Variables created within a function not getting stored in data frame - r

Question

I am writing a function to create some predicted variables within an existing data set that I am using to run some ML models. My function looks like this:
doall <- function(x1, x2){
J48 <- J48(ML, data=df1)
#summary(J48)
X1 <- predict(J48, df1, type="class")
X2 <- predict(J48, df2, type="class")
#return(X1)
}
doall(df1$DT_predict, df2$DT_predict1)
J48 is a decision tree model (via RWeka). The code works (doall(df1$DT_predict1, df2$DT_predict1)) properly, I believe, because when I include the return function, it returns the values of X1. However, the predicted variables are not getting generated/stored in the data frames (df1 and df2). Ideally, I would like to have the dataframe names within the function, but that's the next step.
Can someone show how can I store the variables X1 and X2 within dataframes df1 and df2 respectively.

Answer 1

Ideally your question would have a bit more information about what your data frames look like, what X1 and X2 look like, and where your data frames are stored. For my answer I am assuming your data frames are stored in the global environment, and you want to modify them through a function.
This question has to do with scoping. For an in-depth description of scoping check out this article http://adv-r.had.co.nz/Functions.html#lexical-scoping
First, by assigning your variables within a function you are assigning them in a local environment. This means that the variables you are assigning do not carry over into the global environment (what you see when you type ls().
I believe you either want change a 'global variable' from within a function. This is done by the
<<-
command
for instance
a <- 2
print(a)
returns 2
change_a<-function(x){
x<-x*4
}
change_a(a)
print(a)
still returns 2
while
change_a<-function(x){
x<<-x*4
}
change_a(a)
print(a)
would return 8
I think you want to use the <<- operator instead of <- to accomplish what you want.
On a related note, it is not generally considered to be best practices to assign and change global variables from within a function.