[Rd] "table(droplevels(aq)$Month)" in manual page of droplevels

[Henric Winell]

(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".


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
>>> 
> 
> 
> 
> 
>