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27 lines
735 B
Haskell
27 lines
735 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 []
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pfactor_ :: (Integral a) => a -> [a] -> [a]
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pfactor_ n factors = do
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let next = pfactor_next n 2
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if next == n
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then factors ++ [n]
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else pfactor_ (n `div` next) (factors ++ [next])
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pfactor_next :: (Integral a) => a -> a -> a
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pfactor_next n factor
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| n == factor = factor
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| n `rem` factor == 0 = factor
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| otherwise = pfactor_next n (factor + 1)
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main = do
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printf "Largest prime factor of 600851475143: %d\n" (last (pfactor 600851475143) :: Int)
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