# Turning a stochastic simulator into a set of deterministic simulators by fixing the seed

**URL:** <https://discourse.julialang.org/t/turning-a-stochastic-simulator-into-a-set-of-deterministic-simulators-by-fixing-the-seed/121642>\
**Category:** General Usage\
**Tags:** question\
**Created:** [October 23, 2024, 1:43pm UTC](https://discourse.julialang.org/t/turning-a-stochastic-simulator-into-a-set-of-deterministic-simulators-by-fixing-the-seed/121642 "2024-10-23T13:43:31Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![weltenbummler](https://avatars.discourse-cdn.com/v4/letter/w/82dd89/32.png) [@weltenbummler](https://discourse.julialang.org/u/weltenbummler)\
**Post date:** [October 23, 2024, 1:43pm UTC](https://discourse.julialang.org/t/turning-a-stochastic-simulator-into-a-set-of-deterministic-simulators-by-fixing-the-seed/121642/1 "2024-10-23T13:43:31Z")

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Hello all,

I am given a stochastic simulator and would like to turn it into `n` deterministic simulators where each operates with a fixed seed. The simulators depend on some parameters that the user can specify.

My question is about the best practice so that I may use the deterministic simulators in parallel (e.g. in a distributed environment or as different threads) without running into issues. Constraints are that I am not allowed to change the given stochastic simulator. I will also have limited ability to inspect the internals of the provided stochastic simulator.

As a simple working example, I am given the following stochastic simulator

```julia
function external_simulator(mu=0, var=1)
    return mu+sqrt(var)*randn()
end

```

that implements a Gaussian random variable with mean `mu` and variance `var`. I am not allowed to change it.

To turn the simulator into a set of deterministic simulators, mathematically, I would draw n\_i from a standard normal via `randn()` and then keep it fixed so that the deterministic simulators would be g\_i(\mu, v) = \mu+\sqrt(v) n\_i, for i=1,..., N where N is the total number of deterministic simulators.

Since I am not allowed to change the provided stochastic simulator, I currently use the following wrapper (closure):

```julia
using Random

function make_deterministic(usersim, id)
    function mysim(params)
        Random.seed!(id)
        return usersim(params...)
    end
    return mysim
end

```

where `id` is some identifier (number).

I then use `make_deterministic` to generate multiple deterministic simulators as follows:

```julia
sim1 = make_deterministic(external_simulator, 1);
sim2 = make_deterministic(external_simulator, 2);
# ...
sim100 = make_deterministic(external_simulator, 100);

```

Calling `sim1` for different means  
`[sim1((mu,1)) for mu in 1:10]`  
gives, for example,  
`0.9294168610461021 1.929416861046102 2.929416861046102 3.929416861046102 4.929416861046102 5.929416861046102 6.929416861046102 7.929416861046102 8.929416861046102 9.929416861046102 `  
which displays the desired behavior: the mean increases while the random bit (after the .) stays the same.

**My questions are**

1. Is such an approach safe when used (a) serially on the same computer, (b) as threads on the same computer, (c) in a distributed environment (multiple compute nodes without shared memory)?
2. Are there any restrictions on how I should define the `id` or can it be any `Int`?
3. Are there any best practices on how to choose the `id`?

Many thanks for your help!