Difference between revisions of "99 questions/Solutions/39"
(adding solution 3 based on primesTME from Q.31) |
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| otherwise = |
| otherwise = |
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(if a <= 2 then [2] else []) ++ |
(if a <= 2 then [2] else []) ++ |
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− | gaps a' (join [[x,x+step..b] | p <- takeWhile (<= z) (tail |
+ | gaps a' (join [[x,x+step..b] | p <- takeWhile (<= z) (tail primesTME) |
, let q = p*p ; step = 2*p |
, let q = p*p ; step = 2*p |
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x = if a' <= q then q else |
x = if a' <= q then q else |
Revision as of 19:38, 31 May 2011
(*) A list of prime numbers.
Given a range of integers by its lower and upper limit, construct a list of all prime numbers in that range.
Solution 1:
primesR :: Integral a => a -> a -> [a]
primesR a b = filter isPrime [a..b]
If we are challenged to give all primes in the range between a and b we simply take all number from a up to b and filter the primes out.
Solution 2:
primes :: Integral a => [a]
primes = let sieve (n:ns) = n:sieve [ m | m <- ns, m `mod` n /= 0 ]
in sieve [2..]
primesR :: Integral a => a -> a -> [a]
primesR a b = takeWhile (<= b) $ dropWhile (< a) primes
Another way to compute the claimed list is done by using the Sieve of Eratosthenes. The primes
function generates a list of all (!) prime numbers using this algorithm and primesR
filter the relevant range out. [But this way is very slow and I only presented it because I wanted to show how nicely the Sieve of Eratosthenes can be implemented in Haskell :)]
Solution 3:
Use the proper Sieve of Eratosthenes from e.g. 31st question's solution (instead of the above sieve of Turner), adjusted to start its multiples production from the given start point:
{-# OPTIONS_GHC -O2 -fno-cse #-}
-- tree-merging Eratosthenes sieve, primesTME of haskellwiki/prime_numbers,
-- adjusted to produce primes in a given range
primesR a b
| b<a || b<2 = []
| otherwise =
(if a <= 2 then [2] else []) ++
gaps a' (join [[x,x+step..b] | p <- takeWhile (<= z) (tail primesTME)
, let q = p*p ; step = 2*p
x = if a' <= q then q else
let r = rem (a'-q) step
in if r==0 then a' else a'-r+step ])
where
a' = if a<=3 then 3 else (if even a then a+1 else a)
z = floor $ sqrt $ fromIntegral b + 1
join (xs:t) = union xs (join (pairs t))
join [] = []
pairs (xs:ys:t) = (union xs ys) : pairs t
pairs t = t
gaps k xs@(x:t) | k==x = gaps (k+2) t
| True = k : gaps (k+2) xs
gaps k [] = [k,k+2..b]
-- duplicates-removing union of two ordered increasing lists
union (x:xs) (y:ys) = case (compare x y) of
LT -> x : union xs (y:ys)
EQ -> x : union xs ys
GT -> y : union (x:xs) ys
union a b = a ++ b
(This turned out to be quite a project, with a few quite subtle points.) It should be much better then taking a slice of a full sequential list of primes, as it won't try to generate any primes between the square root of b and a. To wit,
> primesR 10100 10200
[10103,10111,10133,10139,10141,10151,10159,10163,10169,10177,10181,10193]
(6038 reductions, 11986 cells)
> takeWhile (<= 10200) $ dropWhile (< 10100) $ primesTME
[10103,10111,10133,10139,10141,10151,10159,10163,10169,10177,10181,10193]
(140313 reductions, 381058 cells)