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26 lines
770 B
Haskell
26 lines
770 B
Haskell
{-
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Find the largest prime factor of a composite number.
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The prime factors of 13195 are 5, 7, 13 and 29.
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What is the largest prime factor of the number 600851475143 ?
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-}
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import Text.Printf
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pfactor :: (Integral a) => a -> [a]
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pfactor n = pfactor_ n [] 2
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pfactor_ n factors start =
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let next = pfactor_next n start in
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if n == next
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then n:factors
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else pfactor_ (n `div` next) (next:factors) next
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pfactor_next :: (Integral a) => a -> a -> a
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pfactor_next n last
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| n <= last = n
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| otherwise = head (filter (\x -> n `rem` x == 0 || n == x) [last..n])
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main = do
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printf "Largest prime factor of 13195: %d\n" (head (pfactor 13195) :: Integer)
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printf "Largest prime factor of 600851475143: %d\n" (head (pfactor 600851475143) :: Integer)
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