Re: [Numpy-discussion] What is the logical value of nan?
On Wed, March 11, 2009 7:50 am, Christopher Barker wrote: Python does not distinguish between True and False -- Python makes the distinction between something and nothing. In that context, NaN is nothing, thus False. Mathematically speaking, NaN is a quantity with undefined value. Closer analysis of a particular case may reveal that it may be some finite number, or an infinity with some direction, or be intrinsically undefined. NaN is something that cannot be defined because its value is not unique. Nothing would be the content of empty set. Pearu ___ Numpy-discussion mailing list Numpy-discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
Re: [Numpy-discussion] What is the logical value of nan?
Charles R Harris wrote: It isn't 0 so it should be True. Any disagreement?... Chuck NaN is not a number equal to 0, so it should be True? NaN is not a number different from 0, so it should be False? Also see Pearu's comment. Why not raise an exception when NaN is evaluated in a boolean context? bool(NaN) has no obvious interpretation, so it should be considered an error. Sturla Molden ___ Numpy-discussion mailing list Numpy-discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
Re: [Numpy-discussion] What is the logical value of nan?
Charles R Harris wrote:
#include math.h
#include stdio.h
int main() {
double nan = sqrt(-1);
printf(%f\n, nan);
printf(%i\n, bool(nan));
return 0;
}
$ ./nan
nan
1
So resolved, it is True.
Unless specified in the ISO C standard, I'd say this is system and
compiler dependent.
Should NumPy rely on a specific binary representation of NaN?
A related issue is the boolean value of Inf and -Inf.
Sturla Molden
___
Numpy-discussion mailing list
Numpy-discussion@scipy.org
http://mail.scipy.org/mailman/listinfo/numpy-discussion
Re: [Numpy-discussion] What is the logical value of nan?
Sturla Molden wrote:
Charles R Harris wrote:
#include math.h
#include stdio.h
int main() {
double nan = sqrt(-1);
printf(%f\n, nan);
printf(%i\n, bool(nan));
return 0;
}
$ ./nan
nan
1
So resolved, it is True.
Unless specified in the ISO C standard, I'd say this is system and
compiler dependent.
Should NumPy rely on a specific binary representation of NaN?
A related issue is the boolean value of Inf and -Inf.
Sturla Molden
___
Numpy-discussion mailing list
Numpy-discussion@scipy.org
http://mail.scipy.org/mailman/listinfo/numpy-discussion
This is one link that shows the different representation of these
numbers in IEEE 754:
http://www.psc.edu/general/software/packages/ieee/ieee.php
It is a little clearer than Wikipedia:
http://en.wikipedia.org/wiki/IEEE_754-1985
Numpy's nan/NaN/NAN, inf/Inf/PINF, and NINF are not nothing so not zero.
Also, I think that conversion to an integer should be an error for all
of these because there is no equivalent representation of these floating
point numbers as integers and I think that using zero for NaN is wrong.
Now for the other two special representations, I would presume that
Numpy's PZERO (positive zero) and NZERO (negative zero) are treated as
nothing. Conversion to integer for these should be zero.
However, I noticed that the standard has just been revised that may
eventually influence Numpy:
http://en.wikipedia.org/wiki/IEEE_754r
http://en.wikipedia.org/wiki/IEEE_754-2008
Note this defines the min/max behavior:
* |min(x,NaN) = min(NaN,x) = x|
* |max(x,NaN) = max(NaN,x) = x|
Bruce
___
Numpy-discussion mailing list
Numpy-discussion@scipy.org
http://mail.scipy.org/mailman/listinfo/numpy-discussion
Re: [Numpy-discussion] What is the logical value of nan?
