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digitalmars.D - -0 assigned to a FP variable

reply bearophile <bearophileHUGS lycos.com> writes:
A bug I've introduced once in my D code, are you able to spot it?

void main() {
    double x = +0;
    double y = -0;
}


The bug: 'y' isn't the desired double -0.0

To avoid this bug DMD may keep the -0 and +0 integer literals represented in
two distinct ways, so they have two different values when/if assigned to a
floating point. (But here D will behave a little differently from C).
An alternative is to just add a warning to DMD.
A third possibility is to just ignore this probably uncommon bug :-)

Bye,
bearophile
Jan 09 2011
next sibling parent reply Jonathan M Davis <jmdavisProg gmx.com> writes:
On Sunday 09 January 2011 16:27:11 bearophile wrote:

 A bug I've introduced once in my D code, are you able to spot it?
 
 void main() {
     double x = +0;
     double y = -0;
 }
 
 
 The bug: 'y' isn't the desired double -0.0
 
 To avoid this bug DMD may keep the -0 and +0 integer literals represented
 in two distinct ways, so they have two different values when/if assigned
 to a floating point. (But here D will behave a little differently from C).
 An alternative is to just add a warning to DMD.
 A third possibility is to just ignore this probably uncommon bug :-)
 
 Bye,
 bearophile


I didn't even know that there _was_ such as thing as + or - 0 (or 0.0). I would 
have considered it a no-op, being identical to 0 or 0.0, and expected it to be 
compiled out completely. I haven't a clue what -0.0 would even mean. But I'm
not 
exactly an expert on floating point values, so presumably, there's some weird 
floating point thing that - affects. Honestly, the more I deal with floating 
points, the more I wish that we were dealing with fixed points. Granted,
there's 
probably a number of reasons why floating points are better than fixed points,
but 
in general, I just don't see it.

- Jonathan M Davis
Jan 09 2011
parent "Robert Jacques" <sandford jhu.edu> writes:
On Sun, 09 Jan 2011 20:39:12 -0500, Jonathan M Davis <jmdavisProg gmx.com>  
wrote:


 On Sunday 09 January 2011 16:27:11 bearophile wrote:

 A bug I've introduced once in my D code, are you able to spot it?

 void main() {
     double x = +0;
     double y = -0;
 }


 The bug: 'y' isn't the desired double -0.0

 To avoid this bug DMD may keep the -0 and +0 integer literals  
 represented
 in two distinct ways, so they have two different values when/if assigned
 to a floating point. (But here D will behave a little differently from  
 C).
 An alternative is to just add a warning to DMD.
 A third possibility is to just ignore this probably uncommon bug :-)

 Bye,
 bearophile


 I didn't even know that there _was_ such as thing as + or - 0 (or 0.0).  
 I would
 have considered it a no-op, being identical to 0 or 0.0, and expected it  
 to be
 compiled out completely. I haven't a clue what -0.0 would even mean. But  
 I'm not
 exactly an expert on floating point values, so presumably, there's some  
 weird
 floating point thing that - affects.


-0.0 is an artifact of the floating point sign bit: i.e. there is a + and  
- for each value, so naturally there's also + and - 0. The difference  
isn't generally something you care about, as they are practically  
identical except in bit pattern (i.e. assert( 0.0 == -0.0 )).
Jan 09 2011
prev sibling next sibling parent Jim <bitcirkel yahoo.com> writes:
This is not really a bug.

I'd expect -0 to be an integer 0. Negative zero doesn't make sense for
integers, and the behavior is consistent with C/C++.

