Hi Stephen, See in line.
On Fri, May 30, 2014 at 4:18 PM, Stephen Meskin <actu...@umbc.edu> wrote: > Greg, Ista, (or anyone else), > Let me take one last run at this problem. > > Consider the following extract from the Appendix A text: > >> x <- 1:20 >> w <- 1+sqrt(x)/2 >> dummy <- data.frame(x=x, y=x+rnorm(x)*w) >> fm <- lm(y~x, data=dummy) >> fm1 <- lm(y~x, data=dummy, weight=1/w^2) > >>attach(dummy) > Make the columns in the data frame visible as variables. > > The following object is masked_by_.GlobalEnv: > > x > > > In the above I have included only one comment, "Make the columns ... visible > as variables." from the text and > only one response, "The following object is masked by ... : x." > > Stuff I don't understand: > > The purpose of "attach" seems to be to make x and y visible but I can > already see them by entering the command "dummy" even after the warning. So > what does "attach do? It makes them "visible" in the sense that you can refer to them without referring to dummy: try rm(list=ls()) ## delete everything from your workspace dummy <- data.frame(x=1:20) # data.frame containing x dummy$w <- 1+sqrt(dummy$x)/2 # add "w" column to dummy dummy$y <- dummy$x + dummy$x + rnorm(dummy$x) * dummy$w # add "y" column # x is not available x #Error: object 'x' not found #...exept as an element of dummy dummy$x attach(dummy) # Make the columns in the data frame visible as variables. x # x is now available as "x", as well as "dummy$x" > What I would like to see is a table with 1st column x; 2nd column y; 3rd and > 4th columns predicted ys from fm and fm1; plus possibly columns of residuals > and other stuff. Such tables don't seem to be available according to the > discussion in ?lm. In R it is common to calculate things only as you need them. The predicted and residual values are not calculated by lm, but after the fact by predict.lm() and residuals.lm(). For example: fm <- lm(y~x, data=dummy) fm1 <- lm(y~x, data=dummy, weight=1/w^2) dummy$yhat.fm <- predict(fm) dummy$yhat.fm1 <- predict(fm1) dummy$yresid <- residuals(fm1) dummy > The warning about "attach" seems to say that there is an "x" in the Global > Environment that will mask the "x" that I am using. But that is not > happening in what I see. Yes it is. If I enter "> x" after the warning, I still get > 1,2,3,.... as before. What is the problem? That 1,2,3 ... you are seeing comes from the "x" that was defined earlier, by x <- 1:20 _not_ from the x in dummy nor the x attached from dummy. Try this: rm(list=ls()) x <- 1:5 w <- 1+sqrt(x)/2 dummy <- data.frame(x=x, y=x+rnorm(x)*w) attach(dummy) You now have three "x" variables: the one created with "x <- 1:5" (in the global environment), the one in dummy, and the attached one copied from dummy. This makes it easy to become confused about which "x" you are getting. Consider: x <- 1 ## changes x in the global workspace, but not dummy$x nor the attached copy of dummy$x dummy$x <- 2 # changes dummy$x but not the attached copy of dummy$x x # this is the x in the global environment # [1] 1 rm(x) x # this is the attached copy of x #[1] 1 2 3 4 5 dummy$x # this is the x in dummy # [1] 2 2 2 2 2 > If I place the above R-script in a folder other than the R-console that > comes up when I first open R will that obviate the attach "problem." I'm not following this one... Best, Ista > > > Stephen A Meskin, PhD, FSA, MAAA > Adjunct Assistant Professor of Mathematics, UMBC > > Most people give you an anticipatory grin when you mention a statistic, > frown doubtingly when you mention the plural statistics, and > grunt and groan in a gurgle when you mention a statistics course. > On 5/30/2014 12:20 PM, Greg Snow wrote: > > If you pay attention and are careful not to use any variables names > that conflict then you do not need a work around (and the conflicts > function can help you see if there are any conflicts that you may need > to worry about). > > Probably the best work around is to use the with or within function > instead of attaching. For a couple of quick commands these work great > and I prefer them to using attach. But, sometimes for a long sequence > of commands attach is much more convenient and is fine to use as long > as you recognize the potential dangers and are careful. > > On Thu, May 29, 2014 at 3:56 PM, Stephen Meskin <actu...@umbc.edu> wrote: > > Thanks Greg for your response. Is there a work around? > > Of course this begs the question as to Why is attach part of the sample > session in App. A of the introductory manual? All the commands are directly > from App. A. Is it possible the configuration of R on my computer is not in > accord with acceptable practice? I.e. Could my configuration be set so that > attach works as App. A intends? > If not then App. A needs to be changed to replace attach. > If so, then App. A needs to provide instructing on appropriate configuration > of R for newbies. > > Stephen Meskin > Sent from my iPad > > On May 29, 2014, at 1:06 PM, Greg Snow <538...@gmail.com> wrote: > > 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.
