Here are some code snippets, which apply the knowledge from the performance
tip section
<http://julia.readthedocs.org/en/latest/manual/performance-tips/>
https://gist.github.com/SimonDanisch/a36dcc824034c43b5d78#file-selectionsort-jl
Spoiler: the fastest version is 15 times faster.
The one with boundchecks is 8-10 times faster, which is on eye level with
java.
That's not a big surprise, since Julia and Java are often equally fast, but
you would need to use the unsafe collection to get rid of boundchecks in
Java, from what I know.
Am Samstag, 29. August 2015 18:20:05 UTC+2 schrieb Ravi Mohan:
>
> So I wrote an inplace selection sort in Julia, to compare its performance
> against a Java counterpart,
> Here is the code
>
> function selectionSort!{T}(v::AbstractVector{T}, f)
>
> n = length(v)
>
> for i=1:n-1
> min = i
> for j=i+1:n
> if f(v[j], v[min])
> min = j
> end
> end
>
> tmp = v[i]
> v[i] = v[min]
> v[min] = tmp
> end
>
> end # selection Sort
>
> The equivalent java code is
>
> public static void sort(Comparable[] a) {
> int N = a.length;
> for (int i = 0; i < N; i++) {
> int min = i;
> for (int j = i + 1; j < N; j++) {
> if (a[j].compareTo(a[min]) < 0) {
> min = j;
> }
> }
> Comparable temp = a[i];
> a[i] = a[min];
> a[j] = temp;
> }
> }
>
> I have the following questions
> (1) the function f passed in to selectionSort should have the type f:
> (T,T) => Boolean, ie it takes two elements of the Vector and tells whether
> they are in correct order or not. This could be < for example. How do I
> state this in Julia iow, how do I specify the function signature?
>
> (2) Are there any horrendous inefficiencies in the Julia code which are
> apparent to experienced Julians? When I benchmark against the equivalent
> Java I'm getting a factor of 10+ slowdown for larger arrays (though both
> seem O(n^2) as it should be)
>
> size(of array, uniform distribution of doubles between 0 and 1),time taken
> to sort in Java (in milliseconds), time taken to sort in Julia. both times
> are an average of 5 runs for each array size
>
> 1000,0.003,0.010
> 2000,0.003,0.041
> 4000,0.011,0.165
> 8000,0.044,0.664
> 16000,0.206,2.660
>
>
> I readily admit I have very little idea of what it takes to write
> performant code in Julia. Any help greatly appreciated.
>
> Of course, I might be doing something really stupid in the first place,
> which happens all to frequently sadly enough. Please point out any mistakes
> in my code.
>
> Thanks in advance,
> Ravi
>
