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…

Accepted Answer

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
```

Object not found error with ddply inside a function

Solution

Problem

Related problems