Re: 2.6.1 - simple division
farsi...@gmail.com wrote: Thanks all, that's very helpful, sorry to waste your time with a common question. I have tried the decimal module and will definitely keep using it if I need to do this kind of calculation again. I have 1 more question that the floating point article that was linked didn't really answer: x = 0.8 x 0.804 x * 5 4.0 Shouldn't I be expecting something like 4.2 ? How different is 4.0? regards Steve -- Steve Holden +1 571 484 6266 +1 800 494 3119 Holden Web LLC http://www.holdenweb.com/ Want to know? Come to PyCon - soon! http://us.pycon.org/ -- http://mail.python.org/mailman/listinfo/python-list
Re: 2.6.1 - simple division
farsi...@gmail.com wrote: 4 / 5.0 0.84 0.8 * 5 4.0 Start here: http://docs.python.org/tutorial/floatingpoint.html Play with these: 4/5.0 0.84 print 4/5.0 0.8 print repr(4/5.0) 0.84 str(4/5.0) '0.8' repr(4/5.0) '0.80004' And learn more here: http://www.lahey.com/float.htm http://en.wikipedia.org/wiki/Floating-point -tkc -- http://mail.python.org/mailman/listinfo/python-list
Re: 2.6.1 - simple division
Duncan Booth wrote:
farsi...@gmail.com wrote:
Thanks all, that's very helpful, sorry to waste your time with a
common question. I have tried the decimal module and will definitely
keep using it if I need to do this kind of calculation again.
Try to remember though that the decimal module simply replaces one source
of inaccuracies with another one:
Decimal(1)/Decimal(3)
Decimal('0.')
_ * 3
Decimal('0.')
1./3.
0.1
_ * 3
1.0
Sometimes you want floating point, sometimes you want Decimal. You need to
understand the advantages and drawbacks of each in order to make an
informed choice.
I have 1 more question that the floating point article that was linked
didn't really answer:
x = 0.8
x
0.804
x * 5
4.0
Shouldn't I be expecting something like 4.2 ?
You should certainly expect that the final result may be a little bit away
from the 'exact' result but rounding errors can work in your favour just as
well as they work against.
You should also keep fractions.Fraction (Rational) number in your
toolbox. Rational is the most accurate, though it still can't represent
irrational numbers[1] (e.g. pi, e, phi, sqrt(2)) and is generally slower
than the others.
[1] actually, neither float nor Decimal can represent irrationals.
PS: Actually, I've been thinking about reducing numbers that can't be
represented as float, Decimals, Rationals by adding such things as surds
and special constants. Basically getting python to have exact arithmetic
library, but I think it'll be too large and complex while not many
people really need exact arithmetic.
--
http://mail.python.org/mailman/listinfo/python-list
2.6.1 - simple division
4 / 5.0 0.84 0.8 * 5 4.0 python 2.6.1 on mac. What the hell is going on here? -- http://mail.python.org/mailman/listinfo/python-list
Re: 2.6.1 - simple division
j = .8 j 0.80004 Python follows the IEEE-754 standard, which doesn't represent the numbers exactly. See http://en.wikipedia.org/wiki/IEEE_754 Mohammad Tayseer http://spellcoder.com/blogs/tayseer From: farsi...@gmail.com farsi...@gmail.com Sent: Sunday, March 8, 2009 4:16:53 PM Subject: 2.6.1 - simple division 4 / 5.0 0.84 0.8 * 5 4.0 python 2.6.1 on mac. What the hell is going on here? -- http://mail.python.org/mailman/listinfo/python-list -- http://mail.python.org/mailman/listinfo/python-list
Re: 2.6.1 - simple division
2009/3/8 farsi...@gmail.com: 4 / 5.0 0.84 0.8 * 5 4.0 python 2.6.1 on mac. What the hell is going on here? I know this has already been answered in detail, but one thing that it's easy for those new to floating point issues to miss is that fractions that can be expressed exactly in decimal can end up as recurring decimals in binary. 0.8 looks nice and tidy, but in binary (if I get this right) it's 0.1100110011001100..., recurring ad infinitum. The computer has to truncate it somewhere. -- Tim Rowe -- http://mail.python.org/mailman/listinfo/python-list
Re: 2.6.1 - simple division
