# What is Julia's maxk(Matlab) that returns the indice of top K largest values?

**URL:** <https://discourse.julialang.org/t/what-is-julias-maxk-matlab-that-returns-the-indice-of-top-k-largest-values/14100>\
**Category:** General Usage\
**Tags:** sort, sortperm\
**Created:** [August 26, 2018, 10:48pm UTC](https://discourse.julialang.org/t/what-is-julias-maxk-matlab-that-returns-the-indice-of-top-k-largest-values/14100 "2018-08-26T22:48:09Z")\
**Posts on this page:** 1\
**Showing post:** 11

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**Author:** ![platawiec](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/platawiec/32/31914_2.png) [@platawiec](https://discourse.julialang.org/u/platawiec)\
**Post date:** [August 27, 2018, 1:22pm UTC](https://discourse.julialang.org/t/what-is-julias-maxk-matlab-that-returns-the-indice-of-top-k-largest-values/14100/11 "2018-08-27T13:22:12Z")

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Yes, looking at previous discussions it seems like Matlab’s sort functions are multi-threaded and heavily optimized, so they will be tough to beat. I did a naive threaded implementation here, which starts to have some gains over single-threaded for large `a`:

```julia
julia> using Base.Threads

julia> nthreads()
2

julia> function maxk_threaded(a, k)
           ix = Vector{Int}(undef, k*nthreads())
           block_size = ceil(Int, length(a)/nthreads())
           @threads for thread_id in 1:nthreads()
               ix_start = (thread_id-1)*block_size
               ix_end = min(length(a), thread_id*block_size)
               ix[((thread_id-1)*k+1):(thread_id*k)] = ix_start .+ partialsortperm(@view(a[(1+ix_start):ix_end]), 1:k, rev=true)
           end
           partialsortperm!(ix, a, 1:k, rev=true, initialized=true)
           @views collect(zip(ix[1:k], a[ix[1:k]]))
       end
maxk_threaded (generic function with 1 method)

julia> a = randn(10000);

julia> @btime maxk($a, 10)
  70.201 μs (9 allocations: 78.73 KiB)
10-element Array{Tuple{Int64,Float64},1}:
 (3840, 3.7524106800393)
 (1359, 3.667162944745407)
 (4738, 3.46128657912246)
 (8532, 3.349067643815953)
 (8314, 3.3363898988561234)
 (3542, 3.3297030468239965)
 (1159, 3.2795246783768923)
 (9436, 3.259918244413647)
 (9418, 3.254388944717796)
 (2198, 3.155524296051535)

julia> @btime maxk_threaded($a, 10)
  53.894 μs (23 allocations: 1.58 KiB)
10-element Array{Tuple{Int64,Float64},1}:
 (3840, 3.7524106800393)
 (1359, 3.667162944745407)
 (4738, 3.46128657912246)
 (8532, 3.349067643815953)
 (8314, 3.3363898988561234)
 (3542, 3.3297030468239965)
 (1159, 3.2795246783768923)
 (9436, 3.259918244413647)
 (9418, 3.254388944717796)
 (2198, 3.155524296051535)

```

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