Evaluate an expression under a temporary random seed

Sets the random seed for the duration of expr and restores the caller's random state afterwards. This makes a single result reproducible without hijacking the random stream of the surrounding script: two calls with the same seed give the same result, and whatever is drawn after them is unaffected by either.

Usage

withSeed(seed, expr)

Arguments

seed: a single number, or NULL to leave the random stream untouched and simply evaluate expr
expr: the expression to evaluate; evaluated lazily, in the caller's environment

Value

the value of expr

Details

The plain idiom set.seed(s); expr lacks the second half. In a script that generates a series of random objects, seeding one of them shifts every draw that follows, so results that were correct before the seed was added silently change.

Examples

set.seed(1)
a <- runif(1)

set.seed(1)
withSeed(99, runif(1))     # unrelated draw in between
#> [1] 0.5847119
identical(runif(1), a)     # the stream continued as if it never happened
#> [1] TRUE

identical(withSeed(7, runif(3)), withSeed(7, runif(3)))
#> [1] TRUE