CODEWITHSUNDEEP
rgloballocalseeding
Manoel Galdino
Manoel Galdino

Reputation: 2396

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?

Upvotes: 31

Views: 4539

Answers (3)

sieste
sieste

Reputation: 9057

To set the seed locally for a chunk of code you can use the with_seed function from the withr package:

withr::with_seed(123, {
  runif(1) # this always gives the same number
})
runif(1)   # this one is different every time

Upvotes: 2

Gwang-Jin Kim
Gwang-Jin Kim

Reputation: 9990

Using @Romain Francois's answer, generalize as function:

withRandom <- function(expr, seed = 1) {
    old <- .Random.seed
    on.exit({.Random.seed <<- old})
    set.seed(seed)
    expr
}

Usage:

runif(2)
withRandom(seed = 2, {
    runif(1)
    runif(1)
})
runif(2)
withRandom(seed = 2, runif(2))
runif(2)

output:

> runif(2)
[1] 0.5776099 0.6309793
> withRandom(seed = 2, {
+     runif(1)
+     runif(1)
+ })
[1] 0.702374
> runif(2)
[1] 0.5120159 0.5050239
> withRandom(seed = 2, runif(2))
[1] 0.1848823 0.7023740
> runif(2)
[1] 0.5340354 0.5572494

Upvotes: 1

Romain Francois
Romain Francois

Reputation: 17642

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

Upvotes: 43

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