# Speed of sampling with replacement

**URL:** <https://discourse.julialang.org/t/speed-of-sampling-with-replacement/5349>\
**Category:** General Usage\
**Tags:** question\
**Created:** [August 12, 2017, 9:19pm UTC](https://discourse.julialang.org/t/speed-of-sampling-with-replacement/5349 "2017-08-12T21:19:18Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![bkamins](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/bkamins/32/208538_2.png) [@bkamins](https://discourse.julialang.org/u/bkamins)\
**Post date:** [August 12, 2017, 9:19pm UTC](https://discourse.julialang.org/t/speed-of-sampling-with-replacement/5349/1 "2017-08-12T21:19:19Z")

</div>

Currently sampling with replacement in Julia seems to be significantly slower than in R (tested on Julia 0.6.0, 64bit). See e.g.:

```julia
> x <- runif(10^7)
> system.time(sample(x, replace=T))
   user system elapsed 
   0.38 0.02 0.39 

```

vs.

```julia
julia> x = rand(10^7);

julia> @benchmark rand($x, 10^7)
BenchmarkTools.Trial:
  memory estimate: 76.29 MiB
  allocs estimate: 3
  --------------
  minimum time: 648.989 ms (0.03% GC)
  median time: 663.557 ms (1.52% GC)
  mean time: 675.775 ms (3.53% GC)
  maximum time: 724.599 ms (9.70% GC)
  --------------
  samples: 8
  evals/sample: 1

```

Sampling with replacement is a common operation so I believe it would be nice to speed it up.

I do not have a full solution, but a partial one could be to make function `rand!(rng::AbstractRNG, A::AbstractArray, r::AbstractArray)` check if `length(r)` is small (so that it fits into 32 bits) and then create `RangeGenerator` with a narrower domain. My preliminary tests show that this gives ~15% reduction of execution time.

Another approach could be:

```julia
julia> @benchmark $x[rand(1:10^7,10^7)]
BenchmarkTools.Trial:
  memory estimate: 152.59 MiB
  allocs estimate: 5
  --------------
  minimum time: 404.157 ms (2.95% GC)
  median time: 409.564 ms (3.99% GC)
  mean time: 420.688 ms (6.34% GC)
  maximum time: 479.673 ms (18.14% GC)
  --------------
  samples: 12
  evals/sample: 1

```

and

```julia
julia> @benchmark $x[rand(Int32(1):Int32(10^7),10^7)]
BenchmarkTools.Trial:
  memory estimate: 114.44 MiB
  allocs estimate: 5
  --------------
  minimum time: 329.700 ms (3.44% GC)
  median time: 338.070 ms (3.81% GC)
  mean time: 345.072 ms (6.22% GC)
  maximum time: 404.187 ms (20.81% GC)
  --------------
  samples: 15
  evals/sample: 1

```

Unfortunately they are more memory hungry (the last one is faster than R), but the question is why `rand(x, 10^7)` is so much slower?