On Thu, 12 Aug 2004, Remi Turk wrote:
On Thu, Aug 12, 2004 at 09:01:03PM +0200, Henning Thielemann wrote:
On Thu, 12 Aug 2004, Remi Turk wrote:
I usually (each time I urgently need to calculate primes ;)) use a simple intSqrt = floor . sqrt . fromIntegral (which will indeed give wrong answers for big numbers)
If I urgently need factors of an integer I check "factor*factor > n" rather than "factor > intSqrt n". :-]
but you'll have to (^2) once every "iteration". The following code only has to sqrt once.
Right. Hm, but if you get a 'div' for free for every 'mod' (e.g. divMod) you could also check "factor > n `div` factor"
Oh, the joy of premature optimization. Or even premature coding ;)
-- Will lie when given stupid questions isPrime 1 = False isPrime 2 = True isPrime n = not $ any (n`isDivBy`) (2:[3,5..intSqrt n]) where n `isDivBy` d = n `rem` d == 0 intSqrt = floor . sqrt . fromIntegral
If the numbers become too big, will 'fromIntegral' round down or up? If it rounds down, then intSqrt might lead to a too small result.