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From: Robin Becker on 13 Jul 2010 12:06 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
From: Emile van Sebille on 13 Jul 2010 12:39
On 7/13/2010 9:06 AM Robin Becker said... > > 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. I generally do the calculations in memory then set the largest to 100 - sum(rounded(others)). (Same as for double entry bookkeeping to ensure debits == credits) Emile |