[dmd-concurrency] How to use parallel foreach to process a part of array?

[Ilya Sazonov]

Sorry, i made the code as short as I could to ask the question and it looks
broken now :(.
It should have been something like:

import std.parallelism;

void main() {
    int[] data = new int[1000_000_00];
    int[] new_data = new int[1000_000_00];

    foreach(i, ref el; parallel(new_data)) {
        if (i > 10 && i < 1000_000_00 - 10)
            el = data[i-10] + data[i+10];
    }
}

I read from one array and write results to another.


Thank you for your answer, it solved my issue, I didn't know about slices.
Am I right, than I'll have to add an offset to index, because foreach
always starts with zero?
Like that?
    foreach(i, ref el; parallel(new_data[10..$-10)) {
         el = data[i + 10 -10] + data[i + 10 +10];
    }

Or there is some way to start foreach with specified index?



On Wed, Oct 30, 2013 at 12:56 AM, Steven Schveighoffer
<schveiguy at yahoo.com>wrote:

> Right, I didn't check over the full code.
>
> The code is broken with or without my change, but with my change it's
> shorter broken code :)
>
> -Steve
>
> On Oct 29, 2013, at 4:33 PM, Brad Roberts <braddr at puremagic.com> wrote:
>
> > That will eliminate the if, but over looks a larger issue.. parallel
> assumes that there's no interdependencies and that the order of execution
> is irrelevant.  That's very much not the case in the code below.  Each
> value in that code depends on other elements.  Unless you notice that all
> values are 0 and can thus just be replaced with the foreach entirely
> removed.
> >
> > On 10/29/13 6:41 AM, Steven Schveighoffer wrote:
> >> Slice it.
> >>
> >> foreach(i, ref el; parallel(data[10..$-10]))
> >>
> >> -Steve
> >>
> >> On Oct 29, 2013, at 3:49 AM, Ilya Sazonov <poxvuibr at gmail.com> wrote:
> >>
> >>> Suppose I have the following piece of code
> >>>
> >>> import std.parallelism;
> >>>
> >>> void main() {
> >>>     int[] data = new int[1000_000_00];
> >>>
> >>>     foreach(i, ref el; parallel(data)) {
> >>>         if (i > 10 && i < 1000_000_00 - 10)
> >>>             data[i] = data[i-10] + data[i+10];
> >>>     }
> >>> }
> >>>
> >>> That's right, I want to process all the elements of the array, except
> for first 10 and last 10.
> >>> Is there a way to do that without if statement?
> >>>
> >>> I mean something like foreach starting from element with index 10 and
> ending with lendth - 10
> >>>
> >
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