Re: a Python bug report

[Peter J. Holzer]

On 2018-05-29 21:34:21 -0400, José María Mateos wrote:
> On Wed, May 30, 2018 at 01:07:38AM +, Ruifeng Guo wrote:
> > We encountered a bug in Python recently, we checked the behavior for Python 
> > version 2.7.12, and 3.1.1, both version show the same behavior. Please see 
> > below the unexpected behavior in "red text".
[...]
> In [3]: 1000 * 1.017
> Out[3]: 1016.9
> 
> So there you have it.

To expand a bit on that, the reason, why 1000 * 1.017 isn't 1017 isn't
that an x86 processor can't multiply, it is that 1017/1000 cannot be exactly
represented in a binary fraction (just like 1/7 cannot be exactly
represented in a decimal fraction).

So when you type 1.017, the computer really stores
1.016904076730672386474907398223876953125 and when you
multiply that by 1000, the result would be
1016.904076730672386474907398223876953125, but that needs a
few bits too much, so it rounded down to
1016.8863131622783839702606201171875.

hp

-- 
   _  | Peter J. Holzer| we build much bigger, better disasters now
|_|_) || because we have much more sophisticated
| |   | h...@hjp.at | management tools.
__/   | http://www.hjp.at/ | -- Ross Anderson 


signature.asc
Description: PGP signature
-- 
https://mail.python.org/mailman/listinfo/python-list

Re: a Python bug report

[Ian Kelly]

On Tue, May 29, 2018 at 7:07 PM, Ruifeng Guo  wrote:
> Hello,
> We encountered a bug in Python recently, we checked the behavior for Python 
> version 2.7.12, and 3.1.1, both version show the same behavior. Please see 
> below the unexpected behavior in "red text".
>
> Thanks,
> Ruifeng Guo
>
> From: Brian Archer
> Sent: Tuesday, May 29, 2018 5:57 PM
> To: Ruifeng Guo 
> Subject: Python Bug
>
> Python 3.1.1 (r311:74480, Nov 20 2012, 09:11:57)
> [GCC 4.2.2] on linux2
> Type "help", "copyright", "credits" or "license" for more information.
 a=1017.0
 print(int(a))
> 1017
 b=1000*1.017
 print(b)
> 1017.0
 int(b)
> 1016
 c=1017.0
 int(c)
> 1017

Try this, and you'll see what the problem is:

>>> repr(b)
'1016.9'

The value of b is not really 1017, but fractionally less as a result
of floating point rounding error, because 1.017 cannot be exactly
represented as a float.

In Python 3.2, the str() of the float type was changed to match the
repr(), so that when you use print() as above you will also get this
result:

>>> print(b)
1016.9

By the way, Python 3.1.1 is really old (six years!). I recommend
upgrading if possible.
-- 
https://mail.python.org/mailman/listinfo/python-list

Re: a Python bug report

[José María Mateos]

On Wed, May 30, 2018 at 01:07:38AM +, Ruifeng Guo wrote:
> Hello,
> We encountered a bug in Python recently, we checked the behavior for Python 
> version 2.7.12, and 3.1.1, both version show the same behavior. Please see 
> below the unexpected behavior in "red text".

Have you tried the round() function, however?

In [1]: round(1000 * 1.017)
Out[1]: 1017.0

This is a floating point precision "issue". int() only gets rid of the 
decimals.

In [2]: int(3.9)
Out[2]: 3

Because:

In [3]: 1000 * 1.017
Out[3]: 1016.9

So there you have it.

Some more reading: 
https://stackoverflow.com/questions/43660910/python-difference-between-round-and-int

Cheers,

-- 
José María (Chema) Mateos
https://rinzewind.org/blog-es || https://rinzewind.org/blog-en
-- 
https://mail.python.org/mailman/listinfo/python-list

a Python bug report

[Ruifeng Guo]

Hello,
We encountered a bug in Python recently, we checked the behavior for Python 
version 2.7.12, and 3.1.1, both version show the same behavior. Please see 
below the unexpected behavior in "red text".

Thanks,
Ruifeng Guo

From: Brian Archer
Sent: Tuesday, May 29, 2018 5:57 PM
To: Ruifeng Guo 
Subject: Python Bug

Python 3.1.1 (r311:74480, Nov 20 2012, 09:11:57)
[GCC 4.2.2] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> a=1017.0
>>> print(int(a))
1017
>>> b=1000*1.017
>>> print(b)
1017.0
>>> int(b)
1016
>>> c=1017.0
>>> int(c)
1017
>>>



-- 
https://mail.python.org/mailman/listinfo/python-list