[R] Simple syntax question (I think)
bgunter.4567 at gmail.com
Wed Jan 20 21:40:16 CET 2016
Thanks to both Bill and Duncan for their help. As I said, my
mal-understanding of the syntax.
"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 11:51 AM, Duncan Murdoch
<murdoch.duncan at gmail.com> wrote:
> On 20/01/2016 2:21 PM, Bert Gunter wrote:
>> Thanks Marc.
>> Actually, I think the cognate construction for a vector (which is what
>> a list is also) is:
>> > vector("numeric",2) <- 3
>> Error in vector("numeric", 2) <- 3 :
>> target of assignment expands to non-language object
>> but this works:
>> > "[<-"(vector("numeric",2),2,3)
>>  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!
> There's a detailed discussion in the Language Reference. Search for
> "complex assignment", or look in section 3.4.4 Subset assignment. The big
> difference between the two expressions you give is in the final assignment
> of the result of the "[<-" function to the referenced variable. In your
> first case there's no referenced variable, just an expression
> vector("numeric",2), so you get the error, just as you would with
> vector("numeric",2) <- c(0, 3)
> Duncan Murdoch
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