I have just become painfully aware that objects of class "difftime", generated by the difference of two POSIXct objects, carry a "units" attribute, which flashes by when the object is printed, for example.
The pain was occasioned when I tried to turn these objects into numberic objects for use elsewhere as a covariate. as.numeric(difftime object) simply turns off the units attribute and provides a numeric object which may represent a number of seconds or a number of days, with no warning as to which. I think this is an unfortunate situation, but I can't see how to rectify it without breaking code that may rely on this quirky feature. My inclination is to suggest a method for as.numeric (ie for as.double) that settles on a single unit, which for consistency with as.numeric(POSIXct object) should probably be seconds: as.double.difftime <- function(x, ...) if(attr(x, "units") == "days") as.vector(x) * 86400 else as.vector(x) but there must now be lots of code out there that has blythely assumed that the difference will always be a number of days and others assume it is always seconds. At the very least I think the help information should carry a big red warning about this rather unusual feature. (It may, I suppose, but I couldn't find it.) Comments? Bill Venables, CMIS, CSIRO Laboratories, PO Box 120, Cleveland, Qld. 4163 AUSTRALIA Phone: +61 7 3826 7251 Fax: +61 7 3826 7304 Mobile: +61 4 1963 4642 Home: +61 7 3286 7700 mailto:[EMAIL PROTECTED] http://www.cmis.csiro.au/bill.venables/ ______________________________________________ R-devel@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