--- On Wed, 3/11/09, Bruce Southey bsout...@gmail.com wrote: From: Bruce Southey bsout...@gmail.com Subject: Re: [Numpy-discussion] What is the logical value of nan? To: Discussion of Numerical Python numpy-discussion@scipy.org Date: Wednesday, March 11, 2009, 10:24 AM This is one link that shows the different representation of these numbers in IEEE 754: http://www.psc.edu/general/software/packages/ieee/ieee.php It is a little clearer than Wikipedia: http://en.wikipedia.org/wiki/IEEE_754-1985 Thanks. Useful sites. Numpy's nan/NaN/NAN, inf/Inf/PINF, and NINF are not nothing so not zero. Agreed. +1 Also, I think that conversion to an integer should be an error for all of these because there is no equivalent representation of these floating point numbers as integers and I think that using zero for NaN is wrong. Another +1 Now for the other two special representations, I would presume that Numpy's PZERO (positive zero) and NZERO (negative zero) are treated as nothing. Conversion to integer for these should be zero. Yet another +1. -- Lou Pecora, my views are my own. ___ Numpy-discussion mailing list Numpy-discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
Re: [Numpy-discussion] What is the logical value of nan?
On Wed, Mar 11, 2009 at 8:24 AM, Bruce Southey bsout...@gmail.com wrote:
Sturla Molden wrote:
Charles R Harris wrote:
#include math.h
#include stdio.h
int main() {
double nan = sqrt(-1);
printf(%f\n, nan);
printf(%i\n, bool(nan));
return 0;
}
$ ./nan
nan
1
So resolved, it is True.
Unless specified in the ISO C standard, I'd say this is system and
compiler dependent.
Should NumPy rely on a specific binary representation of NaN?
A related issue is the boolean value of Inf and -Inf.
Sturla Molden
___
Numpy-discussion mailing list
Numpy-discussion@scipy.org
http://mail.scipy.org/mailman/listinfo/numpy-discussion
This is one link that shows the different representation of these
numbers in IEEE 754:
http://www.psc.edu/general/software/packages/ieee/ieee.php
It is a little clearer than Wikipedia:
http://en.wikipedia.org/wiki/IEEE_754-1985
Numpy's nan/NaN/NAN, inf/Inf/PINF, and NINF are not nothing so not zero.
Also, I think that conversion to an integer should be an error for all
of these because there is no equivalent representation of these floating
point numbers as integers and I think that using zero for NaN is wrong.
Now for the other two special representations, I would presume that
Numpy's PZERO (positive zero) and NZERO (negative zero) are treated as
nothing. Conversion to integer for these should be zero.
However, I noticed that the standard has just been revised that may
eventually influence Numpy:
http://en.wikipedia.org/wiki/IEEE_754r
http://en.wikipedia.org/wiki/IEEE_754-2008
Note this defines the min/max behavior:
* |min(x,NaN) = min(NaN,x) = x|
* |max(x,NaN) = max(NaN,x) = x|
We have this behavior in numpy with the fmax/fmin functions.
Chuck
___
Numpy-discussion mailing list
Numpy-discussion@scipy.org
http://mail.scipy.org/mailman/listinfo/numpy-discussion
Re: [Numpy-discussion] What is the logical value of nan?
Sturla Molden wrote: Why not raise an exception when NaN is evaluated in a boolean context? bool(NaN) has no obvious interpretation, so it should be considered an error. +1 Though there is clearly a lot of legacy around this, so maybe it's best to follow C convention (sigh). Bruce Southey wrote: Also, I think that conversion to an integer should be an error for all of these because there is no equivalent representation of these floating point numbers as integers and I think that using zero for NaN is wrong. +1 A silent wrong conversion is MUCH worse than an exception! As for MATLAB, it was entirely doubles for a long time -- I don't think it's a good example of well thought-out float-integer interactions. Now for the other two special representations, I would presume that Numpy's PZERO (positive zero) and NZERO (negative zero) are treated as nothing. Conversion to integer for these should be zero. +1 Note this defines the min/max behavior: * |min(x,NaN) = min(NaN,x) = x| * |max(x,NaN) = max(NaN,x) = x| nice -- it's nice to have these defined -- of course, who knows how long it will be (never?) before compilers/libraries support this. -Chris -- Christopher Barker, Ph.D. Oceanographer Emergency Response Division NOAA/NOS/ORR(206) 526-6959 voice 7600 Sand Point Way NE (206) 526-6329 fax Seattle, WA 98115 (206) 526-6317 main reception chris.bar...@noaa.gov ___ Numpy-discussion mailing list Numpy-discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
Re: [Numpy-discussion] What is the logical value of nan?
