This is a warning and in your case would not be a problem, but it is good to think about and the reason why it is suggested that you avoid using attach and be very careful when you do use attach. What is happening is that you first created a vector named 'x' in your global workspace, you then create a data frame that contains a column that is a copy of 'x' that is also named 'x' and the data frame also has another column named 'y'. You then later attach the data frame to the search list (if you run the 'search()' command you will see your search list). This is convenient in that you can now access 'y' by typing its name instead of something like 'dummy$y', but what happens if you just type x? The issue is that there are 2 objects on your search path with that same name. For your example it will not matter much because they have the same value, but what if you run a command like 'x <- 3', now you will see a single value instead of a vector of length 20 which can lead to hard to find errors. This is why R tries to be helpful by warning you that there are multiple objects named 'x' and therefore you may not be accessing the one that you think. If you use attach without being careful it is possible to plot (or regress or ...) one variable from one dataset against another variable from a completely unrelated dataset and end up with meaningless results. So, if you use attach, be careful. You may also want to look at the followng functions for help with dealing with these issues: conflicts, find, get, with, within
On Wed, May 28, 2014 at 11:55 PM, Stephen Meskin <actu...@umbc.edu> wrote: > While following the suggestion in the manual "An Introduction to R" to > begin with Appendix A, I ran into the problem shown below about 3/4 of > the way down the 1st page of App. A. > > After using the function /attach/, I did not get visible columns in the > data frame as indicated but the rather puzzling message emphasized below. > > I am running R version 3.1.0 (2014-04-10) using Windows XP. Thanks in > advance for your help. > >> > x<-1:20 >> > w<-1+sqrt(x)/2 >> > dummy<-data.frame(x=x, y=x+rnorm(x)*w) >> > dummy >> x y >> 1 1 2.885347 >> ... >> > fm<- lm(y ~ x, data=dummy) >> > summary(fm) >> >> Call: >> ... >> > fm1<- lm(y ~ x, data=dummy,weight=1/w^2) >> > summary(fm1) >> >> Call: >> ... >> > attach(dummy) >> *_The following object is masked _by_ .GlobalEnv: >> >> x_**_ >> _* > > -- > /Stephen A Meskin/, PhD, FSA, MAAA > Adjunct Assistant Professor of Mathematics, UMBC > > **// > > [[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. -- Gregory (Greg) L. Snow Ph.D. 538...@gmail.com ______________________________________________ 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.
