Jiefei Wang
2021-Mar-27 14:49 UTC
[Rd] Surprising behavior when using the reference class with the dollar symbol
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.duncan at 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 at r-project.org mailing list
> > https://stat.ethz.ch/mailman/listinfo/r-devel
> >
>
Bill Dunlap
2021-Mar-27 19:25 UTC
[Rd] Surprising behavior when using the reference class with the dollar symbol
> > 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" > `$`(L, i)NULL The 2nd argument is treated as a symbol and is never evaluated. If it is given as a string literal then it is converted to a symbol - that is a holdover from the old days before backticks could be used to make symbol literals for symbols that included odd things like spaces and dollar signs. -Bill On Sat, Mar 27, 2021 at 7:49 AM Jiefei Wang <szwjf08 at gmail.com> wrote:> > Thanks, Duncan. Below is the repost of my question in plain text mode. > > > I'm trying to get the field value of a reference object by the field > name, but the dollar symbol behaves quite unusual. See example below > > .foo <- setRefClass( > "foo", > fields = list( > a = "integer" > ) > ) > x <- .foo$new(a=1L) > ## This is OK > x$a > ## This is OK > `$`(x, "a") > ## But this is not OK > i <- "a" > `$`(x, i) > > For the last line of code I get this error > > Error in envRefInferField(x, what, getClass(class(x)), selfEnv) : > ?i? is not a valid field or method name for reference class ?foo? > > It looks like when calling the dollar symbol using the function > format, it treats the input argument as a character literal and does > not evaluate it inside the function. I know we have the function > `field` to get the slot variable, but I wonder if this is designed on > purpose as the above example works for both list and S4 system. > > Best, > Jiefei > > > On Sat, Mar 27, 2021 at 10:20 PM Duncan Murdoch > <murdoch.duncan at gmail.com> wrote: > > > > On 27/03/2021 10:16 a.m., Jiefei Wang wrote: > > > Hi all, > > > > > > I'm trying to get the field value of a reference object by the field name, > > > but the dollar symbol behaves quite unusual. See example below > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > > *.foo <- setRefClass( "foo", fields = list( a = "integer" > > > ))x <- .foo$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 at r-project.org mailing list > > > https://stat.ethz.ch/mailman/listinfo/r-devel > > > > > > > ______________________________________________ > R-devel at r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-devel