# Random Sampler behavior

**URL:** <https://discourse.julialang.org/t/random-sampler-behavior/69512>\
**Category:** General Usage\
**Tags:** random\
**Created:** [October 10, 2021, 2:00pm UTC](https://discourse.julialang.org/t/random-sampler-behavior/69512 "2021-10-10T14:00:33Z")\
**Posts on this page:** 5\
**Page:** 1

<div class="post-metadata">

**Author:** ![goncharovdk](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/goncharovdk/32/29254_2.png) [@goncharovdk](https://discourse.julialang.org/u/goncharovdk)\
**Post date:** [October 10, 2021, 2:00pm UTC](https://discourse.julialang.org/t/random-sampler-behavior/69512/1 "2021-10-10T14:00:33Z")

</div>

Hello. I’m now rewriting some Monte Carlo simulation code from Java to Julia. It probably was a bad idea to implement it in Java in the first place, but now I use results calculated by Java implementation to verify the correctness of my Julia implementation. Classic Mersenne Twister (MT19937) is used in both variants. And I found a difference in results, let me show it.

Java code:

```nohighlight
public static void main(String[] args) {
    MersenneTwister twister = new MersenneTwister(0);

    for (int i = 1; i <= 5; i++) {
        System.out.println( twister.random());
    }

    for (int i = 1; i <= 10; i++) {
        System.out.print((twister.chooseFrom(10) + 1) + " ");
    }
}

```

Julia code:

```nohighlight
using Random
using RandomNumbers.MersenneTwisters

RNG = MT19937(0)
for i in 1:5 println(rand(RNG)) end

s = Random.Sampler(RNG, 1:10)
for i in 1:10 print(rand(RNG, s), " ") end

```

Java output:  
0.548813502304256  
0.5928446163889021  
0.715189364971593  
0.8442657440900803  
0.6027633703779429  
9 6 9 5 7 7 4 5 3 9

Julia output:  
0.548813502304256  
0.5928446163889021  
0.715189364971593  
0.8442657440900803  
0.6027633703779429  
6 5 7 5 9 10 4 8 6 6

So, as the first 5 lines of outputs prove, both implementations start from the same pseudo-random sequence. But Random.Sampler in Julia provides only even-numbered elements of the sequence:

```julia
9 6 9 5 7 7 4 5 3 9 
  6 5 7 5 9 10 4 8 6 6

```

So it looks like the Sampler makes extra calls to RNG. Is it so? Why this is needed? Does this affect the performance? Is there a way to get rid of that?

Thanks in advance.

---

<div class="post-metadata">

**Author:** ![jling](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jling/32/212909_2.png) [@jling](https://discourse.julialang.org/u/jling)\
**Post date:** [October 10, 2021, 2:14pm UTC](https://discourse.julialang.org/t/random-sampler-behavior/69512/2 "2021-10-10T14:14:03Z")

</div>

this is because Java defaults to Int32:

```julia
julia> RNG = MT19937(0)
MT19937(UInt32[0x00000000, 0x00000001, 0x6c078967, 0x714acb41, 0x48077044, 0x93e7383e, 0xf0f54bb2, 0x141f95dc, 0xc0acdbd4, 0x3cb9cadc … 0xad4200bc, 0xdaa1fb5d, 0x78ff7c7e, 0x71e51784, 0x499b76e3, 0xd522db95, 0x075dea9a, 0x8ccffb2f, 0xb5b4302f, 0x6b1a1930], 625)

julia> for i in 1:5 println(rand(RNG)) end
0.548813502304256
0.5928446163889021
0.715189364971593
0.8442657440900803
0.6027633703779429

julia> for i in 1:10 print(rand(RNG, Int32(1):Int32(10)), " ") end
9 6 9 5 7 7 4 5 3 9 

```

so you want to change to:

```julia
s = Random.Sampler(RNG, Int32(1):Int32(10))

```

---

<div class="post-metadata">

**Author:** ![goncharovdk](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/goncharovdk/32/29254_2.png) [@goncharovdk](https://discourse.julialang.org/u/goncharovdk)\
**Post date:** [October 10, 2021, 2:46pm UTC](https://discourse.julialang.org/t/random-sampler-behavior/69512/3 "2021-10-10T14:46:07Z")

</div>

Thank you very much. Do I understand correctly that in my code the Sampler generates 64 random bits at each call but discards half of the result then? If so, it probably means that for the 1:10 range the type could be restricted even further (Int8). Do such things make any significant performance gain?

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<div class="post-metadata">

**Author:** ![jling](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jling/32/212909_2.png) [@jling](https://discourse.julialang.org/u/jling)\
**Post date:** [October 10, 2021, 2:47pm UTC](https://discourse.julialang.org/t/random-sampler-behavior/69512/4 "2021-10-10T14:47:12Z")

</div>

if you care about performance, don’t use MT to begin with, as the document would tell you: [Mersenne Twisters - RandomNumbers.jl](https://juliarandom.github.io/RandomNumbers.jl/stable/man/mersenne-twisters/)

---

<div class="post-metadata">

**Author:** ![goncharovdk](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/goncharovdk/32/29254_2.png) [@goncharovdk](https://discourse.julialang.org/u/goncharovdk)\
**Post date:** [October 10, 2021, 3:21pm UTC](https://discourse.julialang.org/t/random-sampler-behavior/69512/5 "2021-10-10T15:21:51Z")

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I won’t. I just need it for the debugging time, for the reproducibility of the results. I’ll change it to Xoroshiro after that. Thanks again.