On Wed, Mar 11, 2009 at 11:06 AM, Christopher Barker chris.bar...@noaa.govwrote: Sturla Molden wrote: Why not raise an exception when NaN is evaluated in a boolean context? bool(NaN) has no obvious interpretation, so it should be considered an error. +1 Though there is clearly a lot of legacy around this, so maybe it's best to follow C convention (sigh). Bruce Southey wrote: Also, I think that conversion to an integer should be an error for all of these because there is no equivalent representation of these floating point numbers as integers and I think that using zero for NaN is wrong. +1 A silent wrong conversion is MUCH worse than an exception! As for MATLAB, it was entirely doubles for a long time -- I don't think it's a good example of well thought-out float-integer interactions. Now for the other two special representations, I would presume that Numpy's PZERO (positive zero) and NZERO (negative zero) are treated as nothing. Conversion to integer for these should be zero. +1 Note this defines the min/max behavior: * |min(x,NaN) = min(NaN,x) = x| * |max(x,NaN) = max(NaN,x) = x| nice -- it's nice to have these defined -- of course, who knows how long it will be (never?) before compilers/libraries support this. Raising exceptions in ufuncs is going to take some work as the inner loops are void functions without any means of indicating an error. Exceptions also need to be thread safe. So I am not opposed but it is something for the future. Casting seems to be implemented in arraytypes.inc.src as void functions also without provision for errors. I would also like to see casting implemented as ufuncs but that is a separate discussion. Chuck ___ Numpy-discussion mailing list Numpy-discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
Re: [Numpy-discussion] What is the logical value of nan?
Charles R Harris wrote: Raising exceptions in ufuncs is going to take some work as the inner loops are void functions without any means of indicating an error. Exceptions also need to be thread safe. So I am not opposed but it is something for the future. I just saw David Cournapeau's post regarding a NPY_NAN macro. As it uses the IEEE754 binary format, at least NPY_NAN should be True in a boolean context. So bool(nan) is True then. And that's what happens now on my computer as well: bool(nan) True I don't like Python exception's raised inside ufuncs. In the future we NumPy might add OpenMP support to ufuncs (multicore CPUs are getting common), and Python exceptions would prevent that, or at least make it difficult (cf. the GIL). S.M. ___ Numpy-discussion mailing list Numpy-discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
Re: [Numpy-discussion] What is the logical value of nan?
On Wed, Mar 11, 2009 at 12:19 PM, Sturla Molden stu...@molden.no wrote: Charles R Harris wrote: Raising exceptions in ufuncs is going to take some work as the inner loops are void functions without any means of indicating an error. Exceptions also need to be thread safe. So I am not opposed but it is something for the future. I just saw David Cournapeau's post regarding a NPY_NAN macro. As it uses the IEEE754 binary format, at least NPY_NAN should be True in a boolean context. So bool(nan) is True then. And that's what happens now on my computer as well: bool(nan) True I don't like Python exception's raised inside ufuncs. In the future we NumPy might add OpenMP support to ufuncs (multicore CPUs are getting common), and Python exceptions would prevent that, or at least make it difficult (cf. the GIL). I think numpy needs someway to raise these errors, but error handling is always tricky. Do you have any suggestions as to how you would like to do it? I was thinking that adding an int return to the loops would provide some way of indicating errors without specifying how they were to be handled at this point. Chuck ___ Numpy-discussion mailing list Numpy-discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
Re: [Numpy-discussion] What is the logical value of nan?
On Thu, Mar 12, 2009 at 3:36 AM, Charles R Harris
charlesr.har...@gmail.com wrote:
On Wed, Mar 11, 2009 at 12:19 PM, Sturla Molden stu...@molden.no wrote:
Charles R Harris wrote:
Raising exceptions in ufuncs is going to take some work as the inner
loops are void functions without any means of indicating an error.
Exceptions also need to be thread safe. So I am not opposed but it is
something for the future.
