[Apologies -- I inadvertently omitted an example, essential for clarity, from the examples below. Now corrected.]
On 25-Oct-09 09:30:51, Ted Harding wrote: > On 25-Oct-09 09:52:42, Patrick Burns wrote: >> 'The R Inferno' page 59. >> >> Patrick Burns >> patr...@burns-stat.com >> +44 (0)20 8525 0696 >> http://www.burns-stat.com >> (home of "The R Inferno" and "A Guide for the Unwilling S User") Which essentially says that If you want the component [x1[comp] of the list x1] to stay there but to be NULL, then do: xl[comp] <- list(NULL) I agree that this can be very puzzling! The essential point is that (moving to Maura's example) myList[2] or, equivalently, myList["second"] is a LIST (whose only component is that component of the original myList). On the other hand, myList[[2]] or, equivalently, myList$second is NOT a list, but is the value of that component of myList: myList[1] # $first # [1] "aaa" myList[[1]] # [1] "aaa" myList["first"] # $first # [1] "aaa" myList[["first"]] # [1] "aaa" myList$first # [1] "aaa" ### the final example added this time, to show that $first is ### the same as [["first]]), i.e. is a value, not a list. Note the statement (under "Recursive (list-like) objects") in ?"$" or, equivalently, ?Extract When either '[[' or '$' is used for replacement, a value of 'NULL' deletes the corresponding item of the list. Therefore changing the value of a comnponent of a list to NULL deletes it. So you have to work at the list level, replacing one list by another list. Hence Patrick's tip. Ted. >> mau...@alice.it wrote: >>> I can define a list containing NULL elements: >>> >>>> myList <- list("aaa",NULL,TRUE) >>>> names(myList) <- c("first","second","third") >>>> myList >>> $first >>> [1] "aaa" >>> $second >>> NULL >>> $third >>> [1] TRUE >>>> length(myList) >>> [1] 3 >>> >>> However, if I assign NULL to any of the list element then such >>> element is deleted from the list: >>> >>>> myList$second <- NULL >>>> myList >>> $first >>> [1] "aaa" >>> $third >>> [1] TRUE >>>> length(myList) >>> [1] 2 >>>> # >>>> myList$first <- NULL >>>> myList >>> $third >>> [1] TRUE >>>> length(myList) >>> [1] 1 >>> >>> Instead vectors cannot include NULL element: >>> >>>> vec <- c(TRUE,NULL,FALSE) >>>> vec >>> [1] TRUE FALSE >>>> length(vec) >>> [1] 2 >>>> vec[1] <- NULL >>> Error in vec[1] <- NULL : replacement has length zero >>> >>> Is the above shown behaviour of list data structures to be expected ? >>> I took me a lot of sweat to figure out this wierd behaviour was the >>> cause of a bug >>> in my big program. >>> In general, if I have a list with some elements initialized to NULL, >>> that can be changed >>> dynamically, then how can I reinitialize such elements to NULL >>> without >>> deleting them >>> from the list ? >>> >>> Thank you in advance, >>> Maura >>> >>> >>> >>> >>> >>> [[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. > > -------------------------------------------------------------------- > E-Mail: (Ted Harding) <ted.hard...@manchester.ac.uk> > Fax-to-email: +44 (0)870 094 0861 > Date: 25-Oct-09 Time: 09:30:45 > ------------------------------ XFMail ------------------------------ > > ______________________________________________ > 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. -------------------------------------------------------------------- E-Mail: (Ted Harding) <ted.hard...@manchester.ac.uk> Fax-to-email: +44 (0)870 094 0861 Date: 25-Oct-09 Time: 09:45:03 ------------------------------ XFMail ------------------------------ ______________________________________________ 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.
