>>>>> Rui Barradas <ruipbarra...@sapo.pt> >>>>> on Wed, 12 Apr 2017 17:07:45 +0100 writes:
> Hello, Inline. > Em 12-04-2017 16:40, Henric Winell escreveu: >> (Let's keep the discussion on-list -- I've added back >> R-devel.) >> >> On 2017-04-12 16:39, Ulrich Windl wrote: >> >>>>> Henric Winell <nilsson.hen...@gmail.com> schrieb am >> 12.04.2017 >>>>> um 15:35 in >>> Nachricht >>> <b66fe849-bb8d-f00d-87e5-553f866d5...@gmail.com>: >>>> On 2017-04-12 14:40, Ulrich Windl wrote: >>>> >>>>> The last line of the example in droplevels' manual >>>>> page seems to be incorrect to me. I think it should >>>>> read: "table(droplevels(aq$Month))". Amazingly (I >>>>> don't understand) both variants seem to produce the >>>>> same result (R 3.3.3): --- >>>> >>>> The manual says that "The function 'droplevels' is used >>>> to drop unused levels from a 'factor' or, more >>>> commonly, from factors in a data frame." and, as >>>> documented, the 'droplevels' generic has methods for >>>> objects of class "data.frame" and "factor". So, your >>>> being amazed is a bit surprising given that 'aq' is a >>>> data frame. >>> >>> The "surprising" thing is the syntax: I was unaware that >>> '$' is a generic operator that can be applied to the >>> result of a function (i.e.: droplevels); I thought it's >>> kind of a special variable syntax. >> >> Then your surprise is unrelated to the use of >> 'droplevels'. >> >> Since the 'droplevels' method for objects of class >> "data.frame" returns a data frame, the extraction >> operator '$' works directly on the resulting object. So, >> 'droplevels(aq)$Month' is essentially the same as >> >> aq <- droplevels(aq) aq$Month >> >> > Isn't there also the syntax >> ``droplevels(aq)["Month"]''? >> >> Sure, and there are even more ways to do subsetting. But >> this is basic stuff and therefore off-topic for R-devel. >> Please see the manual (?Extract) or, e.g., Chapter 3 of >> Hadley Wickham's "Advanced R". > But note that droplevels(aq)["Month"] and > droplevels(aq)$Month are _not_ the same. The first returns > a data.frame (with just one vector), the latter returns a > vector. To return just a vector you could also use > droplevels(aq)[["Month"]] > which is preferable for programming, by the way. The '$' > operator should be reserved for interactive use only. > Hope this helps, Indeed, we hope.. Thanks to the helpers! Ulrich, please note that in the end this was all because you're still learning to understand R (e.g., data frames !) better. As such this was completely inappropriate for R-devel and should have gotten to the R help list R-help. With regards, Martin Maechler, ETH Zurich > Rui Barradas >> >> >> Henric Winell >>> >>> Regards, Ulrich >>> >>>> >>>> >>>> Henric Winell >>>> >>>> >>>> >>>>> aq <- transform(airquality, Month = factor(Month, labels = >>>>> month.abb[5:9])) aq <- subset(aq, Month != "Jul") >>>>> table(aq$Month) >>>>> >>>>> May Jun Jul Aug Sep 31 30 0 31 30 >>>>> table(droplevels(aq)$Month) >>>>> >>>>> May Jun Aug Sep 31 30 31 30 >>>>> table(droplevels(aq$Month)) >>>>> >>>>> May Jun Aug Sep 31 30 31 30 >>>>>> >>>>> --- For the sake of learners, try to keep the examples >>>>> simple and useful, even though you experts want to >>>>> impress the newbees... >>>>> >>>>> Ulrich >>>>> >>>>> ______________________________________________ >>>>> R-devel@r-project.org mailing list >>>>> https://stat.ethz.ch/mailman/listinfo/r-devel >>>>> >>> >>> >>> >>> >>> >> >> ______________________________________________ >> R-devel@r-project.org mailing list >> https://stat.ethz.ch/mailman/listinfo/r-devel > ______________________________________________ > R-devel@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-devel ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel