# 99 questions/Solutions/31

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< 99 questions | Solutions(Difference between revisions)

(putting in a better alternative for allPrimes) |
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isPrime :: Integral a => a -> Bool | isPrime :: Integral a => a -> Bool | ||

isPrime p = p > 1 && | isPrime p = p > 1 && | ||

− | (all ( | + | (all ((/= 0).(p `rem`)) $ candidateFactors p) |

− | + | ||

+ | candidateFactors p = takeWhile ((<= p).(^2)) [2..] | ||

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

− | Well, a natural number ''p'' is a prime number | + | Well, a natural number ''p'' is a prime number if it is larger than '''1''' and no natural number ''n >= 2'' with ''n^2 <= p'' is a divisor of ''p''. That's exactly what is implemented: we take the list of all integral numbers starting with '''2''' as long as their square is at most ''p'' and check that for all these ''n'' there is a non-zero remainder concerning the division of ''p'' by ''n''. |

− | However, we don't actually need to check all natural numbers <= sqrt P. We need only check the | + | However, we don't actually need to check all natural numbers ''<= sqrt P''. We need only check the ''primes <= sqrt P'': |

<haskell> | <haskell> | ||

-- Infinite list of all prime numbers | -- Infinite list of all prime numbers | ||

− | + | {-# OPTIONS_GHC -O2 -fno-cse #-} | |

− | + | candidateFactors p = let z = floor $ sqrt $ fromIntegral p + 1 in | |

+ | takeWhile (<= z) primesTME | ||

− | + | -- tree-merging Eratosthenes sieve | |

− | + | primesTME = 2 : gaps 3 (join [[p*p,p*p+2*p..] | p <- primes']) | |

− | + | where | |

− | + | primes' = 3 : gaps 5 (join [[p*p,p*p+2*p..] | p <- primes']) | |

− | + | join ((x:xs):t) = x : union xs (join (pairs t)) | |

− | + | pairs ((x:xs):ys:t) = (x : union xs ys) : pairs t | |

− | + | gaps k xs@(x:t) | k==x = gaps (k+2) t | |

− | + | | True = k : gaps (k+2) xs | |

− | + | -- 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 | ||

+ | </haskell> | ||

− | + | The tree-merging Eratosthenes sieve here seems to strike a good balance between efficiency and brevity. More at [[Prime numbers]] haskellwiki page. |

## Revision as of 07:58, 31 May 2011

(**) Determine whether a given integer number is prime.

isPrime :: Integral a => a -> Bool isPrime p = p > 1 && (all ((/= 0).(p `rem`)) $ candidateFactors p) candidateFactors p = takeWhile ((<= p).(^2)) [2..]

Well, a natural number *p* is a prime number if it is larger than **1** and no natural number *n >= 2* with *n^2 <= p* is a divisor of *p*. That's exactly what is implemented: we take the list of all integral numbers starting with **2** as long as their square is at most *p* and check that for all these *n* there is a non-zero remainder concerning the division of *p* by *n*.

However, we don't actually need to check all natural numbers *<= sqrt P*. We need only check the *primes <= sqrt P*:

-- Infinite list of all prime numbers {-# OPTIONS_GHC -O2 -fno-cse #-} candidateFactors p = let z = floor $ sqrt $ fromIntegral p + 1 in takeWhile (<= z) primesTME -- tree-merging Eratosthenes sieve primesTME = 2 : gaps 3 (join [[p*p,p*p+2*p..] | p <- primes']) where primes' = 3 : gaps 5 (join [[p*p,p*p+2*p..] | p <- primes']) join ((x:xs):t) = x : union xs (join (pairs t)) pairs ((x:xs):ys:t) = (x : union xs ys) : pairs t gaps k xs@(x:t) | k==x = gaps (k+2) t | True = k : gaps (k+2) xs -- 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

The tree-merging Eratosthenes sieve here seems to strike a good balance between efficiency and brevity. More at Prime numbers haskellwiki page.