On 12/07/2010 19:59, Mark Dickinson wrote:
............
It does look inconsistent however, and it seems to me rounding and
interpolation should behave similarly.
Agreed. In this case it's a minor bug that round(-9.85, 1)
produces -9.9 instead of -9.8; both string formatting
and round should give -9.8. This bug is fixed in Python
2.7 and in Python 3.x.
Note that in 2.7 there's still a legitimate difference: round
rounds halfway cases away from 0, while string formatting
rounds them to even. So the following results are correct:
Python 2.7 (r27:82500, Jul 11 2010, 22:38:53)
[GCC 4.2.1 (Apple Inc. build 5659)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
round(1.25, 1)
1.3
'%.1f' % 1.25
'1.2'
(1.25 *is* an exact halfway case, since it's exactly
representable as a binary float.)
In Python 3.x, round always does round-half-to-even, so
string formatting and round should agree (and if they don't,
it's definitely a bug: please report it!)
With all this said, asking for *decimal* rounding of
*binary* approximations to *decimal* halfway cases to give
the results you expect is ... optimistic, to say the least.
Use the decimal module if you care about which way
your (almost) halfway cases get rounded.
[I already replied to this earlier through Google groups, but
I'm not sure whether it went through properly. Apologies
for the duplication, if so.]
yes thanks I saw that, but no problem about the dup. I suspect we might end up
using some kind of fixed point.
Anyhow does anyone have a good algorithm for ensuring rounded percentages do add
up to 100%? :) How about grouped percentages ie ensure the group sums and the
groups display correctly in rounded form.
--
Robin Becker
--
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