How do I modify arguments inside a function?

Question

I have a series of lines of code that replace the contents of an existing column based on the contents of another column (i.e. I am creating a categorical variable where the 'cut' function is not applicable). I am new to R and want to write a function that will perform this task on all data.frames without having to insert and customize 50 lines of code each time.

X is the data frame, Y is the categorical variable, and Z is the other (string) variable. This code works:

X$Y <- ""
X <- transform(X, Y=ifelse(Z=="Alameda",20,""))
... (many more lines)

For example I do:

d.f$loc <- ""
d.f <- transform(d.f, loc=ifelse(county=="Alameda",20,""))
# ... and so on

Now I want to do this for several dataframes and different columns instead of loc and county. However, neither of these functions produces the desired results:

ab<-function(Y,Z,env=X) {
env$Y<-transform(env,Y=ifelse(Z=="Alameda",20,""))
...
}

abc<-function(X,Y,Z) {
X<-transform(X,Y=ifelse(Z=="Alameda",20,""))
...
}

Both of these functions run without error but do not alter the data frame X in any way. Am I doing something wrong in calling the environment or using a function within another function? It seems like a simple question and I would not post if I had not already spent 5+ hours trying to learn this. Thanks in advance!

Answer 1

R uses "call by value" for all objects. Only the return value goes back to the calling enviroment. parameter passing mechanism in R
You can do 

ab <- function(X, Y, Z) { 
   X <- transform(X, Y=ifelse(Z=="Alameda",20,"")) 
   ... 
   return(X) 
}

If your dataframes are in a list L you can do lapply(L, ab) or eventually lapply(L, ab, Y=..., Z=...) As a result you will get a list of the modified dataframes. BTW: Have also a look at with() and within(), e.g. X$Y <- with(X, ifelse(Z=="Alameda",20,""))

implicit returning the value

There is no need for an explicit call of return(...) - you can do it implicit, i.e. using the issue that a function returns the value of its last calculated expression:

ab <- function(X, Y, Z) { 
   X <- transform(X, Y=ifelse(Z=="Alameda",20,"")) 
   ... 
   X ### <<<<< last expression
}

Here is example how you can do it for your situation:

ab <- function(X, Y, Z) { 
  X[, Y] <- ifelse(X[,Z]>12,20,99) 
  # ... 
  X ### <<<<< last expression
}
B <- BOD # BOD is one of the dataframes which come with R
ab(B, "loc", "demand")