On 27/02/2015 9:04 AM, Dimitri Liakhovitski wrote: > I know how to get the output I need, but I would benefit from an > explanation why R behaves the way it does. > > # I have a data frame x: > x = data.frame(a=1:10,b=2:11,c=c(1,NA,3,NA,5,NA,7,NA,NA,10)) > x > # I want to toss rows in x that contain values >=6. But I don't want > to toss my NAs there. > > subset(x,c<6) # Works correctly, but removes NAs in c, understand why > x[which(x$c<6),] # Works correctly, but removes NAs in c, understand why > x[-which(x$c>=6),] # output I need > > # Here is my question: why does the following line replace the values > of all rows that contain an NA # in x$c with NAs? > > x[x$c<6,] # Leaves rows with c=NA, but makes the whole row an NA. Why??? > x[(x$c<6) | is.na(x$c),] # output I need - I have to be super-explicit > > Thank you very much!
Most of your examples (except the ones using which()) are doing logical indexing. In logical indexing, TRUE keeps a line, FALSE drops the line, and NA returns NA. Since "x$c < 6" is NA if x$c is NA, you get the third kind of indexing. Your last example works because in the cases where x$c is NA, it evaluates NA | TRUE, and that evaluates to TRUE. In the cases where x$c is not NA, you get x$c < 6 | FALSE, and that's the same as x$c < 6, which will be either TRUE or FALSE. Duncan Murdoch ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
