[R] Dynamic regex/sub changes to function

[Duncan Murdoch]

On 22/09/2014, 7:38 PM, Daniel Fuka wrote:
> Thanks everyone for the help. I need to step back and refresh my
> memory on expressions as I am still unclear as to why I can not
> directly edit:
> body(nsong)[[2]]
> # Which can be located from a grep:
> body(nsong)[[grep("fuka",body(nsong))]]
> # though I believe
> class(body(nsong)[[2]])
> [1] "="
> # is trying to give me a pretty blatant hint... {: -)

Here's another hint:  everything in R is a function call.  You're
looking at a call to the function named "=".

Duncan Murdoch

> 
> On Mon, Sep 22, 2014 at 1:24 PM, Duncan Murdoch
> <murdoch.duncan at gmail.com> wrote:
>> 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.
>>>>
>>>>
>>
>>