A warning, like you proposed, could possibly help avoiding a mistake.



bearophile Wrote:


 A bug I've introduced once in my D code, are you able to spot it?
 
 void main() {
     double x = +0;
     double y = -0;
 }
 
 
 The bug: 'y' isn't the desired double -0.0
 
 To avoid this bug DMD may keep the -0 and +0 integer literals represented in
two distinct ways, so they have two different values when/if assigned to a
floating point. (But here D will behave a little differently from C).
 An alternative is to just add a warning to DMD.
 A third possibility is to just ignore this probably uncommon bug :-)
 
 Bye,
 bearophile
Jan 09 2011
prev sibling next sibling parent Walter Bright <newshound2 digitalmars.com> writes:
bearophile wrote:

 A bug I've introduced once in my D code, are you able to spot it?
 
 void main() { double x = +0; double y = -0; }
 
 
 The bug: 'y' isn't the desired double -0.0
 
 To avoid this bug DMD may keep the -0 and +0 integer literals represented in
 two distinct ways, so they have two different values when/if assigned to a
 floating point. (But here D will behave a little differently from C). An
 alternative is to just add a warning to DMD. A third possibility is to just
 ignore this probably uncommon bug :-)



This is not a bug in the language or compiler. While there is such a thing as 
-0.0, there is no such thing as -0 as being distinct from 0. If you want to
flag 
mathematical no-ops as errors, what's next? a+0 is an error, too?
Jan 09 2011
prev sibling next sibling parent Justin Johansson <noreply jj.com> writes:
On 10/01/11 11:27, bearophile wrote:

 A bug I've introduced once in my D code, are you able to spot it?

 void main() {
      double x = +0;
      double y = -0;
 }


 The bug: 'y' isn't the desired double -0.0

 To avoid this bug DMD may keep the -0 and +0 integer literals represented in
two distinct ways, so they have two different values when/if assigned to a
floating point. (But here D will behave a little differently from C).
 An alternative is to just add a warning to DMD.
 A third possibility is to just ignore this probably uncommon bug :-)

 Bye,
 bearophile


Hi bearophile,

This time it really looks like you are trying to pull the wings off of a 
butterfly.
But in this case me thinks the butterfly wins.

Best regards and Happy New Year,

Justin Johansson
Jan 10 2011
prev sibling next sibling parent Justin Johansson <noreply jj.com> writes:
On 10/01/11 11:27, bearophile wrote:

 A bug I've introduced once in my D code, are you able to spot it?

 void main() {
      double x = +0;
      double y = -0;
 }


 The bug: 'y' isn't the desired double -0.0

 To avoid this bug DMD may keep the -0 and +0 integer literals represented in
two distinct ways, so they have two different values when/if assigned to a
floating point. (But here D will behave a little differently from C).
 An alternative is to just add a warning to DMD.
 A third possibility is to just ignore this probably uncommon bug :-)

 Bye,
 bearophile



 A bug I've introduced once in my D code, are you able to spot it?

 void main() {
      double x = +0;
      double y = -0;
 }


The butterfly answers back to you.

There is no bug.  Did you mean

<code>
void main() {
      double x = +0.0;
      double y = -0.0;
}
</code>  ?

Are you able to spot it?

Is it still a bug?

Cheers
Justin
Jan 10 2011
prev sibling parent Olivier Pisano <olivier.pisano laposte.net> writes:
Le 10/01/2011 01:27, bearophile a écrit :

 A bug I've introduced once in my D code, are you able to spot it?

 void main() {
      double x = +0;
      double y = -0;
 }


 The bug: 'y' isn't the desired double -0.0

 To avoid this bug DMD may keep the -0 and +0 integer literals represented in
two distinct ways, so they have two different values when/if assigned to a
floating point. (But here D will behave a little differently from C).
 An alternative is to just add a warning to DMD.
 A third possibility is to just ignore this probably uncommon bug :-)

 Bye,
 bearophile


Hi,

Do you suggest the compiler should guess you want -0.0 and convert 0 to 
double before applying the unary minus operator ?

I suppose such a fix would be likely to create many more bugs than the 
one it is suppose to solve.

I don't have a clue on what kind of user code needs to differentiate 0.0 
and -0.0 fp litterals, but I humbly suppose that it must be kind of rare.
Jan 10 2011