[R] Chaining multiple replacement functions in R

[Daniel Gabrieli]

That is really helpful.  I am trying to get the rmatch function to return
the position of the 'name' instead of the value.  So rmatch(app, "FirsKey")
would return c(3,3,1).  Then app[[c(3,3,1)]] <- 'new value' would be
perfect.



On Tue, Dec 30, 2014 at 6:43 AM, Duncan Murdoch <murdoch.duncan at gmail.com>
wrote:

> On 29/12/2014 4:41 PM, Daniel Gabrieli wrote:
> > I cross posted this on Stack Overflow:
> >
> http://stackoverflow.com/questions/27694466/chaining-multiple-replacement-functions-in-r
> >
> >
> > I am using R to work with a large JS object (using the library rjsonio).
> As
> > such, I have a lot of nested lists, which are getting somewhat cumbersome
> > to work with. I have a simplified example below. I am trying to work with
> > this object by creating some form of ‘getter’ and ‘setter’ functions.
> After
> > looking around, I have found a pretty nice ‘getter’ function that
> recurses
> > through the object and returns the value for the first matching label.
> This
> > is especially great because it lends itself to chaining functions
> together.
> > However, I can not figure out a way to get the same effect for a ‘setter’
> > function. Any thoughts on how to create a ‘setter’ function that can be
> > chained together in a similar fashion?
>
> I haven't worked through the details here so this might not work, but
> the assignment function could add extra information saying which part of
> the object was modified.  In the example below, "Firstkey" is
> app[[c(3,3,1)]], so a function that modified it could attach c(3,3,1) as
> an attribute, and later functions that wanted to do more things to it
> could start looking there.
>
> I guess the tricky part would be getting rid of that attribute if you
> didn't want to pass it along the chain, e.g. the final call shouldn't
> return it.
>
> Duncan Murdoch
>
> >
> > #example, simplified, object
> > app = list(
> >   1,
> >   2,
> >   d=list(a=123,
> >          b=456,
> >          list(
> >            FirstKey=list(attr1='good stuff', attr2=12345),
> >            SecondKey=list(attr1='also good stuff', attr2=4321)
> >            )
> >          )
> >   )
> >
> >
> > #Return a function that returns the value
> > #associated with first label that matches 'name'
> > getByName <- function(name){
> >   rmatch <- function(x) {
> >     pos <- match(name, names(x))
> >     if (!is.na(pos))
> >       return(x[[pos]])
> >     for (el in x) {
> >       if (class(el) == "list") {
> >         out <- Recall(el)
> >         if (!is.null(out)) return(out)
> >       }
> >     }
> >   }
> >   rmatch
> > }
> >
> > getFirstKey <- getByName("FirstKey")
> > getAttr1 <- getByName("attr1")
> > getAttr2 <- getByName("attr2")
> >
> > #I like that I can chain these functions together
> > getAttr1(getFirstKey(app))
> > getAttr2(getFirstKey(app))
> >
> > # I would like to be able to do something like this
> > # But this won't work
> > ###    getAttr1(getFirstKey(app)) <- 9876
> >
> > # This does work,,, but I loose the ability to chain functions together
> > # Closure around a replacement function
> > setterKeyAttr <- function(keyName, attr){
> >   function(x, value){
> >     x$d[[3]][[keyName]][[attr]] <- value
> >     x
> >   }
> > }
> >
> > `setFirstKeyAttr2<-` <- setterKeyAttr("FirstKey", "attr2")
> > setFirstKeyAttr2(app) <- 22222
> > #check the answer is correct
> > getAttr2(getFirstKey(app))
> >
> >
> >
> > references:
> >
> >
> http://stackoverflow.com/questions/23124096/r-decorator-to-change-both-input-and-output
> >
> >
> http://r.789695.n4.nabble.com/How-to-get-a-specific-named-element-in-a-nested-list-td3037430.html
> >
> > http://adv-r.had.co.nz/Functions.html
> >
> >       [[alternative HTML version deleted]]
> >
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> > and provide commented, minimal, self-contained, reproducible code.
> >
>
>

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