I am reading as fast as I can! Just started with R five days ago. I found the following in the documentation:
"Although the default for 'drop' is 'TRUE', the default behaviour when only one _row_ is left is equivalent to specifying 'drop = FALSE'. To drop from a data frame to a list, 'drop = FALSE' has to (sic) specified explicitly." I think the exception mentioned in the first sentence is the reason for my confusion. I also think the second sentence is wrong and should have 'TRUE' instead of 'FALSE'. While it is true that a data frame is a list, it is not a list of numbers, but rather a list of columns, which, if I understand correctly, can be either vectors or matrices. So regardless of the value assigned to 'drop' the returned object is a list. When I asked "why isn't sw[1, ] a list?" I should have asked instead "why isn't sw[1, ] a list of vectors?" I did some experiments with a data frame a, where the columns are vectors (no matrix columns): > is.data.frame(a) # just checking [1] TRUE > a1<- a[3, ] > (is.data.frame(a1)) [1] TRUE (did not sop being a data frame) > (is.list(a1)) [1] TRUE (but it is a list) > a2<- a[3, , drop=T] > (is.data.frame(a2)) [1] FALSE (no longer a data frame) > (is.list(a2)) [1] TRUE (but it is a list) > a3<- a[3, , drop=F] > (is.data.frame(a3)) [1] TRUE (still a data frame) > (is.list(a3)) [1] TRUE (but it is a list) I also tried: > a2[1] $dates.num [1] 477032400 > a3[1] dates.num 3 477032400 (notice the row name) > attributes(a3[1]) $names [1] "dates.num" $class [1] "data.frame" $row.names [1] "3" > attributes(a2[1]) $names [1] "dates.num" FS On 4/16/05, Prof Brian Ripley <[EMAIL PROTECTED]> wrote: > On Sat, 16 Apr 2005, Prof Brian Ripley wrote: > > > Perhaps Fernando will also note that is documented in ?"[.data.frame", > > a slightly more appropriate reference than Bill's. > > > > It would be a good idea to read a good account of R's indexing: Bill > > Venables > > and I know of a couple you will find in the R FAQ. > > BTW, > > sw <- swiss > sw[1,,drop=TRUE] *is* a list (not as claimed, but as documented) > sw[1, ] is a data frame > sw[, 1] is a numeric vector. > > I should have pointed out that "[.data.frame" is in the See Also of Bill's > reference. > > BTW to Andy: a list is a vector, and Kurt and I recently have been trying > to correct documentation that means `atomic vector' when it says `vector'. > (Long ago lists in R were pairlists and not vectors.) > > > is.vector(list(a=1)) > [1] TRUE > > > > On Sat, 16 Apr 2005, Liaw, Andy wrote: > > > >> Because a data frame can hold different data types (even matrices) in > >> different variables, one row of it can not be converted to a vector in > >> general (where all elements need to be of the same type). > >> > >> Andy > >> > >>> From: Fernando Saldanha > >>> > >>> Thanks, it's interesting reading. > >>> > >>> I also noticed that > >>> > >>> sw[, 1, drop = TRUE] is a vector (coerces to the lowest dimension) > >>> > >>> but > >>> > >>> sw[1, , drop = TRUE] is a one-row data frame (does not convert it into > >>> a list or vector) > >>> > >>> FS > >>> > >>> > >>> On 4/16/05, [EMAIL PROTECTED] <[EMAIL PROTECTED]> wrote: > >>>> You should look at > >>>> > >>>>> ?"[" > >>>> > >>>> and look very carefully at the "drop" argument. For your example > >>>> > >>>>> sw[, 1] > >>>> > >>>> is the first component of the data frame, but > >>>> > >>>>> sw[, 1, drop = FALSE] > >>>> > >>>> is a data frame consisting of just the first component, as > >>>> mathematically fastidious people would expect. > >>>> > >>>> This is a convention, and like most arbitrary conventions > >>> it can be very > >>>> useful most of the time, but some of the time it can be a very nasty > >>>> trap. Caveat emptor. > >>>> > >>>> Bill Venables. > >>>> > >>>> -----Original Message----- > >>>> From: [EMAIL PROTECTED] > >>>> [mailto:[EMAIL PROTECTED] On Behalf Of > >>> Fernando Saldanha > >>>> Sent: Saturday, 16 April 2005 1:07 PM > >>>> To: Submissions to R help > >>>> Subject: [R] Getting subsets of a data frame > >>>> > >>>> I was reading in the Reference Manual about Extract.data.frame. > >>>> > >>>> There is a list of examples of expressions using [ and [[, with the > >>>> outcomes. I was puzzled by the fact that, if sw is a data > >>> frame, then > >>>> > >>>> sw[, 1:3] > >>>> > >>>> is also a data frame, > >>>> > >>>> but > >>>> > >>>> sw[, 1] > >>>> > >>>> is just a vector. > >>>> > >>>> Since R has no scalars, it must be the case that 1 and 1:1 > >>> are the same: > >>>> > >>>>> 1 == 1:1 > >>>> [1] TRUE > >>>> > >>>> Then why isn't sw[,1] = sw[, 1:1] a data frame? > >>>> > >>>> FS > >>>> > >>>> ______________________________________________ > >>>> R-help@stat.math.ethz.ch mailing list > >>>> https://stat.ethz.ch/mailman/listinfo/r-help > >>>> PLEASE do read the posting guide! > >>>> http://www.R-project.org/posting-guide.html > >>>> > >>> > >>> ______________________________________________ > >>> R-help@stat.math.ethz.ch mailing list > >>> https://stat.ethz.ch/mailman/listinfo/r-help > >>> PLEASE do read the posting guide! > >>> http://www.R-project.org/posting-guide.html > >>> > >>> > >>> > >> > >> ______________________________________________ > >> R-help@stat.math.ethz.ch mailing list > >> https://stat.ethz.ch/mailman/listinfo/r-help > >> PLEASE do read the posting guide! > >> http://www.R-project.org/posting-guide.html > >> > > > > -- > > Brian D. Ripley, [EMAIL PROTECTED] > > Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ > > University of Oxford, Tel: +44 1865 272861 (self) > > 1 South Parks Road, +44 1865 272866 (PA) > > Oxford OX1 3TG, UK Fax: +44 1865 272595 > > > > -- > Brian D. Ripley, [EMAIL PROTECTED] > Professor of Applied Statistics, http://www.stats.ox.ac.uk/~ripley/ > University of Oxford, Tel: +44 1865 272861 (self) > 1 South Parks Road, +44 1865 272866 (PA) > Oxford OX1 3TG, UK Fax: +44 1865 272595 > ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html
