numpy-discussion

[Numpy-discussion] exp behaviour: is it a bug ?

[Czerminski, R. <ry...@ar...>]

Is the behaviour illustrated below a bug
or the Python's feature ?

Python 1.5.2 (#4, Mar  3 2000, 15:04:36)  [GCC 2.8.1] on irix6
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
>>>
>>> from math import exp
>>> x = -706.
>>> while x > -900:
...    x = x - 1
...    print 'x, exp(x) = %g %g' % (x, exp(x))
... 
x, exp(x) = -707 8.99086e-308
x, exp(x) = -708 3.30755e-308
x, exp(x) = -709 0
[...]
x, exp(x) = -745 0
Traceback (innermost last):
  File "<stdin>", line 3, in ?
OverflowError: math range error
>>>

Ryszard Czerminski   phone: (781)994-0479
ArQule, Inc.         email:ry...@ar...
19 Presidential Way  http://www.arqule.com
Woburn, MA 01801     fax: (781)994-0679


-----Original Message-----
From: Jean-Bernard Addor [mailto:jb...@ph...]
Sent: Friday, June 02, 2000 10:19 AM
To: Num...@li...
Subject: [Numpy-discussion] Is it a bug ?


Hey Numeric people!

I am just upgrading to a more recent version of Numeric and observe a new
behaviour:

Python 1.5.2 (#9, May 30 2000, 15:08:12)  [GCC 2.95.2 19991024 (release)]
on linux2
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
Hello from .pythonrc.py
>>> import Numeric
>>> Numeric.__version__
'11'
>>> Numeric.arange(2)*1j
Segmentation fault

Python 1.5.1 (#1, Dec 17 1998, 20:58:15)  [GCC 2.7.2.3] on linux2
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
Hello from .pythonrc.py
>>> import Numeric
>>> Numeric.__version__ 
'1.7'
>>> Numeric.arange(2)*1j
array([ 0.+0.j,  0.+1.j])

I also saw:
              Numerical Python - Bug Tracking
              Viewing Open Bugs
                Bug ID
                                           Summary
               102277
                     CFLOAT/DOUBLE_setitem crashes when accessing imag.
part 

Am I hitting that bug?

CU

Jean-Bernard


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Re: [Numpy-discussion] exp behaviour: is it a bug ?

[Jon S. <js...@wm...>]

No, it works correctly for me (other Python revision, anyway):
Python 1.5.2 (#3, Sep 27 1999, 15:02:20)  [GCC egcs-2.91.66 19990314
(egcs-1.1.2
  on linux2
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
>>> from math import *
>>> x=-706.
>>> while x>-900:
...     x=x-1
...     print 'x, exp(x) = %g %g' % (x, exp(x))
... 
x, exp(x) = -707 8.99086e-308
x, exp(x) = -708 3.30755e-308
x, exp(x) = -709 1.21678e-308
.....
x, exp(x) = -898 0
x, exp(x) = -899 0
x, exp(x) = -900 0
>>>


Jon Saenz.				| Tfno: +34 946012470
Depto. Fisica Aplicada II               | Fax:  +34 944648500
Facultad de Ciencias.   \\ Universidad del Pais Vasco \\
Apdo. 644   \\ 48080 - Bilbao  \\ SPAIN

On Fri, 2 Jun 2000, Czerminski, Ryszard wrote:

