The difference is the same as the difference between a set with 1 element and a single element from a set.
The single [ extracts/replaces/assigns a subset of the list elements, but the piece is still a list (even if it is one element). So when you are assigning using [ you need to give it a list, not a single object. If you create a list then do something like mode( mylist[1] ) you will see that it is still a list. The double [[ exctracts/replaces/assigns a single element of the list, it does not work on anything more than a single element, but it works with that element as its own object, not a list (unless it is a list). So [[ can be used to assign a single element without needing to create a list (but can only do a single element where [ can do 1 or more). If you do mode( mylist[[1]] ) then you will see that the single element is no longer a list. Hope that helps, -- Gregory (Greg) L. Snow Ph.D. Statistical Data Center Intermountain Healthcare greg.s...@imail.org 801.408.8111 > -----Original Message----- > From: r-help-boun...@r-project.org [mailto:r-help-boun...@r- > project.org] On Behalf Of Filoche > Sent: Tuesday, October 12, 2010 9:29 AM > To: r-help@r-project.org > Subject: Re: [R] List or matrix of object > > > Hi again everyone. > > I found I could use a list with > > l = list() > l[[1]] = myObj > > instead of > > l[1] = myObj > > Anyone can explain me why the use of double [] is required? > > Regards, > Phil > > -- > View this message in context: http://r.789695.n4.nabble.com/List-or- > matrix-of-object-tp2992101p2992121.html > Sent from the R help mailing list archive at Nabble.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. ______________________________________________ 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.