dt[row.num,][[col.name]] is indeed the solution. And works of course for data.frames, too. Maybe it should be added to the FAQ #1.3. Thank you!
On May 8, 2013, at 4:00 PM, Frank Erickson <[email protected]> wrote: > For your first question, this should work: > > dt[row.num,][[col.name]] > > For the second question, I guess your problem goes away if you aren't using > an (all but) NULL data.table. > > dt <- data.table(x=1,y=1) > nr <- data.table(NA,NA) > > rbind(dt,nr,use.names=FALSE) > # x y > #1: 1 1 > #2: NA NA > > So, if you're dynamically growing your data.table from nothing, you'll only > have to assign the colnames once, after the data.table becomes non-empty. > I've read that R is pretty inefficient at dynamically growing things, ...as > you say, it's a copy operation, right? > > I hope this helps. > > Best, > > Frank > > > On Wed, May 8, 2013 at 11:31 AM, David Kulp <[email protected]> wrote: > I must be doing something stupid. I'd like to get a vector from a data.frame > column using with=FALSE instead of a single-column data.table. > > dt <- data.table(x=1:10,y=letters[1:10]) > col.name <- 'y' > row.num <- 5 > print(dt[row.num,y]) # returns a vector with the letter 'e'. OK. > print(dt[row.num,list(y)]) # returns a data.table. OK. > print(dt[row.num, col.name ,with=FALSE]) # returns a data.table... no list > syntax here but I don't get a vector back. Not OK. > > The best I can do is > > unlist(as.list(dt[row.num, col.name ,with=FALSE])) > > which seems rather hackish. > > I've read the FAQ and I'm stymied. v1.8.8. Any help? > > ---- > > While I've got your attention, I might as well ask another stupid question. > I can't insert new rows automagically. > > dt[11] <- c(11,'k') > > Although I can do > > df <- as.data.frame(dt) > df[11,] <- c(11,'k') > > So I figure you want me to use rbind, even though rbind.data.table is > probably a copy operation. > > dt <- rbind(dt, list(x=11,y='k')) > > But I'd like to start with an empty data.table and programmatically add > chunks of rows as I run out of space. So I generate a data.table of NA > values and rbind. E.g., here I want to add 5 new rows to the 2 column table. > > dt <- data.table(x=numeric(), y=character()) > new.rows <- lapply(1:2, function(c) { rep(NA, 5) }) > > dt <- rbind(dt, new.rows, use.names=FALSE) > > According to the documentation, rbind is supposed to copy by position if > use.names=FALSE, but it doesn't retain the column names. This worked in > v1.8.2. Then I upgraded and it stopped working. I know I can fix this by > labeling the columns of new.rows, but I'm guessing that there's a much better > way to simply allocate a new chunk of rows to a growing table and I didn't > see any info online. > > Thanks in advance!! > > _______________________________________________ > datatable-help mailing list > [email protected] > https://lists.r-forge.r-project.org/cgi-bin/mailman/listinfo/datatable-help >
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