> > > > >>> print "%10.3f" % 1.2345 # seems like a bug > > > > 1.234 > > > > the first one, print knows enough to recognize and print it as 1.2345. > > however, in the second line, when it is round off, it doesn't know it > > is 1.2345 any more.
That is because it isn't 1.2345 anymore. 1.2345 cannot be represented exactly in radix-2 floating point arithmetic. > I think maybe this is the reason: the first one, print will print it > out with a rounding to the 11th decimal point, therefore hiding any > floating point imperfection. > > however, in the second one, print will not first round it off to the > 11th decimal point with a subsequent rounding off to the 3rd decimal > point. In that case, the floating point imperfection is manifested. > (by thinking it is 1.2344999999999999) > > a question is: since print can nicely hide and smooth out the floating > point imperfection, and probably most people prefer it that way, how I wouldn't prefer it that way. ;) > come the implementation of print "%10.3f" doesn't also do that -- > eliminating the imperfection first, and then print it out accordingly. > I think one argument is the loss of precision, but we only print it, > rather than modify the number or the variable itself... hm... say, if > a bank uses python to print out the "change that should be returned to > the customer" using print "%20.2f", then there will be a cent missing > here and there... why not smooth out that imperfection and return that > penny to the customer? (not that they really care). The decimal module will do what you want. One of its primary motivations was correct rounding for financial activities. casevh -- http://mail.python.org/mailman/listinfo/python-list
