[R] Simple syntax question (I think)

[William Dunlap]

Note that the expression
   x[1] <- 10
is equivalent not to
   `[<-`(x, 1, value=10)
but to
   x <- `[<-`(x, 1, value=10)
so there is no conflict between your two expressions.

Saying
   c(1,2,3) <- `[<-`(c(1,2,3), 1, value=10)
is not allowed because there is no name to assign something to.

There are a few cases where an expression without a name on
the left side would make sense, as in
   environment()[["x"]] <- 12
instead of
   thisEnvir <- environment()
   thisEnvir[["x"]] <- 12
but that is not allowed (in the interests of having consistent rules).



Bill Dunlap
TIBCO Software
wdunlap tibco.com

On Wed, Jan 20, 2016 at 11:21 AM, Bert Gunter <bgunter.4567 at gmail.com>
wrote:

> Thanks Marc.
>
> Actually, I think the cognate construction for a vector (which is what
> a list is also) is:
>
> > vector("numeric",2)[2] <-  3
> Error in vector("numeric", 2)[2] <- 3 :
>   target of assignment expands to non-language object
>
> but this works:
>
> > "[<-"(vector("numeric",2),2,3)
> [1] 0 3
>
> I would have thought the 2 versions should be identical, but as you
> allude, there are apparently subtleties in the parsing/evaluation that
> I do not understand, so that the explicit functional form is parsed
> and evaluated differently than the implicit one. The obvious message,
> though, is: don't do this!
>
> I suspect there is a reference to this somewhere in the R Language
> definition  or elsewhere, and if so, I would appreciate someone
> referring me to it -- RTFM certainly applies!
>
> Cheers,
> Bert
>
> Bert Gunter
>
> "The trouble with having an open mind is that people keep coming along
> and sticking things into it."
> -- Opus (aka Berkeley Breathed in his "Bloom County" comic strip )
>
>
> On Wed, Jan 20, 2016 at 10:57 AM, Marc Schwartz <marc_schwartz at me.com>
> wrote:
> >
> >> On Jan 20, 2016, at 12:26 PM, Bert Gunter <bgunter.4567 at gmail.com>
> wrote:
> >>
> >> Could someone please explain to me my mal-understanding of the
> >> following, which I expected to give the same results without errors.
> >>
> >> TIA.
> >>
> >> -- Bert
> >>
> >>> z <-  list(x=1)
> >>> z[[2]] <- 3
> >>> z
> >> $x
> >> [1] 1
> >>
> >> [[2]]
> >> [1] 3
> >>
> >>> list(x = 1)[[2]] <- 3
> >> Error in list(x = 1)[[2]] <- 3 :
> >>  target of assignment expands to non-language object
> >
> >
> > Bert,
> >
> > I will take a stab at this.
> >
> > In the first case, you are adding a new element to an existing list
> object, so works as expected:
> >
> > # Create a new list 'z'
> > z <-  list(x = 1)
> >
> >> z
> > $x
> > [1] 1
> >
> >
> > # Now, add a new unnamed element in the list
> > z[[2]] <- 3
> >
> >> z
> > $x
> > [1] 1
> >
> > [[2]]
> > [1] 3
> >
> >
> > In the second case, you are attempting to subset a list that does not
> yet exist and assign a value to an element of a non-existent object:
> >
> >> list(x = 1)[[2]]
> > Error in list(x = 1)[[2]] : subscript out of bounds
> >
> >> list(x = 1)[[2]] <- 3
> > Error in list(x = 1)[[2]] <- 3 :
> >   target of assignment expands to non-language object
> >
> >
> > If this was to work, the parser would have to evaluate the command in a
> left to right fashion, first creating the list with an element 'x' and then
> adding the new element to it as a second step, much as you did explicitly
> in the first approach.
> >
> > You get somewhat similar behavior with a vector, albeit the error is
> perhaps a bit more clear:
> >
> >> Vec
> > Error: object 'Vec' not found
> >
> >> Vec[2] <- 3
> > Error in Vec[2] <- 3 : object 'Vec' not found
> >
> > Vec <- 1
> >
> >> Vec
> > [1] 1
> >
> > Vec[2] <- 2
> >
> >> Vec
> > [1] 1 2
> >
> >
> > Regards,
> >
> > Marc
> >
>
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