Flatten a list: Difference between revisions

(Note that Julia versions prior to 0.5 automatically flattened nested arrays.)

The following version of flatten makes use of the higher order function ''mapreduce''.

<lang julia>isflat(x) = isempty(x) || first(x) === x

function flat_mapreduce(arr)

mapreduce(vcat, arr, init=[]) do x

isflat(x) ? x : flat(x)

end

end</lang>

<lang julia>function flat_recursion(arr)

res = []

function grep(v)

for x in v

if x isa Array

grep(x)

else

push!(res, x)

end

end

res

end</lang>

Using the Iterators library from the Julia base:

<lang julia>function flat_iterators(arr)

while any(a -> a isa Array, arr)

arr = collect(Iterators.flatten(arr))

end

arr

end</lang>

Benchmarking these three functions using the BenchmarkTools package yields the following results:

<lang julia>using BenchmarkTools

@show flat_mapreduce(arr)

@show flat_recursion(arr)

@show flat_iterators(arr)

@btime flat_mapreduce($arr)

@btime flat_recursion($arr)

@btime flat_iterators($arr)</lang>{{out}}

flat_mapreduce(arr) = Any[1, 2, 3, 4, 5, 6, 7, 8]

flat_recursion(arr) = Any[1, 2, 3, 4, 5, 6, 7, 8]

flat_iterators(arr) = [1, 2, 3, 4, 5, 6, 7, 8]

14.163 μs (131 allocations: 4.27 KiB)

500.824 ns (4 allocations: 176 bytes)

28.223 μs (133 allocations: 4.33 KiB)