I am rather new to Julia, so please do remind me if I missed anything
important.
I was trying to write a function which would operate on the elements in an
array, and return an array. For the sake of simplicity, let's say
calculating the squares of an array of real numbers. I designed four
functions, each implementing the task using a different style:
function tf1{T<:Real}(x::AbstractArray{T}) return r = x .^ 2 end #
vectorised power operator
function tf2{T<:Real}(x::AbstractArray{T}) r = Array(T, length(x)); for i
in 1:length(x) r[i] = x[i] ^ 2 end; return r end # plain for loop
function tf3{T<:Real}(x::AbstractArray{T}) return [i ^ 2 for i in x] end #
array comprehension
function tf4{T<:Real}(x::AbstractArray{T}) return map(x -> x ^ 2, x) end #
using the "map" function
And I timed the operations with tic() and toc(). The results varies from
each run, but the following is a typical set of results after warming up:
tic(); tf1( 1:1000000 ); toc()
elapsed time: 0.011582169 seconds
tic(); tf2( 1:1000000 ); toc()
elapsed time: 0.016566094 seconds
tic(); tf3( 1:1000000 ); toc()
elapsed time: 0.004038817 seconds
tic(); tf4( 1:1000000 ); toc()
elapsed time: 0.065989988 seconds
I understand that the map function should run slower than the rest, but why
is the comprehension version so much faster than the vectorised "^"
operator? Does this mean array comprehensions should be used in favour of
all other styles whenever possible?
P.S. version of Julia:
_
_ _ _(_)_ | A fresh approach to technical computing
(_) | (_) (_) | Documentation: http://docs.julialang.org
_ _ _| |_ __ _ | Type "help()" for help.
| | | | | | |/ _` | |
| | |_| | | | (_| | | Version 0.3.9 (2015-05-30 11:24 UTC)
_/ |\__'_|_|_|\__'_| | Official http://julialang.org/ release
|__/ | x86_64-w64-mingw32
This is on a Windows 7 machine.
