> From: Thomas Lumley > > On Thu, 7 Apr 2005, Liaw, Andy wrote: > > > - It seems counter-intuitive to allow non-integer values as > operand for ":". > > It also seems a bit odd that 1:2 returns something with storage.mode > > integer, whereas 0.5:2 gives doubles. Would it make sense > to disallow > > non-integer operands to ":"? I can't think of a situation > when non-integer > > operands for ":" would be good... > > > > It depends on what you mean by non-integer. You certainly > want to allow > operand with storage.mode "double". You might want to exclude > non-integers, but then you have problems with rounding: > > a<-sqrt(2) > b<-sqrt(3) > (a^2):(b^2) > > would fail. Allowing non-integers up to some tolerance would > be needed,
I think the fact that we don't need to do as.integer(1):as.integer(3) in order for ":" to return an integer vector must means that there's some tolerance built-in. (I know, I should just check the source...) Here are some experiments (R-devel 2005-03-11 on WinXPPro): > storage.mode((1 + .Machine$double.eps):3) [1] "double" > storage.mode((1 - .Machine$double.eps/4):3) [1] "integer" > storage.mode((1 + .Machine$double.eps/4):3) [1] "integer" > storage.mode((1 + .Machine$double.eps/3):3) [1] "integer" > storage.mode((1 - .Machine$double.eps/3):3) [1] "double" > but then there isn't much reason to forbid 0.5:2, and someone > might have a use for it. IMHO that's more of a trap than a `feature'. At least to me it's not immediately obvious what one should expect when the first operand is an `obvious' non-integer such as 0.5. For such sequences, I'd say one should really use seq() instead of ":". (I'd go as far as saying that something like 0.5:2 is an abuse.) BTW, here's something that surprised me: > x <- 1:10 > i <- (1 + 1e-15):5 > i [1] 1 2 3 4 5 > x[i] [1] 1 2 3 4 5 > i <- (1 - 1e-15):5 > i [1] 1 2 3 4 5 > x[i] [1] 1 2 3 4 The reason for this, of course, is that (as ?"[" explains): > as.integer(i) [1] 0 1 2 3 4 My wish would be for ":" to coerce its operands to integers, perhaps with a bit more tolerance, and always return object of integer storage mode. Cheers, Andy > > -thomas > > > ______________________________________________ R-devel@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
