On Thu, Apr 5, 2012 at 1:40 PM, Peter Meilstrup <peter.meilst...@gmail.com> wrote: > Consider the data.frame: > > df <- data.frame(A = c(1,4,2,6,7,3,6), B= c(3,7,2,7,3,5,4), C = > c(2,7,5,2,7,4,5), index = c("A","B","A","C","B","B","C")) > > I want to select the column specified in 'index' for every row of 'df', to > get > > goal <- c(1, 7, 2, 2, 3, 5, 5) > > This sounds a lot like the indexing-by-a-matrix you can do with arrays; > > df[cbind(1:nrow(df), df$index)] > > but this returns me values that are all characters where I want numbers.
str(df[,-4][cbind(1:nrow(df),df$index)]) num [1:7] 1 7 2 2 3 5 5 > (it seems that indexing by an array isn't well supported for data.frames.) No, it's just that the index column in df is a factor so as.matrix(df) return a matrix of characters > > What is a better way to perform this selection operation? > Not that I know of Cheers > Peter > > [[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.