I just saw David Cournapeau's post regarding a NPY_NAN macro. As it uses
the IEEE754 binary format, at least NPY_NAN should be True in a boolean
context. So bool(nan) is True then.
And that's what happens now on my computer as well:
bool(nan)
True
I don't like Python exception's raised inside ufuncs. In the future we
NumPy might add OpenMP support to ufuncs (multicore CPUs are getting
common), and Python exceptions would prevent that, or at least make it
difficult (cf. the GIL).
I think numpy needs someway to raise these errors, but error handling is
always tricky. Do you have any suggestions as to how you would like to do
it? I was thinking that adding an int return to the loops would provide some
way of indicating errors without specifying how they were to be handled at
this point.
I think that we should think carefully about how to set up a good
error system within numpy. If we keep adding ad-hoc error handling, I
am afraid it will be hard to read and maintain. We could have
something like:
typedef struct
{
int error ;
const char *str ;
} ErrorStruct ;
static
ErrorStruct UfuncErrors [] =
{ {CODE1, error 1 string}, ...};
and the related functions to get strings from code. Currently, we
can't really pass errors through several callees because we don't have
a commonly agreed set of errors. If we don't use an errno, I don't
think there are any other options,
David
___
Numpy-discussion mailing list
Numpy-discussion@scipy.org
http://mail.scipy.org/mailman/listinfo/numpy-discussion
[Numpy-discussion] What is the logical value of nan?
It isn't 0 so it should be True. Any disagreement?... Chuck ___ Numpy-discussion mailing list Numpy-discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
Re: [Numpy-discussion] What is the logical value of nan?
I think it should be considered as roughly (the numerical) equivalent to None, therefore False Nadav. -הודעה מקורית- מאת: numpy-discussion-boun...@scipy.org בשם Charles R Harris נשלח: ג 10-מרץ-09 21:08 אל: numpy-discussion נושא: [Numpy-discussion] What is the logical value of nan? It isn't 0 so it should be True. Any disagreement?... Chuck winmail.dat___ Numpy-discussion mailing list Numpy-discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
Re: [Numpy-discussion] What is the logical value of nan?
Tue, 10 Mar 2009 13:08:17 -0600, Charles R Harris wrote: It isn't 0 so it should be True. Any disagreement? +1 Nonzero Python object, hence True. Moreover, it's also True in Python: import numpy as np type(np.nan) type 'float' bool(np.nan) True IMHO, we should follow Python here, otherwise unnecessary confusion may arise. -- Pauli Virtanen ___ Numpy-discussion mailing list Numpy-discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
Re: [Numpy-discussion] What is the logical value of nan?
2009/3/10 Pauli Virtanen p...@iki.fi:
Nonzero Python object, hence True. Moreover, it's also True in Python:
Also in C:
#include math.h
#include stdio.h
int main() {
double nan = sqrt(-1);
printf(%f\n, nan);
printf(%i\n, bool(nan));
return 0;
}
$ ./nan
nan
1
Cheers
Stéfan
___
Numpy-discussion mailing list
Numpy-discussion@scipy.org
http://mail.scipy.org/mailman/listinfo/numpy-discussion
Re: [Numpy-discussion] What is the logical value of nan?
On Tue, Mar 10, 2009 at 2:49 PM, Charles R Harris charlesr.har...@gmail.com
wrote:
On Tue, Mar 10, 2009 at 3:16 PM, Stéfan van der Walt ste...@sun.ac.zawrote:
2009/3/10 Pauli Virtanen p...@iki.fi:
Nonzero Python object, hence True. Moreover, it's also True in Python:
Also in C:
#include math.h
#include stdio.h
int main() {
double nan = sqrt(-1);
printf(%f\n, nan);
printf(%i\n, bool(nan));
return 0;
}
$ ./nan
nan
1
So resolved, it is True.
I appear to be late to the party, but IMO it should raise an exception in
those cases where it's feasible to do so.
--
. __
. |-\
.
. tim.hochb...@ieee.org
___
Numpy-discussion mailing list
Numpy-discussion@scipy.org
http://mail.scipy.org/mailman/listinfo/numpy-discussion
Re: [Numpy-discussion] What is the logical value of nan?
