[R] Function argument and scope

jeremiah rounds roundsjeremiah at gmail.com
Mon Nov 14 09:16:38 CET 2016


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.

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.


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
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