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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