code to run combination of strings in parallel

Question

public static IEnumerable<IEnumerable<T>> Combinations<T>(this IEnumerable<T> elements, int k)
{
    return k == 0 
        ? new[] { new T[0] } 
        : elements.SelectMany((e, i) => elements
              .Skip(i + 1).Combinations(k - 1)
              .Select(c => (new[] { e }).Concat<T>(c)));
    }

The code currently runs combination of strings in series e.g list of a,b,c in two ways would give ab,ac,bc which is very correct. Is there a way I can modify the code to run the combination in parallel (i.e to max my cpu processor) because I intend to run combination on large volume of strings.

use `elements.AsParallel().SelectMany...` – Tim Schmelter Jul 12 '17 at 13:25 — Tim Schmelter, Jul 12 '17 at 13:25

mjwills · Accepted Answer · 2017-07-12T23:34:00.767

0

This should introduce some parallelism. Note in particular the AsParallel.

public static IEnumerable<IEnumerable<T>> Combinations<T>(this IEnumerable<T> elements, int k)
{
    return k == 0 ? (IEnumerable<IEnumerable<T>>)new[] { new T[0] } : elements.AsParallel().SelectMany((e, i) => elements.Skip(i + 1).Combinations(k - 1).Select(c => (new[] { e }).Concat<T>(c)));
}

You could also consider using Enumerable.Empty rather than new T[0].

edited Jul 12 '17 at 23:34

answered Jul 12 '17 at 13:25

mjwills

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the method would not compile by trying to introduce parallelism that way – Osamuyimen Osagiede Dennis Jul 12 '17 at 14:55
There is no implicit conversion between 'T[][]' and System.Linq.ParallelQuery I still had this same error after replacing new T[0] with Enumerable.Empty – Osamuyimen Osagiede Dennis Jul 12 '17 at 23:06
It would be excellent if you could provide timings for this method now. – Enigmativity Jul 12 '17 at 23:38
please Enigmativity can u explain further, i do not understand what you mean – Osamuyimen Osagiede Dennis Jul 12 '17 at 23:44
He is suggesting you actually compare the speeds of your original code vs the "parallelized" version, and ensure that the new code is actually faster than the old code. – mjwills Jul 13 '17 at 04:12
I have not been able to get the parallelized version to work, i have still not able to compile it yet – Osamuyimen Osagiede Dennis Jul 13 '17 at 07:50
my bad, i did not rebuild the solution that why i got the compilation errors, introducing the As Parallel in the code did not introduce any form of parallelism. If one could help me with a code that can do combination of string in parallel i would be very grateful. – Osamuyimen Osagiede Dennis Jul 13 '17 at 15:38
What make you think it `did not introduce any form of parallelism` @OsamuyimenOsagiedeDennis ? – mjwills Jul 14 '17 at 03:31
my cpu monitor dd not spike up like it normally does anytime i am running a parallel code, and when i set timing between series and the edited parallel method, i observed the two timings were almost the same thing. – Osamuyimen Osagiede Dennis Jul 14 '17 at 09:17
`Parallel` doesn't always mean faster @OsamuyimenOsagiedeDennis . – mjwills Jul 14 '17 at 09:25

code to run combination of strings in parallel

1 Answers1