[R] error in passing an argument to do.call when do.call is in a for statement

[Smits N]

Dear R-users,

I am having a problem with passing an argument to a function in a do.call function which itself is in a for statement.


I am building a function to upload question pools to the blackboard learning environment.
This function which transforms questions to XML style output should be generic and allow for any function to generate questions. Therefore I use do.call in this function. However, because I make multiple versions of a single type of item I use a for statement. Now, when specifying a path for a jpeg (which is part of a question), this path is no longer found by the do.call statement when it is in the for loop. Consequently no jpegs are being created (my created dirs remain empty). I guess it has something to do with different environments inside and outside the do.call. I don't know how to provide them to the do.call properly. I tried to solve it by giving the path a `<<-' assignment but then only one jpeg is being created the first time, all other jpegs are not created. Maybe anyone of you can help?

Here is a simplified version of the code I have been using (looks a bit silly, but the  drawpic function is a bunch of code with all kinds of other functionalities, which work fine).


#This function is unimportant; it generates random paths
randomstring<-function(a=1,n=31){
z<-vector(length=a)
for (h in 1:a){
x<-"n"
for (i in 1:n){
if (runif(1)>0.5){
y<-round(runif(1,-.5,9.5))
    }
    else {y<-letters[round(runif(1,.5,24.5))]
    }                 
x<-paste(x,y, sep = "")
  }
z[h]<-x  }
z
}



#this is a simple function to generate jpegs
dp<-function(path){
jpeg(filename = path, width = 480, height = 480,
pointsize = 12, quality = 85, bg = "white",
res = NA, restoreConsole = TRUE)
x<-rnorm(1000)
hist(x)   
dev.off()}

#this mimicks the generic function
drawpic<-function(method,methodArgs,rep=5){
dir.create(paste("C:\\BB\\"))
time<-format(Sys.time(), "%Y-%m-%d-%H%M")
dir.create(paste("C:\\BB\\",time,sep=""))
supdir<-paste("C:\\BB\\",time,sep="")
for (i in 1:rep){
subdir<-randomstring()
dir.create(paste(supdir,"\\",subdir,sep=""))
Path<<-paste(supdir,"\\",subdir,"\\jpg.jpg",sep="")#using `<-' instead of `<<-'
                                                   #does not produce any pictures
x<-do.call(method,methodArgs)
}}


#run the small function within generic function:
drawpic(method=dp,methodArgs=list(path=Path))


Hope to hear from you people,

Cheers!




Dr Niels Smits
Taskforce Methods & Statistics
Faculty of Psychology and Education
Vrije Universiteit
The Netherlands