R - evaluate the expression as a string

Question

Is is possible to do something like,

x = function(n,v) paste("<rel name=\"",quote(n),"\" value=\"",quote(v),"\"/>",sep="")

so that x(y,1) produces,

"<rel name=\"y\" value=\"1\"/>"

of course this doesn't work and instead produces,

"<rel name=\"n\" value=\"v\"/>"

Also I have a nagging feeling that this kind of operation has a technical name, anybody know what it is?

Essentially, it would be nice if I didn't have do x("y","1").

Answer 1

You're looking for substitute :

x = function(n,v) paste("<rel name=\"",substitute(n),"\" value=\"",
                        substitute(v),"\"/>",sep="")

x(y,1)
#[1] "<rel name=\"y\" value=\"1\"/>"

Or if you're going to have more complex expressions, deparse(substitute( :

x = function(n,v) paste("<rel name=\"",deparse(substitute(n)),"\" value=\"",
                        deparse(substitute(v)),"\"/>",sep="")

x(y + 2, 3)
#[1] "<rel name=\"y + 2\" value=\"3\"/>"

Answer 2

You could use deparse(substitute() or match.call . Note I have used sprintf as I find it easier to decipher than paste in these situations.

 xx <- function(n,v){
       x <- sapply(as.list(match.call())[-1],deparse)
        sprintf(fmt ='<rel name=\"%s\" value=\"%s\">',x['n'],x['v'])}
 xx(y,2)
 ## [1] "<rel name=\"y\" value=\"2\">"
 xx(y, fun(x,b,v))
 ## [1] "<rel name=\"y\" value=\"fun(x, b, v)\">"

Note that x(y,fun(p;d)) will not parse as it is not a valid R expression (it won't get past the language intepreter to even start