[R] Function argument and scope

[jeremiah rounds]

Hi,

Didn't bother to run the code because someone else said it might do what
you intended, and also your problem description was complete unto itself.

The issue is that R copies on change.  You are thinking like you have a
reference, which you do not.  That is not very R like in style, but it
certainly can be accomplished if you want via change of input class (See
new.env()).  A typical R style would be to make the modifications to the
input argument, return it, and then assign it back to the input object.

e.g.
test = myFunction(test)

If you really have some reason to want to change the data.frame in a
function without re-assigning it then check out data.table, which has that
as a side effect of how it operates.

Thanks,



On Sun, Nov 13, 2016 at 2:09 PM, Bernardo Doré <berdore at gmail.com> wrote:

> Hello list,
>
> my first post but I've been using this list as a help source for a while.
> Couldn't live without it.
>
> I am writing a function that takes a dataframe as an argument and in the
> end I intend to assign the result of some computation back to the
> dataframe. This is what I have so far:
>
> myFunction <- function(x){
>   y <- x[1,1]
>   z <- strsplit(as.character(y), split = " ")
>   if(length(z[[1]] > 1)){
>     predictedWord <- z[[1]][length(z[[1]])]
>     z <- z[[1]][-c(length(z[[1]]))]
>     z <- paste(z, collapse = " ")
>   }
>   x[1,1] <- z
> }
>
> And lets say I create my dataframe like this:
> test <- data.frame(var1=c("a","b","c"),var2=c("d","e","f"))
>
> and then call
> myFunction(test)
>
> The problem is when I assign x[1,1] to y in the first operation inside the
> function, x becomes a dataframe inside the function scope and loses the
> reference to the dataframe "test" passed as argument. In the end when I
> assign z to what should be row 1 and column 1 of the "test" dataframe, it
> assigns to x inside the function scope and no modification is made on
> "test".
>
> I hope the problem statement is clear.
>
> Thank you,
>
> Bernardo Doré
>
>         [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> 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.

	[[alternative HTML version deleted]]