# 99 questions/Solutions/39

### From HaskellWiki

Line 9: | Line 9: | ||

</haskell> | </haskell> | ||

− | 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 the primes | + | 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 <code>isPrime</code> tests by ''trial division'' using (up to<math>\sqrt b</math>) a memoized primes list produced by sieve of Eratosthenes to which it refers internally. So it'll be slower, but immediate. | ||

'''Solution 2:''' | '''Solution 2:''' | ||

Line 21: | Line 23: | ||

</haskell> | </haskell> | ||

− | Another way to compute the claimed list is done by using the ''Sieve of Eratosthenes''. The <code>primes</code> function generates a list of all (!) prime numbers using this algorithm and <code>primesR</code> 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 :)] | + | Another way to compute the claimed list is done by using the ''Sieve of Eratosthenes''. The <code>primes</code> function generates a list of all (!) prime numbers using this algorithm and <code>primesR</code> 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:''' | '''Solution 3:''' | ||

Line 57: | Line 61: | ||

union a b = a ++ b | union a b = a ++ b | ||

</haskell> | </haskell> | ||

− | ''(This turned out to be quite a project, with | + | ''(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, |

<haskell> | <haskell> | ||

> primesR 10100 10200 -- Sol.3 | > primesR 10100 10200 -- Sol.3 | ||

Line 79: | Line 83: | ||

(testing with Hugs of Nov 2002). | (testing with Hugs of Nov 2002). | ||

− | This is | + | 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 <math>a < \sqrt{b}</math>, we're better off just using the primes sequence itself, without any post-processing: | ||

<haskell> | <haskell> | ||

− | primes = primesTME | + | primes :: Integral a => [a] |

− | + | primes = primesTME -- of Q.31 | |

+ | |||

+ | primesR :: Integral a => a -> a -> [a] | ||

+ | primesR a b = takeWhile (<= b) . dropWhile (< a) $ primes | ||

</haskell> | </haskell> |

## Revision as of 10:59, 2 June 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 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) 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 = if a' <= q then q else snapUp a' q step ]) where primes' = tail primesTME -- external unbounded list of primes 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] snapUp v origin step = let r = rem (v-origin) step in if r==0 then v else v-r+step -- 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] (5,428 reductions, 11,310 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 , 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