Thank you very much, Duncan. All this being said: What would you say is the most elegant and most safe way to solve such a seemingly simple task?
Thank you! On Fri, Feb 27, 2015 at 10:02 AM, Duncan Murdoch <murdoch.dun...@gmail.com> wrote: > 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 >>> >> >> >> > -- Dimitri Liakhovitski ______________________________________________ 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.
