On 27/02/2015 9:49 AM, Dimitri Liakhovitski wrote: > So, Duncan, do I understand you correctly: > > When I use x$x<6, R doesn't know if it's TRUE or FALSE, so it returns > a logical value of NA.
Yes, when x$x is NA. (Though I think you meant x$c.) > When this logical value is applied to a row, the R says: hell, I don't > know if I should keep it or not, so, just in case, I am going to keep > it, but I'll replace all the values in this row with NAs? Yes. Indexing with a logical NA is probably a mistake, and this is one way to signal it without actually triggering a warning or error. BTW, I should have mentioned that the example where you indexed using -which(x$c>=6) is a bad idea: if none of the entries were 6 or more, this would be indexing with an empty vector, and you'd get nothing, not everything. Duncan Murdoch > > On Fri, Feb 27, 2015 at 9:13 AM, Duncan Murdoch > <murdoch.dun...@gmail.com> wrote: >> 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.
