[julia-users] Help: how to improve the function performance

[Weicheng Zhu]

Hi there,
I write a function to calculate the distance for each row of a two 
dimensional array and I compared it with the `pairwise` function in the 
Distance module.
Does anyone can help me to find out the reason why my function is slower 
than the pairwise function? I only keep the triangle elements of the 
distance matrix which I thought should be faster. Thanks in advance for any 
help:)

Here is the code:

Module Tmp

import DataFrames: DataFrame

function dist(x::Matrix)

    x = x'

    n = size(x, 2)

    ij::UInt = 0

    d = zeros(convert(Int, (n-1)*n/2))

    for i in 1:n

        for j in (i+1):n

            ij += 1

            d[ij] = norm(x[:,i] - x[:,j])

        end

    end

    return d

end


function dist(x::DataFrame)

    dist(convert(Array, dat))

end

export dist

end


using Tmp

using Distances

x = rand(100,2)

@time dist(x)

# 0.001581 seconds (29.71 k allocations: 1.399 MB)

@time pairwise(Euclidean(), x')

# 0.000318 seconds (310 allocations: 91.984 KB)