negative = 1;
see above.
>
However, what if I am to support an implementation that only provides
C89 (no long long, no signbit())?
see above.
>
Anybody got a reliable, portable method of handling this?
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