[R] function closure

[William Dunlap]

Change your 'fileIndex <- fileIndex + 1' to 'fileIndex <<- fileIndex + 1'
so the fileIndex in the environment of the call to generateFiles() is altered
(instead of creating and forgetting about a variable called fileIndex
in the function created by generateFiles.

  generateFiles <- function(path, pattern="\\.xml$") {
      files <- dir(path, pattern=pattern, full.names=TRUE, recursive=TRUE, no..=TRUE)
      total <- length(files)
      fileindex <- 1
      fileGen <- function() {
          if (fileindex < total) {
              fileToReturn <- files[[fileindex]]
              print(fileindex)
              fileindex <<- fileindex + 1
              fileToReturn
          }
      }
      fileGen
  }

used as:

  > nextFile <- generateFiles("c:/temp", pattern="\\.R$")
  > nextFile()
  [1] 1
  [1] "c:/temp/4209/test.R"
  > nextFile()
  [1] 2
  [1] "c:/temp/a.R"

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com


> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf
> Of Ashwani Rao
> Sent: Monday, August 12, 2013 9:45 AM
> To: r-help at r-project.org
> Subject: [R] function closure
> 
> Hi,
> 
> Just for fun, I wanted to try function closures.
> Here  is simple function definition.
> The following "generateFiles" function return function "fileGen" which will
> return a a regular file in directory , every time a call it (fileGen).
> 
> 
> generateFiles <- function(path, pattern="*.xml")
> 
> {
> 
>         files <- dir(path, pattern="*.xml", full.names=TRUE, recursive=TRUE,
> no..=TRUE)
> 
> total <- length(files)
> 
> fileindex = 1
> 
>         fileGen <- function()
> 
> {
> 
> if (fileindex > total)
> 
> {
> 
> return(NULL)
> 
> }
> 
> fileToReturn <- files[[fileindex]]
> 
> print(fileindex)
> 
> fileindex <- fileindex + 1
> 
> return(fileToReturn)
> 
> }
> 
> return(fileGen)
> 
> }
> 
> 
> I am incrementing the value of variable "fileindex" in the function
> "fileGen" , everytime I call this function.
> 
> But the value of fileindex remains 1 , no matter how many times i call
> "fileGen()".
> 
> Can you please tell me how to change the value of fileIndex so as in
> subsequent call to "fileGen" will reflect this.
>  --
> अश्वनी प्रताप
> 
> 	[[alternative HTML version deleted]]