Thank you for the clarification! Wow, I do not know the same rule applies to the list object. Maybe that's because the list object will give NULL not an error. I might have incorrectly used it in my package.
Best, Jiefei On Sun, Mar 28, 2021 at 3:26 AM Bill Dunlap <williamwdun...@gmail.com> wrote: > > > > It looks like when calling the dollar symbol using the function format, it > > > treats the input argument as a character literal and does not evaluate it > > > inside the function. I know we have the function `field` to get the slot > > > variable, but I wonder if this is designed on purpose as the above example > > > works for both list and S4 system. > > It works almost the same way with lists, the difference being that an > unknown name with a list gives NULL and not an error. > > > L <- list(A=101) > > `$`(L, A) > [1] 101 > > `$`(L, "A") > [1] 101 > > i <- "A" > > `$`(L, i) > NULL > > The 2nd argument is treated as a symbol and is never evaluated. If it > is given as a string literal then it is converted to a symbol - that > is a holdover from the old days before backticks could be used to make > symbol literals for symbols that included odd things like spaces and > dollar signs. > > -Bill > > On Sat, Mar 27, 2021 at 7:49 AM Jiefei Wang <szwj...@gmail.com> wrote: > > > > Thanks, Duncan. Below is the repost of my question in plain text mode. > > > > > > I'm trying to get the field value of a reference object by the field > > name, but the dollar symbol behaves quite unusual. See example below > > > > .foo <- setRefClass( > > "foo", > > fields = list( > > a = "integer" > > ) > > ) > > x <- .foo$new(a=1L) > > ## This is OK > > x$a > > ## This is OK > > `$`(x, "a") > > ## But this is not OK > > i <- "a" > > `$`(x, i) > > > > For the last line of code I get this error > > > > Error in envRefInferField(x, what, getClass(class(x)), selfEnv) : > > ‘i’ is not a valid field or method name for reference class “foo” > > > > It looks like when calling the dollar symbol using the function > > format, it treats the input argument as a character literal and does > > not evaluate it inside the function. I know we have the function > > `field` to get the slot variable, but I wonder if this is designed on > > purpose as the above example works for both list and S4 system. > > > > Best, > > Jiefei > > > > > > On Sat, Mar 27, 2021 at 10:20 PM Duncan Murdoch > > <murdoch.dun...@gmail.com> wrote: > > > > > > On 27/03/2021 10:16 a.m., Jiefei Wang wrote: > > > > Hi all, > > > > > > > > I'm trying to get the field value of a reference object by the field > > > > name, > > > > but the dollar symbol behaves quite unusual. See example below > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > *.foo <- setRefClass( "foo", fields = list( a = "integer" > > > > ))x <- .foo$new(a=1L)## This is OKx$a## This is OK`$`(x, "a")## But > > > > this is > > > > not OKi <- "a"`$`(x, i)* > > > > > > This is really hard to read. Please post it again, but don't use HTML. > > > > > > Duncan Murdoch > > > > > > > > > > > For the last line of code I get this error > > > > > > > > > > > > *Error in envRefInferField(x, what, getClass(class(x)), selfEnv) : > > > > ‘i’ is > > > > not a valid field or method name for reference class “foo”* > > > > > > > > It looks like when calling the dollar symbol using the function format, > > > > it > > > > treats the input argument as a character literal and does not evaluate > > > > it > > > > inside the function. I know we have the function `field` to get the slot > > > > variable, but I wonder if this is designed on purpose as the above > > > > example > > > > works for both list and S4 system. > > > > > > > > Best, > > > > Jiefei > > > > > > > > [[alternative HTML version deleted]] > > > > > > > > ______________________________________________ > > > > R-devel@r-project.org mailing list > > > > https://stat.ethz.ch/mailman/listinfo/r-devel > > > > > > > > > > > ______________________________________________ > > R-devel@r-project.org mailing list > > https://stat.ethz.ch/mailman/listinfo/r-devel ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
