Denis Chabot <[EMAIL PROTECTED]> writes: > Hi, > > I'm wondering why > > 48 %/% 2 gives 24 > but > 4.8 %/% 0.2 gives 23... > I'm not trying to round up here, but to find out how many times > something fits into something else, and the answer should have been > the same for both examples, no?
Well, you can't trust floating point numbers to give you an exact result: > 4.8 / 0.2 - 24 [1] -3.552714e-15 and even > (48/10) / (2/10) - 24 [1] -3.552714e-15 the basic issue being that tenths are not exactly representable in binary floating point. I think very few people even expected you to use integer division on non-integers, but I note that the claim on the help page actually holds: > 0.2 * 4.8 %/% 0.2 + 4.8 %% 0.2 == 4.8 [1] TRUE > On a different topic, I like the behavior of NAs better in R than in > SAS (at least they are not considered the smallest value for a > variable), but at the same time I am surprised that the sum of NAs is > 0 instead of NA. > > The sum of a vector having at least one NA but also valid data gives > NA if we do not specify na.rm=T. But with na.rm=T, we are telling sum > to give the sum of valid data, ignoring NAs that do not tell us > anything about the value of a variable. I found out while getting the > sum of small subsets of my data (such as when subsetting by several > variables), sometimes a "cell" only contained NAs for my response > variable. I would have expected the sum to be NA in such cases, as I > do not have a single data point telling me the value of my response > here. But R tells me the sum was zero in that cell! Was this behavior > considered "desirable" when sum was built? If not, any hope it will be > fixed? Yes it was, and no there isn't. In math, the sum over an empty index set is zero, which has some nice consistency properties (the sum over a disjoint union of sets is the sum of the sums over each set, for instance. -- O__ ---- Peter Dalgaard Blegdamsvej 3 c/ /'_ --- Dept. of Biostatistics 2200 Cph. N (*) \(*) -- University of Copenhagen Denmark Ph: (+45) 35327918 ~~~~~~~~~~ - ([EMAIL PROTECTED]) FAX: (+45) 35327907 ______________________________________________ [email protected] mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
