rgloballocalseeding

setting seed locally (not globally) in R


I'd like to set seeds in R only locally (inside functions), but it seems that R sets seeds not only locally, but also globally. Here's a simple example of what I'm trying (not) to do.

myfunction <- function () {
  set.seed(2)
}

# now, whenever I run the two commands below I'll get the same answer
myfunction()
runif(1)

So, my questions are: why does R set the seed globally and not only inside my function? And how I can make R to set the seed only inside my function?


Solution

  • Something like this does it for me:

    myfunction <- function () {
      old <- .Random.seed
      set.seed(2)
      res <- runif(1)
      .Random.seed <<- old
      res
    }
    

    Or perhaps more elegantly:

    myfunction <- function () {
      old <- .Random.seed
      on.exit( { .Random.seed <<- old } )
      set.seed(2)
      runif(1)
    }
    

    For example:

    > myfunction()
    [1] 0.1848823
    > runif(1)
    [1] 0.3472722
    > myfunction()
    [1] 0.1848823
    > runif(1)
    [1] 0.4887732