[R] Dynamic regex/sub changes to function

[Duncan Murdoch]

On 22/09/2014 11:34 AM, Daniel Fuka wrote:
> Howdy Duncan,
>
> Thanks for the quick reply!  I must be missing something
> simple/obvious. I need to have the "sub()" not return quoted and
> escaped characters to "just edit the language expression". In my
> problem, there is a function that is supported from a different
> package. So I always want to use the supported function as my base...
> but a url in the supported function needs to be changed dynamically
> for my application, which is easiest using "sub()".
>
> I am trying to do what you correctly indicate I would need to do:
> "just edit the language expression that body(fsong) gives you, and
> assign it back"
> BUT, using sub, I get back a quoted string in my example if I just use sed:
>
> > fsong
> function(x){
>   song=paste("my name is fuka,",x)
>   return(song)
> }
> # Using "sub()" becomes:
> > nsong
> function (x)
> {
>      "song = paste(\"my name is muka,\", x)"
>      return(song)
> }

You didn't do it right :-).   With fsong as above, the string to edit is 
body(fsong)[[c(2,3,2)]].  (Why c(2,3,2)?  Because that's where the 
string is in the parse tree.  Try looking at variations on 
body(fsong)[[c(2,3,2)]] to figure it out, e.g.
body(fsong)[[c(2,3)]], or body(fsong)[[c(2,3,3)]], etc.)

So this code would work:

orig <- body(fsong)[[c(2,3,2)]]
new <- sub("fuka", "muka", orig)

# Now put it back in nsong:
nsong <- fsong
body(nsong)[[c(2,3,2)]] <- new

But as Bill said, this is a really bad idea.  If you just *think* about 
changing that fsong function, it will break.

Duncan Murdoch

>
> Thanks again for the quick reply and help you are giving me!
> dan
>
> On Mon, Sep 22, 2014 at 10:37 AM, Duncan Murdoch
> <murdoch.duncan at gmail.com> wrote:
> > On 22/09/2014 9:16 AM, Daniel Fuka wrote:
> >>
> >> Howdy,
> >>
> >> I have searched the lists and can not seem to find a solution to my
> >> problem. I need to be able to dynamically modify a string inside a
> >> function to build a new function. "sub" replaces with a quoted
> >> string... and "parse" of "sub" returns expression... How can I get an
> >> unquoted string from a regex to stick into a "body" of a function?
> >
> >
> > It's possible to do what you want, though you don't want to be using
> > parse(), you can just edit the language expression that body(fsong) gives
> > you, and assign it back.  But that's a messy way to solve your problem.
> >
> > Why not create a new function containing the new string?  e.g.
> >
> > makefsong <- function(name = "fuka") {
> >   line1 <- paste("my name is", name)
> >   function(x) {
> >     song <- paste(line1, x)
> >     return(song)
> >   }
> > }
> >
> > f1 <- makefsong()
> > f1("I live on the second floor")
> > f2 <- makefsong("muka")
> > f2("I live on the second floor")
> >
> > Duncan Murdoch
> >
> >>
> >> Thanks for your help!
> >> dan
> >>
> >> # Original Function
> >> fsong=function(x){
> >>   song=paste("my name is fuka,",x)
> >>   return(song)
> >> }
> >> fsong("I live on the second floor")
> >> #
> >> # Copy and modify using "sub" returns quoted string with escaped quotes
> >> #   internally... as expected.. which can not be evaluated.
> >> nsong=fsong
> >> body(nsong)[[grep("fuka",body(nsong))]]=
> >>     sub("fuka","muka",list(body(fsong)[[grep("fuka",body(fsong))]]))
> >>
> >> nsong("I live on the second floor") # broken
> >>
> >> #
> >> # Copy and modify with "parse" of  "sub",  returns expression.. but
> >> without quotes,
> >> # o getting closer.
> >> #
> >> nsong=fsong
> >> body(nsong)[[grep("fuka",body(nsong))]]=
> >>
> >> parse(text=sub("fuka","muka",list(body(fsong)[[grep("fuka",body(fsong))]])))
> >>
> >> ______________________________________________
> >> R-help at r-project.org mailing list
> >> https://stat.ethz.ch/mailman/listinfo/r-help
> >> PLEASE do read the posting guide
> >> http://www.R-project.org/posting-guide.html
> >> and provide commented, minimal, self-contained, reproducible code.
> >
> >