r - Object not found error with ddply inside a function

Question

This has really challenged my ability to debug R code.

I want to use ddply() to apply the same functions to different columns that are sequentially named; eg. a, b, c. To do this I intend to repeatedly pass the column name as a string and use the eval(parse(text=ColName)) to allow the function to reference it. I grabbed this technique from another answer.

And this works well, until I put ddply() inside another function. Here is the sample code:

# Required packages:
library(plyr)

myFunction <- function(x, y){
    NewColName = "a"
    z = ddply(x, y, summarize,
            Ave = mean(eval(parse(text=NewColName)), na.rm=TRUE)
    )
    return(z)
}

a = c(1,2,3,4)
b = c(0,0,1,1)
c = c(5,6,7,8)
df = data.frame(a,b,c)
sv = c("b")

#This works.
ColName = "a"
ddply(df, sv, summarize,
        Ave = mean(eval(parse(text=ColName)), na.rm=TRUE)
)

#This doesn't work
#Produces error: "Error in parse(text = NewColName) : object 'NewColName' not found"
myFunction(df,sv)

#Output in both cases should be
#  b Ave
#1 0 1.5
#2 1 3.5

Any ideas? NewColName is even defined inside the function!

I thought the answer to this question, loops-to-create-new-variables-in-ddply, might help me but I've done enough head banging for today and it's time to raise my hand and ask for help.

question from:https://stackoverflow.com/questions/6955128/object-not-found-error-with-ddply-inside-a-function

Answer 1

You can do this with a combination of do.call and call to construct the call in an environment where NewColName is still visible:

myFunction <- function(x,y){
NewColName <- "a"
z <- do.call("ddply",list(x, y, summarize, Ave = call("mean",as.symbol(NewColName),na.rm=TRUE)))
return(z)
}

myFunction(d.f,sv)
  b Ave
1 0 1.5
2 1 3.5