[R-pkg-devel] Save and restoring random number seed in a package function

[Noah Greifer]

Yes, set.seed() cannot accept .Random.Seed as an input; it can only take a
single integer. As said in this answer
<https://stackoverflow.com/a/13997608/6348551>, there is a one-way
relationship between set.seed() and .Random.Seed. My understanding is that
the recommended way to restore the seed is to assign the saved seed to
.Random.Seed in the global environment, though this is the method that is
not allowed by the CRAN policy. Unfortunately saving it in the environment
of the inner function is not sufficient.

One potential inconsistency with CRAN's policy is that generating a random
number itself changes the global environment by changing the value of
.Random.Seed. The boot.array() code just does it manually using assign().
Indeed, the boot.array() code does less damage to the global environment in
that it resets the seed to what it would have been had boot.array() not
been run.

Noah

On Wed, Sep 14, 2022 at 10:39 AM James Pustejovsky <jepusto using gmail.com>
wrote:

> I'm interested in this question too. Noah, is there a reason you are using
> assign(".Random.seed",...) rather than set.seed()?
>
> On Wed, Sep 14, 2022 at 9:31 AM Noah Greifer <noah.greifer using gmail.com>
> wrote:
>
>> Hello fellow developers,
>>
>> I am attempting to solve the problem of saving the state of the random
>> generator so that the state can be recovered in a future call.
>> Essentially,
>> my function generates random numbers, performs an operation on them
>> (saving
>> the result), and throws them out (saving them would require too much
>> memory). A second function is meant to take the output of the first
>> function, generate the same random numbers, and perform a different
>> operation on them.
>>
>> This is exactly what happens in the *boot* package: the boot() function
>> saves the random seed (extracted from .Random.Seed), and the boot.array()
>> function extracts the saved seed from the boot() output, sets the seed to
>> that value, re-generates the same set of random numbers, and then
>> re-sets the seed to what it was before boot.array() was called. This has
>> the following benefits: 1) it allows the same random numbers to be drawn;
>> 2) the random numbers don't need to be saved, which is good because they
>> would take up a lot of memory and boot.array() is an optional function (it
>> is used in boot.ci() with type = "bca" for those curious); and 3) the
>> seed
>> carries on from where it left off before boot.array() was called instead
>> of
>> being set to what it was after boot() was called.
>>
>> This is implemented in boot in the following way (code abbreviated):
>>
>> boot <- function(...) {
>>   seed <- .Random.Seed
>>   #Random numbers generated
>>   out <- list(seed = seed
>>                   #Other stuff is in this list
>>               )
>>   out
>> }
>>
>> boot.array <- function(boot.out) {
>>   #Save current random seed in `temp`
>>   if (exists(".Random.seed", envir = .GlobalEnv, inherits = FALSE))
>>     temp <- get(".Random.seed", envir = .GlobalEnv, inherits = FALSE)
>>   else temp <- NULL
>>
>>   #Assign saved seed from boot.out
>>   assign(".Random.seed", boot.out$seed, envir = .GlobalEnv)
>>
>>   #Generate same random numbers from boot() call
>>
>>   #Restore random seed to what it was before boot.array() call
>>   if (!is.null(temp))
>>     assign(".Random.seed", temp, envir = .GlobalEnv)
>>   else rm(.Random.seed, pos = 1)
>> }
>>
>> This seems to work as intended. However, this violates the CRAN policy of
>> changing the global environment. When I used this exact code in a package
>> I
>> submitted, the package was rejected for it. The message I received was
>>
>> > Please do not modify the .GlobalEnv (e.g.: by changing the .Random.seed
>> > which is part of the global environment). This is not allowed by the
>> CRAN
>> > policies.
>>
>>
>> I'm curious what you think the best course of action might be, and what
>> the
>> current policy means for the *boot* package. Thanks for your help.
>>
>> Noah
>>
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>>
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>>
>

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