std.parallelism and map questions

[Ali Çehreli]

On 09/15/2012 08:12 PM, freeman wrote:
 > std.parallelism uses the foreach loop for parallel(). As such,
 > how might I implement a parallel while-loop?

That doesn't sound right to me because the while loop condition is 
normally a result of the operations inside the loop itself. And that's a 
race condition, violating a fundamental rule of parallelism that the 
operations are indepentent (having no side-effects).

Perhaps I am wrong; then I would like to learn about the answer.

 > Can I use an
 > ordinary for-loop with parallel()?

I think this works:

import std.stdio;
import std.parallelism;
import core.thread;

void foo(size_t i)
{
     writefln("Working on %s", i);
     Thread.sleep(dur!("msecs")(500));
}

void main()
{
     auto workers = new TaskPool(2);

     for (size_t i = 0; i != 10; ++i) {
         if (i != 7) {
             auto t = task!foo(i);
             workers.put(t);
         }
     }

     workers.finish();
}

 > Also, how can I map array elements with a conditional like if()?

You can pass the return range of std.algorithm.filter to map:

import std.stdio;
import std.parallelism;
import std.range;
import std.algorithm;

int tenTimes(int i)
{
     return i * 10;
}

void main()
{
     auto result = taskPool.map!tenTimes(iota(10).filter!(a => a % 2)(), 5);
     writeln(result);
}

Ali