[R] Help needed with eval parse

[Duncan Murdoch]

On 18/09/2019 8:43 a.m., Huzefa Khalil wrote:
> Hello R-users,
> 
> I have been running a script which produces objects based on the
> column names of a data.frame. The column names are of the form CB_1-1,
> CB_1-2, etc. Now this calculation was rather long and memory
> intensive, so I would rather not have to do it again after fixing the
> column names using "make.names". As a consequence, I am left with a
> bunch of R objects with `-` in the name.
> Accessing them is proving challenging and any help would be appreciated.
> 
> Reproducible example:
> `cb_1-2` <- "hello world"
> t <- "cb_1-2"
> t <- as.name(t)
> t <- eval(parse(text = t))
> 
> Error in eval(parse(text = t)) : object 'cb_1' not found

After t <- as.name(t), you already have language:  no need to parse it 
again.  So

   eval(t)

works.  If you have more complicated expressions, use call() to set them 
ụp.  For example, call("paste0", t, "!") evaluates to

   paste0(`cb_1-2`, "!")

and evaluating that expression via

   eval(call("paste0", t, "!"))

gives

[1] "hello world!"

Don't go back and forth between language objects and text 
representations of them, because it's hard to do that without 
introducing changes.  In other words, don't use eval(parse()).

Duncan Murdoch