Re: round function error???
On Jul 19, 12:20 am, John Machin [EMAIL PROTECTED] wrote:
On Jul 19, 8:05 am, Mark Dickinson [EMAIL PROTECTED] wrote:
for more information. But I'm guessing that you're
questioning the fact that a value that's apparently
*less* than 3499.35 is rounded up to 3499.4, rather
than down to 3499.3. ?
apparently being the operative word.
Well, it's not just an apparent problem: the closest
floating-point number to 3499.35 really *is* less than
3499.35. A nice way to check this is using the new
fractions module in 2.6, which allows exact conversions
of floats and Decimals into rational numbers:
Python 2.6b2+ (trunk:65155, Jul 20 2008, 15:39:46)
[GCC 4.0.1 (Apple Inc. build 5484)] on darwin
Type help, copyright, credits or license for more information.
from decimal import Decimal
from fractions import Fraction
x = 3499.35
y = Decimal('3499.35')
Fraction.from_float(x)
Fraction(7695152029315891, 219902322)
Fraction.from_decimal(y)
Fraction(69987, 20)
[54933 refs]
Fraction.from_float(x) Fraction.from_decimal(y)
True
Mark
--
http://mail.python.org/mailman/listinfo/python-list
Re: round function error???
On Jul 21, 12:56 am, Mark Dickinson [EMAIL PROTECTED] wrote: On Jul 19, 12:20 am, John Machin [EMAIL PROTECTED] wrote: On Jul 19, 8:05 am, Mark Dickinson [EMAIL PROTECTED] wrote: for more information. But I'm guessing that you're questioning the fact that a value that's apparently *less* than 3499.35 is rounded up to 3499.4, rather than down to 3499.3. ? apparently being the operative word. Well, it's not just an apparent problem: the closest floating-point number to 3499.35 really *is* less than 3499.35. I'm well aware of that. My point is that I hope that you weren't planning on changing that behaviour in an unannounced unstaged manner. -- http://mail.python.org/mailman/listinfo/python-list
round function error???
Isn't this a mistake??? round(3499.349439034,44) 3499.3494390340002 round(_,2) 3499.34999 round(_,1) 3499.40001 My Python 2.5.1 spat that out.. -- http://mail.python.org/mailman/listinfo/python-list
Re: round function error???
On Jul 18, 10:17 pm, Anthony [EMAIL PROTECTED] wrote: Isn't this a mistake??? Which 'this'? That is, what were you expecting? If you're objecting to the fact that the second result produces 3499.34999 instead of 3499.35, then no, that's not a mistake; see http://www.python.org/doc/faq/general/#why-are-floating-point-calculations-so-inaccurate for more information. But I'm guessing that you're questioning the fact that a value that's apparently *less* than 3499.35 is rounded up to 3499.4, rather than down to 3499.3. ? Then yes, I'd agree that's less than ideal, though I don't consider it a particularly serious bug. It's been on my list of things to fix for a while. (See http://bugs.python.org/issue1869 ). Contributions welcome! Mark -- http://mail.python.org/mailman/listinfo/python-list
Re: round function error???
Anthony wrote: Isn't this a mistake??? round(3499.349439034,44) 3499.3494390340002 round(_,2) 3499.34999 round(_,1) 3499.40001 My Python 2.5.1 spat that out.. No, round() return binary floats that, in general, cannot represent decimal floats exactly. Formatted printing gives what you expect. '%8.2f' % x ' 3499.35' -- http://mail.python.org/mailman/listinfo/python-list
Re: round function error???
On Jul 18, 11:15 pm, Terry Reedy [EMAIL PROTECTED] wrote: No, round() return binary floats that, in general, cannot represent decimal floats exactly. Formatted printing gives what you expect. '%8.2f' % x ' 3499.35' Sure. But it's still true that the second printed value (printed as 3499.34999) is strictly less than 3499.35, so when rounding to 1 decimal place an ideal rounding routine would round it *down* to 3499.3 instead of up to 3499.4. This is what '%.1f' does, for example, on a platform where printf does correct rounding: Python 2.5.1 (r251:54863, Jan 17 2008, 19:35:17) [GCC 4.0.1 (Apple Inc. build 5465)] on darwin Type help, copyright, credits or license for more information. x = 3499.35 x 3499.34999 print '%.1f' % x 3499.3 print round(x, 1) 3499.4 Mark -- http://mail.python.org/mailman/listinfo/python-list
