[Rd] Why does not the command 'length(a <- 1:5) <- 4' not work?
murdoch at stats.uwo.ca
Tue Jan 16 20:12:39 CET 2007
On 1/16/2007 1:24 PM, Charles Dupont wrote:
> when running the command
> > length(a <- 1:5) <- 4
> there are two responses.
> If 'a' does not exist then the response is
> Error in length(a <- 1:5) <- 4 : object "a" not found
> If 'a' does exist then the response is
> Error in length(a <- 1:5) <- 4 : could not find function "<-<-"
> I would assume that 'length(a <- 1:5) <- 4' should work because
> 'length(a <- 1:5)' does work.
I'm guessing you are assuming it would mean the same as
a <- 1:5
length(a) <- 4
But how would R know the name of the variable whose length is set in the
second line? In "a <- 1:5" the "a" is just part of a larger expression,
it's not special as in some other languages, i.e. R sees that as
So if you rewrote your original as
length("<-"(a, 1:5)) <- 4
it is a lot less clear that you really mean to create and then change "a".
In general things like
foo(a) <- b
are evaluated as "foo<-"(a, b), where "foo<-" is a function that expects
a variable reference as its first argument. There's some magic going on
to allow things like
a <- matrix(1:4, 2,2)
names(dim(a)) <- letters[1:2]
and I think the parser is trying to set things up for that kind of magic
in your expression, though I haven't traced through the execution path
to explain exactly why you saw the error messages you saw.
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