Haskell 002

haskell/e002.hs

@@ -0,0 +1,27 @@
+{-
+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)