# Customize a random function to sample 3 out of a list of 4097 real numbers

**URL:** <https://discourse.julialang.org/t/customize-a-random-function-to-sample-3-out-of-a-list-of-4097-real-numbers/100876>\
**Category:** Performance\
**Tags:** question\
**Created:** [June 27, 2023, 7:12am UTC](https://discourse.julialang.org/t/customize-a-random-function-to-sample-3-out-of-a-list-of-4097-real-numbers/100876 "2023-06-27T07:12:47Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![Lian\_Yunlong](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lian_yunlong/32/14437_2.png) [@Lian\_Yunlong](https://discourse.julialang.org/u/Lian_Yunlong)\
**Post date:** [June 27, 2023, 7:12am UTC](https://discourse.julialang.org/t/customize-a-random-function-to-sample-3-out-of-a-list-of-4097-real-numbers/100876/1 "2023-06-27T07:12:47Z")

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I need a very fast random sampler to choose `n` numbers from a precomputed 1D array, say `X`. We can assume that `n` is much smaller than `length(X)`. I have tried `rand(X,n)`, `[rand(X) for i=1:n]` , but neither gives satisfactory performance, even if `n` is as small as 3. The profiling shows that the bottleneck is in Random/generation.jl line 421

```julia
rand(rng::AbstractRNG, sp::SamplerSimple{<:AbstractArray,<:Sampler}) =
    @inbounds return sp[][rand(rng, sp.data)]

```

then eventually line 342

```julia
function rand(rng::AbstractRNG, sp::SamplerRangeNDL{U,T}) where {U,T}
    s = sp.s
    x = widen(rand(rng, U))
    m = x * s
    l = m % U
    if l < s
        t = mod(-s, s) # as s is unsigned, -s is equal to 2^L - s in the paper
        while l < t
            x = widen(rand(rng, U))
            m = x * s
            l = m % U
        end
    end
    (s == 0 ? x : m >> (8*sizeof(U))) % T + sp.a
end

```

I wish to customize the sampler down to a very particular case of sampling three `Float64` out of a `Vector{Float64}`, which may look like

```julia
function my_rand_3(rng::AbstractRNG, X::Vector{Float64})::Vec3
    ...
    ...
    return Vec3(...)
end

```

Can anyone give me a hint on how to do that?

---

<div class="post-metadata">

**Author:** ![Lian\_Yunlong](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lian_yunlong/32/14437_2.png) [@Lian\_Yunlong](https://discourse.julialang.org/u/Lian_Yunlong)\
**Post date:** [June 27, 2023, 7:15am UTC](https://discourse.julialang.org/t/customize-a-random-function-to-sample-3-out-of-a-list-of-4097-real-numbers/100876/2 "2023-06-27T07:15:29Z")

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I am doing a simulator for vibrating atoms in a solid, this sampler is very useful and very frequently called.

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**Author:** ![fatteneder](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/fatteneder/32/33991_2.png) [@fatteneder](https://discourse.julialang.org/u/fatteneder)\
**Post date:** [June 27, 2023, 7:38am UTC](https://discourse.julialang.org/t/customize-a-random-function-to-sample-3-out-of-a-list-of-4097-real-numbers/100876/3 "2023-06-27T07:38:04Z")

</div>

IDK what your current benchmarks say, but here is an alternative version that just populates a preallocated array:

```julia
using Random
using BenchmarkTools

x = rand(Float64, 4079)
xx = zeros(Float64, 3)

@benchmark rand!($xx, $x)

```

On my laptop this gives

```julia
julia> @benchmark rand!($xx, $x)
BenchmarkTools.Trial: 10000 samples with 998 evaluations.
 Range (min … max): 17.124 ns … 46.397 ns ┊ GC (min … max): 0.00% … 0.00%
 Time (median): 17.813 ns ┊ GC (median): 0.00%
 Time (mean ± σ): 17.937 ns ± 0.669 ns ┊ GC (mean ± σ): 0.00% ± 0.00%

          ▂▄▆█▆▃▁
  ▂▂▂▂▃▄▆▇████████▇▆▅▅▅▅▄▄▄▃▃▃▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▂▁▂▂▂▂▂ ▃
  17.1 ns Histogram: frequency by time 20.4 ns <

 Memory estimate: 0 bytes, allocs estimate: 0.

```

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<div class="post-metadata">

**Author:** ![Dan](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/dan/32/42581_2.png) [@Dan](https://discourse.julialang.org/u/Dan)\
**Post date:** [June 27, 2023, 8:36am UTC](https://discourse.julialang.org/t/customize-a-random-function-to-sample-3-out-of-a-list-of-4097-real-numbers/100876/4 "2023-06-27T08:36:17Z")

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Is the sampling with or without replacement?  
Also, the subject-line specifies sampling 3 out of 4097, these are specific numbers and a method can be optimized for them, or is the method required more general?

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<div class="post-metadata">

**Author:** ![Lian\_Yunlong](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lian_yunlong/32/14437_2.png) [@Lian\_Yunlong](https://discourse.julialang.org/u/Lian_Yunlong)\
**Post date:** [July 7, 2023, 6:38am UTC](https://discourse.julialang.org/t/customize-a-random-function-to-sample-3-out-of-a-list-of-4097-real-numbers/100876/5 "2023-07-07T06:38:27Z")

</div>

> [@fatteneder](#):
>
> `rand!`

What a cute exclamation mark! Thank you so much!

---

<div class="post-metadata">

**Author:** ![Lian\_Yunlong](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/lian_yunlong/32/14437_2.png) [@Lian\_Yunlong](https://discourse.julialang.org/u/Lian_Yunlong)\
**Post date:** [July 7, 2023, 6:42am UTC](https://discourse.julialang.org/t/customize-a-random-function-to-sample-3-out-of-a-list-of-4097-real-numbers/100876/6 "2023-07-07T06:42:28Z")

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The number 3 comes from the demand of generating random three-dimensional vector. The number 4079 is the size of the pre-computed amplitudes, which satisfy some distribution. A more refined pre-computation would yield more numbers.

I think the sampling is just to take and consume numbers, without modification of the pool.