add fourth exercise of lecture 4

1 files changed, 16 insertions, 0 deletions

diff --git a/lec04/hw.hs b/lec04/hw.hs
index 923ec01..c96ef44 100644
--- a/lec04/hw.hs
+++ b/lec04/hw.hs
@@ -1,3 +1,4 @@
+import Data.List
 -- Exercise 1
 
 fun1 :: [Integer] -> Integer
@@ -48,3 +49,18 @@ map' f = foldr (\x s -> f x : s) []
 
 myFoldl :: (a -> b -> a) -> a -> [b] -> a
 myFoldl f base xs = foldr (flip f) base (reverse xs)
+
+-- Exercise 4
+
+cartProd :: [a] -> [b] -> [(a, b)]
+cartProd xs ys = [(x,y) | x <- xs, y <- ys]
+
+sieveSundaram :: Integer -> [Integer]
+sieveSundaram n =
+    take (fromInteger n)
+    . map (\x -> (2*x) + 1)
+    . (\\) [1..n]
+    . filter (<=n)
+    . map (\(i,j) -> i + j + (2*i*j))
+    . filter (\(i,j) -> i <= j)
+    $ cartProd [1..n] [1..n]