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Haskell 002

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Correl Roush 2011-10-10 23:58:48 -04:00
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{-
Find the sum of all the even-valued terms in the Fibonacci sequence which do not exceed four million.
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:
1, 2, 3, 5, 8, 13, 21, 34, 55, 89, ...
Find the sum of all the even-valued terms in the sequence which do not exceed four million.
-}
import Text.Printf
fib :: (Integral a) => (a, a) -> a
fib (x, y) = x + y
fibnext :: (Integral a) => (a, a) -> (a, a)
fibnext (x, y) = (y, x + y)
fibsumeven :: (Integral a) => a -> a
fibsumeven max = fibsumeven_ max 0 (0,1)
fibsumeven_ :: (Integral a) => a -> a -> (a, a) -> a
fibsumeven_ max total pair
| fib pair > max = total
| even (fib pair) = fibsumeven_ max (total + (fib pair)) (fibnext pair)
| otherwise = fibsumeven_ max total (fibnext pair)
main = do
printf "Sum of even fibonacci terms <= 4,000,000: %d\n" (fibsumeven 4000000 :: Int)