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is the bankers' round() algorithm implemented well?

php.internals

Ron Korving

21 years ago
I understood that round() is using the so called Bankers' round algorithm. But I notice it's inconsistent. The banker's algorithm is magically activated only for numbers beyond 2047.0. Before 2047, the "classic" 5-is-rounded-up method is used. 5 is rounded up for: round(9.005, 2) gives 9.01 round(9.015, 2) gives 9.02 round(9.025, 2) gives 9.03 round(9.035, 2) gives 9.04 round(2047.005, 2) gives 2047.01 round(2047.015, 2) gives 2047.02 round(2047.025, 2) gives 2047.03 round(2047.035, 2) gives 2047.04 bankers' rounding: round(2048.005, 2) gives 2048.01 round(2048.015, 2) gives 2048.01 round(30000.005, 2) gives 30000.01 round(30000.015, 2) gives 30000.01 This can't be intentional, can it? Someone tell me if this is a bug or a feature please. Ron

Sara Golemon

21 years ago
> I understood that round() is using the so called Bankers' round algorithm. > But I notice it's inconsistent. The banker's algorithm is magically > activated only for numbers beyond 2047.0. Before 2047, the "classic" > 5-is-rounded-up method is used. >
Floating point numbers are........ imprecise. What you see as 2047.005, the computer might see as 2047.0049999999999999123869128 or perhaps as 2047.005000000000000012396291 When I only care about a certain accuracy (such as with monetary amounts) I 'solve' this quirk of the CPU by adding a miniscule fudge factor: round($someval + 0.000000001, 2) PHP doesn't do this automatically because sometimes a plus-fudge is appropriate, sometimes a negative-fudge is better. -Sara

Ron Korving

21 years ago
I just did a long test from 1 - 10000 and I noticed that every time the number was increased with one bit (starting with 2048, then 4096, then 8192) behaviour changes. You were right, my bad. Makes me wonder though, what's the point of a bankers' algorithm if .005 can never be reached, only approached? :) This unpredictable rounding behaviour is a real b*tch, but what can you do :) Ron "Sara Golemon" <pollita@php.net> wrote in message news:20050207204629.63824.qmail@lists.php.net...
> > I understood that round() is using the so called Bankers' round
algorithm.
> > But I notice it's inconsistent. The banker's algorithm is magically > > activated only for numbers beyond 2047.0. Before 2047, the "classic" > > 5-is-rounded-up method is used. > > > > Floating point numbers are........ imprecise. > > What you see as 2047.005, the computer might see as > 2047.0049999999999999123869128 or perhaps as
2047.005000000000000012396291
> > When I only care about a certain accuracy (such as with monetary amounts)
I

messju mohr

21 years ago
On Mon, Feb 07, 2005 at 10:04:46PM +0100, Ron Korving wrote:
> I just did a long test from 1 - 10000 and I noticed that every time the > number was increased with one bit (starting with 2048, then 4096, then 8192) > behaviour changes. You were right, my bad. > > Makes me wonder though, what's the point of a bankers' algorithm if .005 can > never be reached, only approached? :) This unpredictable rounding behaviour > is a real b*tch, but what can you do :)
you want to use integers or bcmath (http://php.net/bc) or gmp (http://php.net/gmp) to get your own control of rounding and accuracy. this makes this thread more appropiate for php-general than for internals. greetings messju