! is a logical operator... it means "not". When you write lidx <- seq_along( mtcars[[ 1 ]] ) %in% train_indices
you end up with a vector of logical values for which ! makes sense. Since R supports logical indexing this can be a very convenient way to select one group or the other. If you give an integer to the ! operator, any non-zero value is treated as TRUE, which can be useful sometimes but not in this case, since all of the train_indices are greater than zero. Look at what !train_indices actually is. As the Introduction to R document says, integer indexing always starts at 1 instead of zero as in many other languages. This makes it feasible to let negative integers as indexes represent the idea of excluding those positions. Thus identical( mtcars[ !lidx, ], mtcars[ -train_indices, ] ) The ItoR document is really quite informative to re-read occasionally. For example, look up indexing with a matrix as the index. -- Sent from my phone. Please excuse my brevity. On April 15, 2017 5:18:43 PM PDT, Carl Sutton via R-help <r-help@r-project.org> wrote: >Hi > > >I normally use package data.table but today was doing some base R >coding. Had a problem for a bit which I finally resolved. I was >attempting to separate a data frame between train and test sets, and in >base R was using the "!" to exclude training set indices from the data >frame. All I was getting was zero observations. Changed to using "-" >and it worked. I recalled that in data.table the "!" function worked, >so created this little bit of code. > ># Base R Functions >str(mtcars) >train_indices <- sample(nrow(mtcars), round(0.75*nrow(mtcars))) >train <- mtcars[train_indices,] >mode(train_indices); class(train_indices) >test <- mtcars[!train_indices,] # the "!" function returning 0 >observations >test_1 <- mtcars[-train_indices,] >identical(test, test_1) > ># Using data.table package >library(data.table) >dt1 <- data.table(mtcars) >train_indices <- sample(nrow(dt1), round(0.75*nrow(dt1))) >train <- dt1[train_indices,] >mode(train_indices); class(train_indices) >test <- dt1[!train_indices,] # the "!" function >test_1 <- dt1[-train_indices,] >identical(test, test_1) >The documentation appears to me to accept "!" in base, so do I have >some kind of ridiculous error or ..?? >Carl Sutton > >______________________________________________ >R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see >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 -- To UNSUBSCRIBE and more, see 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.