On Tue, Mar 10, 2009 at 4:19 PM, Timothy Hochberg tim.hochb...@ieee.orgwrote:
On Tue, Mar 10, 2009 at 2:49 PM, Charles R Harris
charlesr.har...@gmail.com wrote:
On Tue, Mar 10, 2009 at 3:16 PM, Stéfan van der Walt ste...@sun.ac.zawrote:
2009/3/10 Pauli Virtanen p...@iki.fi:
Nonzero Python object, hence True. Moreover, it's also True in Python:
Also in C:
#include math.h
#include stdio.h
int main() {
double nan = sqrt(-1);
printf(%f\n, nan);
printf(%i\n, bool(nan));
return 0;
}
$ ./nan
nan
1
So resolved, it is True.
I appear to be late to the party, but IMO it should raise an exception in
those cases where it's feasible to do so.
That also seems reasonable to me. There is also the unresolved issue of
whether casting nan to an integer should raise an exception, currently it is
just converted to 0.
Chuck
___
Numpy-discussion mailing list
Numpy-discussion@scipy.org
http://mail.scipy.org/mailman/listinfo/numpy-discussion
Re: [Numpy-discussion] What is the logical value of nan?
Charles R Harris wrote:
On Tue, Mar 10, 2009 at 4:19 PM, Timothy Hochberg
tim.hochb...@ieee.org mailto:tim.hochb...@ieee.org wrote:
On Tue, Mar 10, 2009 at 2:49 PM, Charles R Harris
charlesr.har...@gmail.com mailto:charlesr.har...@gmail.com wrote:
On Tue, Mar 10, 2009 at 3:16 PM, Stéfan van der Walt
ste...@sun.ac.za mailto:ste...@sun.ac.za wrote:
2009/3/10 Pauli Virtanen p...@iki.fi mailto:p...@iki.fi:
Nonzero Python object, hence True. Moreover, it's also
True in Python:
Also in C:
#include math.h
#include stdio.h
int main() {
double nan = sqrt(-1);
printf(%f\n, nan);
printf(%i\n, bool(nan));
return 0;
}
$ ./nan
nan
1
So resolved, it is True.
I appear to be late to the party, but IMO it should raise an
exception in those cases where it's feasible to do so.
That also seems reasonable to me. There is also the unresolved issue
of whether casting nan to an integer should raise an exception,
currently it is just converted to 0.
I think it is reasonable as well - but I am worried about the
integration with seterr (not just for this case, but in general in our
way toward better handling of this kind of things). I note that matlab
convert nan to 0 as well - presumably they did not handle it besides
what C guarantees (that is not much in that case I believe):
a = nan;
int32(a); % gives 0
in C:
#define _ISOC99_SOURCE
#include
stdio.h
#include
math.h
int
main(void)
{
printf(nan is %f\n,
NAN);
printf(nan is %d\n,
(int)NAN);
return
0;
}
prints nan and 0 respectively - it may well be implementation dependent,
but it seems that (int)nan simply gives back the nan binary representation.
cheers,
David
___
Numpy-discussion mailing list
Numpy-discussion@scipy.org
http://mail.scipy.org/mailman/listinfo/numpy-discussion
Re: [Numpy-discussion] What is the logical value of nan?
Pauli Virtanen wrote: Tue, 10 Mar 2009 13:08:17 -0600, Charles R Harris wrote: It isn't 0 so it should be True. Any disagreement? - 1 Nonzero Python object, hence True. Empty sequences are False also. There was a lot of discussion about all this when Guido added Bool types to python. Personally, I don't think zero should be false, I think only False (and maybe None) should be false -- is it so hard to write: if x != 0:, rather than if x:? but there is a LOT of legacy to 0 being False! Anyway, Laura Creighton wrote a great post about it, with this basic thesis: Python does not distinguish between True and False -- Python makes the distinction between something and nothing. In that context, NaN is nothing, thus False. my $0.02 -Chris ___ Numpy-discussion mailing list Numpy-discussion@scipy.org http://mail.scipy.org/mailman/listinfo/numpy-discussion
