Rui Barradas
2017-Apr-12 16:07 UTC
[Rd] "table(droplevels(aq)$Month)" in manual page of droplevels
Hello, Inline. Em 12-04-2017 16:40, Henric Winell escreveu:> (Let's keep the discussion on-list -- I've added back R-devel.) > > On 2017-04-12 16:39, Ulrich Windl wrote: > >>>>> Henric Winell <nilsson.henric at gmail.com> schrieb am 12.04.2017 >>>>> um 15:35 in >> Nachricht <b66fe849-bb8d-f00d-87e5-553f866d57e0 at gmail.com>: >>> On 2017-04-12 14:40, Ulrich Windl wrote: >>> >>>> The last line of the example in droplevels' manual page seems to >>>> be incorrect to me. I think it should read: >>>> "table(droplevels(aq$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, 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$Month)) >>>> >>>> May Jun Aug Sep 31 30 31 30 >>>>> >>>> --- For the sake of learners, try to keep the examples simple >>>> and useful, even though you experts want to impress the >>>> newbees... >>>> >>>> Ulrich >>>> >>>> ______________________________________________ >>>> R-devel at r-project.org mailing list >>>> https://stat.ethz.ch/mailman/listinfo/r-devel >>>> >> >> >> >> >> > > ______________________________________________ > R-devel at r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-devel
Martin Maechler
2017-Apr-13 14:16 UTC
[Rd] "table(droplevels(aq)$Month)" in manual page of droplevels
>>>>> Rui Barradas <ruipbarradas at 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.henric at gmail.com> schrieb am>> 12.04.2017>>>>> um 15:35 in>>> Nachricht >>> <b66fe849-bb8d-f00d-87e5-553f866d57e0 at 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 at r-project.org mailing list >>>>> https://stat.ethz.ch/mailman/listinfo/r-devel >>>>> >>> >>> >>> >>> >>> >> >> ______________________________________________ >> R-devel at r-project.org mailing list >> https://stat.ethz.ch/mailman/listinfo/r-devel > ______________________________________________ > R-devel at r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-devel
Ulrich Windl
2017-Apr-18 06:24 UTC
[Rd] Antw: Re: "table(droplevels(aq)$Month)" in manual page of droplevels
>>> Martin Maechler <maechler at stat.math.ethz.ch> schrieb am 13.04.2017 um 16:16 inNachricht <22767.34875.318482.421150 at stat.math.ethz.ch>: [...]> Ulrich, please note that in the end this was all because you're > still learning to understand R (e.g., data frames !) better.Hi! I understood that I did not understand the example due to my lack of R knowledge. That is the point you made. However I read a book on R, the introduction on R, and several tutorials on R, and I still never saw the use pattern presented in the example. Thus I think my proposal to present more comprehensible examples is a valid one. Alternatively the introduction to R could be extended to a level that allows the user to understand the example presented. Honestly there are many example I don't understand (plus descriptions that I hope to understand from the examples). While I understand that R developers prefer to be left alone, not hearing about such issues, I think even R developers could benefit from hearing of beginner's problems. Anyway R developers are free to forward messages to R-help if they think more appropriate for R-help.> > As such this was completely inappropriate for R-devel and should > have gotten to the R help list R-help.Regards, Ulrich> > With regards, > Martin Maechler, ETH Zurich[...]