Guys, many thanks. lapply works. Did not occur to me as I thought lapply returns a list and the receiving entity is a data frame.
H -----Original Message----- From: William Dunlap [mailto:wdun...@tibco.com] Sent: Tuesday, June 08, 2010 2:22 PM To: Horace Tso; r-help@r-project.org Subject: RE: [R] type conversion with apply or not The short answer is, don't use apply() on data.frame's. Use lapply to loop over the columns of a data.frame. > -----Original Message----- > From: r-help-boun...@r-project.org > [mailto:r-help-boun...@r-project.org] On Behalf Of Horace Tso > Sent: Tuesday, June 08, 2010 2:19 PM > To: r-help@r-project.org > Subject: [R] type conversion with apply or not > > Folks, i thought it should be straightforward but after a few > hours poking around, I decided it's best to post my question > on this list. > > I have a data frame consisting of a (large) number of date > columns, which are read in from a csv file as character > string. I want to convert them to Date type. Following is an > example, where the first column is of integer type, while the > rest are type character. > > > head(df) > TRANSNO TRANS.START_DATE TRANS.END_DATE DIVISION FASB > 1 250897 7/1/2010 7/31/2010 PSTRUCT Z > 2 250617 8/1/2010 8/31/2010 PSTRUCT Z > 3 250364 4/1/2011 6/30/2011 PLR Z > 4 250176 4/1/2011 6/30/2011 PLR Z > 5 250176 4/1/2011 6/30/2011 PLR Z > 6 250364 4/1/2011 6/30/2011 PLR Z > > sapply(df, class) > TRANSNO TRANS.START_DATE TRANS.END_DATE > DIVISION FASB > "integer" "character" "character" > "character" "character" > I thought it's just a matter of applying with a as.Date, > > df[,c(2,3)] = apply(df[,c(2,3)], 2, function(x)as.Date(x,"%m/%d/%Y")) > > Well, the Date conversion fails and I got, > > TRANSNO TRANS.START_DATE TRANS.END_DATE DIVISION FASB > 1 250897 14791 14821 PSTRUCT Z > 2 250617 14822 14852 PSTRUCT Z > 3 250364 15065 15155 PLR Z > 4 250176 15065 15155 PLR Z > 5 250176 15065 15155 PLR Z > 6 250364 15065 15155 PLR Z > The character columns are indeed converted, but they became > integer, not Date type. OK, that's strange and so I started > reading the help pages. > > It turns out in apply, the result is coerced to some basic > vector types. And apparently Date is not one of the basic > vector types. > > "In all cases the result is coerced by > as.vector<http://127.0.0.1:19182/library/base/help/as.vector> > to one of the basic vector types before the dimensions are > set, so that (for example) factor results will be coerced to > a character array. " > > The question then is how type conversion can be carried out > on some columns of a data frame without using a loop. > > Thanks. > > Horace Tso > > > > > > > > > [[alternative HTML version deleted]] > > ______________________________________________ > R-help@r-project.org mailing list > 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. > ______________________________________________ R-help@r-project.org mailing list 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.
