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27 lines
948 B
Haskell
27 lines
948 B
Haskell
{-
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Find the sum of all the even-valued terms in the Fibonacci sequence which do not exceed four million.
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Each new term in the Fibonacci sequence is generated by adding the previous two terms. By starting with 1 and 2, the first 10 terms will be:
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1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
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Find the sum of all the even-valued terms in the sequence which do not exceed four million.
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-}
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import Text.Printf
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fib :: (Integral a) => (a, a) -> a
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fib (x, y) = x + y
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fibnext :: (Integral a) => (a, a) -> (a, a)
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fibnext (x, y) = (y, x + y)
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fibsumeven :: (Integral a) => a -> a
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fibsumeven max = fibsumeven_ max 0 (0,1)
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fibsumeven_ :: (Integral a) => a -> a -> (a, a) -> a
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fibsumeven_ max total pair
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| fib pair > max = total
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| even (fib pair) = fibsumeven_ max (total + (fib pair)) (fibnext pair)
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| otherwise = fibsumeven_ max total (fibnext pair)
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main = do
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printf "Sum of even fibonacci terms <= 4,000,000: %d\n" (fibsumeven 4000000 :: Int)
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