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. To make sure we are on the same page, I have added this project to my subversion on google code. Here is the link: http://gil-contributions.googlecode.com/svn/trunk/prime_calculator Regards, Christian