[R] Is there a way to edit a specific line in a function (e.g: doing function->text->edit->function) ?

[Duncan Murdoch]

On 16/03/2010 5:31 PM, Tal Galili wrote:
> Hello,
> 
> Let's say we have the following function:
> 
> foo <- function(x)
> 
> {
> 
> 
>     line1 <- x
> 
> 
>     line2 <- 0
> 
> 
>     line3 <- line1 + line2
> 
> 
>     return(line3)
> 
> }
> 
>  And that we want to change the second line to be:
> 
>     line2 <- 2
> 
>  How would you do that?
> 
> The two ways I know of are either to use
> 
> fix(foo)
> 
>  And change the function.
> 
> Or to just write the function again.

That's the best way.
> 
> Is there another way?

You can use findLineNum to find where that line of source appears in 
your workspace.  For example, if I source your input from the above, the 
line that interests you is line 9 of my foo.R input file.  Then

 > source("c:/temp/foo.R")
 > findLineNum("foo.R#9")
c:\temp\foo.R#9:
  foo step 3 in <environment: R_GlobalEnv>
 > body(foo)[[3]]
line2 <- 0

Now I can replace step 3 using

body(foo)[[3]] <- quote(line2 <- 2)

If the line you want is nested deep within a block structure, you might 
need to use a vector index to get to it, e.g. after wrapping that line 
in an if() { }, I see

 > findLineNum("foo.R#10")
c:\temp\foo.R#10:
  foo step 3,3,2 in <environment: R_GlobalEnv>
 > body(foo)[[c(3,3,2)]] <- quote(line2 <- 2)
 > foo
function (x)
{
     line1 <- x
     if (x > 0) {
         line2 <- 2
     }
     line3 <- line1 + line2
     return(line3)
}

The findLineNum() function depends on having source references in the 
function, so it won't work on functions in packages unless you build 
them with the option to keep package source refs.  (And then you need 
special contortions to tell it to look in the namespace for the 
function, and to edit a function in the namespace.  Better not to do that.)

Duncan Murdoch




> 
> *What I would like* is for some way to represent the function as a vector of
> strings (well, characters), then change one of it's values, and then turn it
> into a function again.
> 
> The reason I am asking is that I just published a post online where I used a
> function to which I did a minor tweak (so to improve it's output for my
> particular case).
> This tweaking was just adding one line of code, to a function who's length
> is 187 lines of code. So instead of repasting all the function on my blog, I
> decided to just explain how to edit it.  But I would rather have a simple
> code that edited the function for the reader.
> 
> 
> Thanks,
> Tal
> 
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