[R] Simple syntax question (I think)

[Marc Schwartz]

> 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