# 99 questions/Solutions/39

(Difference between revisions)

(*) 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 numbers from a up to b and filter all the primes through.

This is good for very narrow ranges as Q.31's isPrime tests by trial division using (up to$\sqrt b$) a memoized primes list produced by sieve of Eratosthenes to which it refers internally. So it'll be slower, but immediate.

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) $dropWhile (< a) primes Another way to compute the claimed list is done by using the Sieve of Eratosthenes. The primes function generates a list of all (!) prime numbers using this algorithm and primesR filter the relevant range out. [But this way is very slow and I only presented it because I wanted to show how nicely the Sieve of Eratosthenes can be implemented in Haskell :)] this is of course a famous case of executable specification, with all the implied pitfalls of inefficiency when (ab)used as if it were an actual code. Solution 3: Use the proper Sieve of Eratosthenes from e.g. 31st question's solution (instead of the above sieve of Turner), adjusted to start its multiples production from the given start point: {-# OPTIONS_GHC -O2 -fno-cse #-} -- tree-merging Eratosthenes sieve, primesTME of haskellwiki/prime_numbers, -- adjusted to produce primes in a given range (inclusive) primesR a b | b<a || b<2 = [] | otherwise = (if a <= 2 then [2] else []) ++ gaps a' (join [[x,x+step..b] | p <- takeWhile (<= z) primes' , let q = p*p ; step = 2*p x = snapUp (max a' q) q step ]) where primes' = tail primesTME -- external unbounded list of primes a' = snapUp (max 3 a) 1 2 z = floor$ sqrt $fromIntegral b + 1 join (xs:t) = union xs (join (pairs t)) join [] = [] pairs (xs:ys:t) = (union xs ys) : pairs t pairs t = t gaps k xs@(x:t) | k==x = gaps (k+2) t | True = k : gaps (k+2) xs gaps k [] = [k,k+2..b] snapUp v origin step = let r = rem (v-origin) step -- rem OK if v>=origin in if r==0 then v else v+(step-r) -- duplicates-removing union of two ordered increasing lists union (x:xs) (y:ys) = case (compare x y) of LT -> x : union xs (y:ys) EQ -> x : union xs ys GT -> y : union (x:xs) ys union a b = a ++ b (This turned out to be quite a project, with some quite subtle points.) It should be much better then taking a slice of a full sequential list of primes, as it won't try to generate any primes between the square root of b and a. To wit, > primesR 10100 10200 -- Sol.3 [10103,10111,10133,10139,10141,10151,10159,10163,10169,10177,10181,10193] (5,497 reductions, 11,382 cells) > takeWhile (<= 10200)$ dropWhile (< 10100) $primesTME -- TME of Q.31 [10103,10111,10133,10139,10141,10151,10159,10163,10169,10177,10181,10193] (140,313 reductions, 381,058 cells) > takeWhile (<= 10200)$ dropWhile (< 10100) $sieve [2..] -- Sol.2 where sieve (n:ns) = n:sieve [ m | m <- ns, m mod n /= 0 ] [10103,10111,10133,10139,10141,10151,10159,10163,10169,10177,10181,10193] (54,893,566 reductions, 79,935,263 cells, 6 garbage collections) > filter isPrime [10100..10200] -- Sol.1 [10103,10111,10133,10139,10141,10151,10159,10163,10169,10177,10181,10193] (15,750 reductions, 29,292 cells) -- isPrime: Q.31 (testing with Hugs of Nov 2002). This solution is faster but not immediate. It has a certain preprocessing stage but then goes on fast to produce the whole range. To illustrate, to produce the 49 primes in 1000-wide range above 120200300100 it takes about 18 seconds on my oldish notebook for the 1st version, with the first number produced almost immediately (~ 0.4 sec); but this version spews up all 49 primes in one go after just under 1 sec. Solution 4. For very wide ranges, specifically when $a < \sqrt{b}$, we're better off just using the primes sequence itself, without any post-processing: primes :: Integral a => [a] primes = primesTME -- of Q.31 primesR :: Integral a => a -> a -> [a] primesR a b = takeWhile (<= b) . dropWhile (< a)$ primes