round() wrong in Python 2.4?

[Nils Grimsmo]

Why did round() change in Python 2.4?

$ python2.3
Python 2.3.5 (#2, Jun 19 2005, 13:28:00)
[GCC 3.3.6 (Debian 1:3.3.6-6)] on linux2

round(0.0225, 3) 0.023 "%.3f" % round(0.0225, 3) '0.023' $ python2.4
Python 2.4.1 (#2, Jul 12 2005, 09:22:25)
[GCC 4.0.1 (Debian 4.0.1-1)] on linux2 round(0.0225, 3) 0.0219999999999 99999 "%.3f" % round(0.0225, 3) '0.022'

(Is this due to the different GCC used?)

How do you correctly output floating-point numbers in 2.4?

I do not like the "print number + EPS" solution, as you would need
different EPS for different exponent sizes. In C you could get it by
taking integer 1, and &-ing in the right exponent, and then casting to
double via void*. This would not be very portable, though.
Klem fra Nils

[Robert Kern]

Grant Edwards wrote:

On 2005-09-14, Robert Kern <rk***@ucsd.edu > wrote:

Antoon Pardon wrote:

0.0225 isn't representable and it happens that the actual number
you get differ. Now which number python should choose when it is
fed 0.0225, I don't know. But expressing the different behaviour
as a change in round, suggest that the O.P. would be wise to
learn about floating point problems

Uhh, Python didn't change anything between 2.3 and 2.4 wrt round().

That's what Antoon Pardon just said. The above paragraph says
that round() didn't change, and the fact that the OP thinks it
did indicates that the OP needs to learn more about FP.

Antoon:
"Python 2.3 isn't rounding 0.0225 up while pyton 2.4 rounds it down."

--
Robert Kern
rk***@ucsd.edu

"In the fields of hell where the grass grows high
Are the graves of dreams allowed to die."
-- Richard Harter

[Reinhold Birkenfeld]

Robert Kern wrote:

Grant Edwards wrote:

On 2005-09-14, Robert Kern <rk***@ucsd.edu > wrote:

Antoon Pardon wrote:

0.0225 isn't representable and it happens that the actual number
you get differ. Now which number python should choose when it is
fed 0.0225, I don't know. But expressing the different behaviour
as a change in round, suggest that the O.P. would be wise to
learn about floating point problems

Uhh, Python didn't change anything between 2.3 and 2.4 wrt round().

That's what Antoon Pardon just said. The above paragraph says
that round() didn't change, and the fact that the OP thinks it
did indicates that the OP needs to learn more about FP.

Antoon:
"Python 2.3 isn't rounding 0.0225 up while pyton 2.4 rounds it down."

Written in Pseudocode:

not (Py2.3 rounding up and Py2.4 rounding down)

Reinhold

[Robert Kern]

Reinhold Birkenfeld wrote:

Robert Kern wrote:

Antoon:
"Python 2.3 isn't rounding 0.0225 up while pyton 2.4 rounds it down."

Written in Pseudocode:

not (Py2.3 rounding up and Py2.4 rounding down)

I presumed the "isn't" was a typo given the "while."

--
Robert Kern
rk***@ucsd.edu

"In the fields of hell where the grass grows high
Are the graves of dreams allowed to die."
-- Richard Harter

[Robert Kern]

Robert Kern wrote:

Reinhold Birkenfeld wrote:

Robert Kern wrote:

Antoon:
"Python 2.3 isn't rounding 0.0225 up while pyton 2.4 rounds it down."

Written in Pseudocode:

not (Py2.3 rounding up and Py2.4 rounding down)

I presumed the "isn't" was a typo given the "while."

Oh never mind. I'm sorry I started this line of conversation.

--
Robert Kern
rk***@ucsd.edu

"In the fields of hell where the grass grows high
Are the graves of dreams allowed to die."
-- Richard Harter

[Antoon Pardon]

Op 2005-09-14, Robert Kern schreef <rk***@ucsd.edu >:

Antoon Pardon wrote:

Op 2005-09-13, Robert Kern schreef <rk***@ucsd.edu >:

Jeremy Sanders wrote:

Nils Grimsmo wrote:

>Why did round() change in Python 2.4?

It the usual floating point representation problem. 0.0225 cannot be
represent ed exactly:

That's not what he's asking about. He's asking why his Python 2.3 rounds
0.0225 *up* to 0.023 while his Python 2.4 rounds *down* to 0.022. It's
the change in behavior that he's concerned with and isn't just the usual
floating point problem.
I would say the usual floating point problem is involved.

Python 2.3 isn't rounding 0.0225 up while pyton 2.4 rounds it down.

0.0225 isn't representable and it happens that the actual number
you get differ. Now which number python should choose when it is
fed 0.0225, I don't know. But expressing the different behaviour
as a change in round, suggest that the O.P. would be wise to
learn about floating point problems

Uhh, Python didn't change anything between 2.3 and 2.4 wrt round().

That is what I said, or at least meant to say.
The
reason he is seeing a difference is because the two executables were
built with different compilers. The fact that the version of Python was
different in the two cases obscures the real cause.
IMO the real cause is unimportant. The real cause can be a different
CPU or a different compilor or a different library. What it boils
down to is that you can't expect 0,0225 to be represented in a
value that will be rounded up.
Saying that 0.0225 can't be represented exactly as a binary floating
point number is entirely true but is an incomplete answer. Yes,
obviously binary floating point representations are involved. But one
could always define a standard representation scheme that always gives
the same answer for the same input.
Can we? I don't think we can, unless you are working with decimal
numbers. If you work with floats you are basically saying that
the program should choose the best approximation it can. That
approximation can differ according to circumstances. So one
must be prepared that round(0.225,3) can give different results
in different circumstances.
The fact is that for some reason
there are two schemes being used. Another fact is that this has nothing
to do with difference in the versions of Python he is using. Most of
Python's floating point behavior is a platform-dependent accident (as
Tim Peters always says), and Nils is using two slightly different platforms.

Yes and he wouldn't have blamed it on round, had he known or thought
about FP representations .

--
Antoon Pardon