> 
> Is the behaviour illustrated below a bug
> or the Python's feature ?
> 
> Python 1.5.2 (#4, Mar  3 2000, 15:04:36)  [GCC 2.8.1] on irix6
> Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
> >>>
> >>> from math import exp
> >>> x = -706.
> >>> while x > -900:
> ...    x = x - 1
> ...    print 'x, exp(x) = %g %g' % (x, exp(x))
> ... 
> x, exp(x) = -707 8.99086e-308
> x, exp(x) = -708 3.30755e-308
> x, exp(x) = -709 0
> [...]
> x, exp(x) = -745 0
> Traceback (innermost last):
>   File "<stdin>", line 3, in ?
> OverflowError: math range error
> >>>
> 
> Ryszard Czerminski   phone: (781)994-0479
> ArQule, Inc.         email:ry...@ar...
> 19 Presidential Way  http://www.arqule.com
> Woburn, MA 01801     fax: (781)994-0679
> 
> 
> -----Original Message-----
> From: Jean-Bernard Addor [mailto:jb...@ph...]
> Sent: Friday, June 02, 2000 10:19 AM
> To: Num...@li...
> Subject: [Numpy-discussion] Is it a bug ?
> 
> 
> Hey Numeric people!
> 
> I am just upgrading to a more recent version of Numeric and observe a new
> behaviour:
> 
> Python 1.5.2 (#9, May 30 2000, 15:08:12)  [GCC 2.95.2 19991024 (release)]
> on linux2
> Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
> Hello from .pythonrc.py
> >>> import Numeric
> >>> Numeric.__version__
> '11'
> >>> Numeric.arange(2)*1j
> Segmentation fault
> 
> Python 1.5.1 (#1, Dec 17 1998, 20:58:15)  [GCC 2.7.2.3] on linux2
> Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
> Hello from .pythonrc.py
> >>> import Numeric
> >>> Numeric.__version__ 
> '1.7'
> >>> Numeric.arange(2)*1j
> array([ 0.+0.j,  0.+1.j])
> 
> I also saw:
>               Numerical Python - Bug Tracking
>               Viewing Open Bugs
>                 Bug ID
>                                            Summary
>                102277
>                      CFLOAT/DOUBLE_setitem crashes when accessing imag.
> part 
> 
> Am I hitting that bug?
> 
> CU
> 
> Jean-Bernard
> 
> 
> _______________________________________________
> Numpy-discussion mailing list
> Num...@li...
> http://lists.sourceforge.net/mailman/listinfo/numpy-discussion
> 
> _______________________________________________
> Numpy-discussion mailing list
> Num...@li...
> http://lists.sourceforge.net/mailman/listinfo/numpy-discussion
> 

[Numpy-discussion] math.exp underflow (was: exp behaviour: is it a bug ?)

[Charles G W. <cg...@fn...>]

Czerminski, Ryszard writes:
 > 
 > Is the behaviour illustrated below a bug or the Python's feature ?

I don't know why you're posting this to the NumPy discussion list,
since the
"math" module is part of the standard Python distribution, not NumPy.

Anyhow, the differing behaviors people are seeing are just coming from
differences in the system math libraries:

On Linux (Intel):

Python 1.6a2 (#9, May 22 2000, 12:34:51)  [GCC 2.95.2 19991024 (release)] on linux2
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
Copyright 1995-2000 Corporation for National Research Initiatives (CNRI)
>>> from math import exp
>>> x = -706.
>>> while x > -900:
...    x = x - 1
...    print 'x, exp(x) = %g %g' % (x, exp(x))
... 
x, exp(x) = -707 8.99086e-308
x, exp(x) = -708 3.30755e-308
<snip>
x, exp(x) = -745 4.94066e-324
x, exp(x) = -746 0
<snip>
x, exp(x) = -899 0
x, exp(x) = -900 0
>>> 


Wheras on Irix (MIPS):
Python 1.6a2 (#8, Jun  1 2000, 20:01:55) [C] on irix646
Copyright 1991-1995 Stichting Mathematisch Centrum, Amsterdam
Copyright 1995-2000 Corporation for National Research Initiatives (CNRI)
>>> from math import exp
>>> x = -706.
>>> while x > -900:
...    x = x - 1
...    print 'x, exp(x) = %g %g' % (x, exp(x))
... 
x, exp(x) = -707 8.99086e-308
x, exp(x) = -708 3.30755e-308
x, exp(x) = -709 0
x, exp(x) = -710 0
<snip>
x, exp(x) = -745 0
Traceback (most recent call last):
  File "<stdin>", line 3, in ?
OverflowError: math range error


From the Irix man page for exp(3M):

     The exp functions return HUGE_VAL when the correct value would overflow,
     and return zero if the correct value would underflow. The -lm and -lmx
     versions set the value of errno to ERANGE for both underflow and
     overflow.

Since the Python math module sees errno set after the call to exp, it
raises an exception.

Whereas on Linux, exp(-very big number) simply returns 0 and does not
set errno.

On the one hand, Python's behavior makes sense because it simply
reflects the behavior of the system math libraries.  On the other
hand, these kinds of differences make it hard to write portable code -
you could test on Linux and think everything is OK, then run on IRIX
and get exceptions.  Maybe that's just the way life is when you are
using floating-point math... Tim Peters may have more to say on this
topic <exp(-900)wink>