round function error???

[Anthony]

Isn't this a mistake???

>>round(3499.34 9439034,44)

3499.3494390340 002

>>round(_,2)

3499.3499999999 999

>>round(_,1)

3499.4000000000 001

My Python 2.5.1 spat that out..

[Mark Dickinson]

Re: round function error???

On Jul 18, 10:17 pm, Anthony <ajdam...@gmail .comwrote:

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 3499.3499999999 999 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

[Terry Reedy]

Re: round function error???



Anthony wrote:

Isn't this a mistake???
>

>>>round(3499.3 49439034,44)

3499.3494390340 002

>>>round(_,2)

3499.3499999999 999

>>>round(_,1)

3499.4000000000 001
>
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'

[Mark Dickinson]

Re: round function error???

On Jul 18, 11:15 pm, Terry Reedy <tjre...@udel.e duwrote:

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 3499.3499999999 999) 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

3499.3499999999 999

>>print '%.1f' % x

3499.3

>>print round(x, 1)

3499.4

Mark

[John Machin]

Re: round function error???

On Jul 19, 8:05 am, Mark Dickinson <dicki...@gmail .comwrote:

On Jul 18, 10:17 pm, Anthony <ajdam...@gmail .comwrote:
>

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 3499.3499999999 999 instead of 3499.35, then
no, that's not a mistake; see
>
http://www.python.org/doc/faq/genera...-point-calcula...
>
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.

>>x = 3499.35
>>repr(x)

'3499.349999999 9999'

>>float(repr(x) ) == x

True

>>float(repr(x) ) < x

False

>
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.

I'd suggest adding this to the list of floating point strangenesses in
the FAQ and/or the appendix to the tutorial, rather than "fixing" it.

(See http://bugs.python.org/issue1869).

The example you give there seems to be somewhat more deserving of a
fix.

Cheers,
John

[Mark Dickinson]

Re: round function error???

On Jul 19, 12:20 am, John Machin <sjmac...@lexic on.netwrote:

On Jul 19, 8:05 am, Mark Dickinson <dicki...@gmail .comwrote:

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('3499.3 5')
>>Fraction.from _float(x)

Fraction(769515 2029315891, 2199023255552)

>>Fraction.from _decimal(y)

Fraction(69987, 20)
[54933 refs]

>>Fraction.from _float(x) < Fraction.from_d ecimal(y)

True

Mark

[John Machin]

Re: round function error???

On Jul 21, 12:56 am, Mark Dickinson <dicki...@gmail .comwrote:

On Jul 19, 12:20 am, John Machin <sjmac...@lexic on.netwrote:
>

On Jul 19, 8:05 am, Mark Dickinson <dicki...@gmail .comwrote:

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.