[R] -> using eval(parse(text= ... ))

[Duncan Murdoch]

On 2/16/2007 7:13 PM, Jon Minton wrote:
> Hi, 
> 
> I'm unsure why I can't use eval(parse(text=...)) to represent the thing I
> want to assign a value into.
> 
> e.g.
> 
>> A <- c("Good")
>> B <- c("Bad")
>> C <- c()
>> X <- c("A", "B", "C")
>> eval(parse(text=X[1]))
> [1] "Good"
>> eval(parse(text=X[2])) -> Z
>> Z
> [1] "Bad"
>> eval(parse(text=X[3])) <- "Ugly"
> Error in file(file, "r") : unable to open connection
> In addition: Warning message:
> cannot open file 'C', reason 'No such file or directory'  
> 
> Why the error message?
> 
> I've tried looking through the archive for a solution but, perhaps because
> I'm not too sure how to phrase this problem, I've not found one...

eval(parse(text=X[3])) is asking R to evaluate the expression C, which 
doesn't exist yet.  The expression

C <- "Ugly"

doesn't evaluate C first (because C doesn't exist yet).  It evaluates a 
call to "<-" with C as one of the arguments, and "<-" treats that 
argument as a name.

There are several things that would work to do what you want, but the 
most straightforward is

assign(X[3], "Ugly")

Duncan Murdoch