On Mar 8, 2:16 pm, farsi...@gmail.com wrote: 4 / 5.0 0.84 0.8 * 5 4.0 python 2.6.1 on mac. What the hell is going on here? Pure curiosity prompted me to try the following: 40 / 5.0 8.0 Strange... -- http://mail.python.org/mailman/listinfo/python-list
Re: 2.6.1 - simple division
En Sun, 08 Mar 2009 12:22:50 -0200, farsi...@gmail.com escribió: On Mar 8, 2:16 pm, farsi...@gmail.com wrote: 4 / 5.0 0.84 0.8 * 5 4.0 python 2.6.1 on mac. What the hell is going on here? Pure curiosity prompted me to try the following: 40 / 5.0 8.0 Strange... See http://docs.python.org/tutorial/floatingpoint.html -- Gabriel Genellina -- http://mail.python.org/mailman/listinfo/python-list
Re: 2.6.1 - simple division
farsi...@gmail.com wrote: On Mar 8, 2:16 pm, farsi...@gmail.com wrote: 4 / 5.0 0.84 This one is a common FAQ. Basically floating point is never to be trusted. This issue is quite language agnostic, however some language decided to hide the issue, python does not. For more information on floating point and its intricacies: http://docs.python.org/tutorial/floatingpoint.html 0.8 * 5 4.0 python 2.6.1 on mac. What the hell is going on here? Pure curiosity prompted me to try the following: 40 / 5.0 8.0 Strange... Strange, I don't see anything strange with that... Perhaps you meant, python returns 4.0 instead of 4? It's because in division with at least one of the divisor or the dividend a floating point will return a floating point value. Perhaps you're confusing it with integer division, in which both divisor and dividend are integers. In python 2.6, this will still return integers, but this will change (edit: have changed) in python 3.x, division of integer by integer will always result in floating point even if the result can be represented exactly by an integer. You can do 'from __future__ import division' to use the new division semantic in python 2.x -- http://mail.python.org/mailman/listinfo/python-list
Re: 2.6.1 - simple division
On Mar 8, 7:22 pm, farsi...@gmail.com wrote: On Mar 8, 2:16 pm, farsi...@gmail.com wrote: 4 / 5.0 0.84 0.8 * 5 4.0 python 2.6.1 on mac. What the hell is going on here? Pure curiosity prompted me to try the following: 40 / 5.0 8.0 Strange... Please see http://docs.python.org/library/decimal.html for explanation of why this happens. To get the proper values, try Decimal(4)/Decimal(5.0) Decimal(0.8) -- http://mail.python.org/mailman/listinfo/python-list
Re: 2.6.1 - simple division
Thanks all, that's very helpful, sorry to waste your time with a common question. I have tried the decimal module and will definitely keep using it if I need to do this kind of calculation again. I have 1 more question that the floating point article that was linked didn't really answer: x = 0.8 x 0.804 x * 5 4.0 Shouldn't I be expecting something like 4.2 ? -- http://mail.python.org/mailman/listinfo/python-list
Re: 2.6.1 - simple division
farsi...@gmail.com wrote:
Thanks all, that's very helpful, sorry to waste your time with a
common question. I have tried the decimal module and will definitely
keep using it if I need to do this kind of calculation again.
Try to remember though that the decimal module simply replaces one source
of inaccuracies with another one:
Decimal(1)/Decimal(3)
Decimal('0.')
_ * 3
Decimal('0.')
1./3.
0.1
_ * 3
1.0
Sometimes you want floating point, sometimes you want Decimal. You need to
understand the advantages and drawbacks of each in order to make an
informed choice.
I have 1 more question that the floating point article that was linked
didn't really answer:
x = 0.8
x
0.804
x * 5
4.0
Shouldn't I be expecting something like 4.2 ?
You should certainly expect that the final result may be a little bit away
from the 'exact' result but rounding errors can work in your favour just as
well as they work against.
--
http://mail.python.org/mailman/listinfo/python-list
Re: 2.6.1 - simple division
Thanks duncan, thats very helpful. I'll be more careful with floating point numbers in future. -- http://mail.python.org/mailman/listinfo/python-list
