Dealing with parentheses within variable names

On 01/03/2013 11:20 AM, William Dunlap wrote:

A core R function that fails with odd names is reformulate():

     > reformulate(c("P/E", "% Growth"), response="+-")

     Error in parse(text = termtext) : <text>:1:16: unexpected input
     1: response ~ P/E+% Growth
                      ^

Thanks.  That one looks relatively easy to fix.

After taking a closer look, I realized that it is actually behaving as 
designed.  The first input to reformulate is supposed to be a character 
vector, not bits of R language.  For example, we want

reformulate("x*w")

to return

~ x*w

not

~ `x*w`

So you might think your example should have been entered as

reformulate(c("`P/E`", "`% Growth`"), response="`+-`")

However, this doesn't quite work:  it ends up with response having 
double backticks.  The way to get what you want is to use

reformulate(c("`P/E`", "`% Growth`"), response=as.name("+-"))

Definitely not my favourite design for a function, but I think it's not 
a code issue.  Maybe something should be added to the help page, but 
this is such an obscure issue that I'm not sure I could make things clearer.

Duncan Murdoch