# The quasi-best MaxN function

**URL:** <https://discourse.julialang.org/t/the-quasi-best-maxn-function/52813>\
**Category:** Performance\
**Created:** [January 4, 2021, 9:32am UTC](https://discourse.julialang.org/t/the-quasi-best-maxn-function/52813 "2021-01-04T09:32:15Z")\
**Posts on this page:** 1\
**Showing post:** 3

<div class="post-metadata">

**Author:** ![rocco\_sprmnt21](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rocco_sprmnt21/32/20127_2.png) [@rocco\_sprmnt21](https://discourse.julialang.org/u/rocco_sprmnt21)\
**Post date:** [January 5, 2021, 9:48am UTC](https://discourse.julialang.org/t/the-quasi-best-maxn-function/52813/3 "2021-01-05T09:48:55Z")

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Thanks. Based on your indication I found [this](https://discourse.julialang.org/t/sorting-algorithms-partial-quicksort-usage/1007) topic which seems to refer to the library function in question.

I tried to use it in order to obtain the first 5 largest values of a list and I made comparisons between different functions including one derived from that of the base library.

```julia
function MaxN4(cr,N)
    maxn=Int[]
    r=copy(cr)
    for x in 1:N
        p=popfirst!(r)
        for (i,e) in enumerate(r)
            if p < e
                r[i],p = p,e
            end
        end
        push!(maxn,p)
    end
    maxn
end

maxN2(s,N)= sort(s,rev=true)[1:N]

MaxNPS(r,N) = sort(r, alg=PartialQuickSort(N),rev=true)[1:N]

```

Below are the results obtained for the following two vectors

```julia
re7=rand(1:10^9, 10^7)

re6=rand(1:10^8, 10^6)

```

per re6:

```julia
using BenchmarkTools

@btime sort(re6,rev=true)[1:5]
# 67.170 ms (3 allocations: 7.63 MiB)
# 5-element Array{Int64,1}:
# 99999902
# 99999827
# 99999775
# 99999613
# 99999409
@btime MaxN4(re6,5)
# 5.630 ms (5 allocations: 7.63 MiB)
# 5-element Array{Int64,1}:
# 99999902
# 99999827
# 99999775
# 99999613
# 99999409

@btime MaxNPS(re6,5)
# 10.661 ms (4 allocations: 7.63 MiB)
# 5-element Array{Int64,1}:
# 99999902
# 99999827
# 99999775
# 99999613
# 99999409

```

per re7:

```julia

@btime sort(re7,rev=true)[1:5]
# 778.337 ms (3 allocations: 76.29 MiB)
# 5-element Array{Int64,1}:
# 999999837
# 999999762
# 999999473
# 999999470
# 999999383

@btime MaxN4(re7,5)
# 62.091 ms (5 allocations: 76.29 MiB)
# 5-element Array{Int64,1}:
# 999999837
# 999999762
# 999999473
# 999999470
# 999999383

@btime MaxNPS(re7,5)
# 79.725 ms (4 allocations: 76.29 MiB)
# 5-element Array{Int64,1}:
# 999999837
# 999999762
# 999999473
# 999999470
# 999999383

```

Given the results I have the doubt of not having done something properly either in the measurement or in the definition of the function that uses the library partialquicksort.

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