Re: [Boost-users] [mapreduce] Prim Calculator

Subject: Re: [Boost-users] [mapreduce] Prim Calculator
From: Craig Henderson (cdm.henderson_at_[hidden])
Date: 2009-08-22 13:11:57

> -----Original Message-----
> From: boost-users-bounces_at_[hidden] [mailto:boost-users-
> bounces_at_[hidden]] On Behalf Of Christian Henning
> Sent: 22 August 2009 17:19
> To: boost-users_at_[hidden]
> Subject: Re: [Boost-users] [mapreduce] Prim Calculator
> 
> Hi Craig,
> 
> I also have rewritten the prime_calculator. I never like that the fact
> that the reduce key type was std::size_t. It should be bool. is_prime
> is now returning a boolean. I also have changed map value type from
> std::pair<std::size_t,std::size_t> to just std::size_t. I hope this is
> correct in terms of the mapreduce methodologies.
> 
> To adopt the mapreduce problem description notation this is what I
> want:
> 
> map: ( number, number ) -----> list( boolean, number )
> reduce: ( boolean, list( number ) ---------> list( number )
> 
> Well, it all compiles and runs but the results is empty. I have
> intercepted the reduce function and the supplied list is correct.
> Meaning all primes are in there. Weird. Dunno what's wrong here.

Did you try defining the job to write the results to file? This will work
until I fix the iterator issue.

A small optimization in the is_prime is to check for %2 - this avoids
expensive sqrt & loops for even numbers that are always not prime

bool const is_prime(long const number)
{
    if (number == 0 || number == 1)
        return false;
    else if (number == 2)
        return true;
    else if (number%2 == 0)
        return false;
    ...
}

Regards
-- Craig