new: Solved problems 11 to 20

Problems 11-20/problem_11.hs

@@ -0,0 +1,20 @@
+data ListItem a = Single a | Multiple Int a deriving (Show)
+
+getListItem :: (Int, a) -> ListItem a
+getListItem (1, x) = Single x
+getListItem (n, x) = Multiple n x
+
+encode :: (Eq a) => [a] -> [(Int, a)]
+encode =
+  foldr
+    ( \x acc -> case acc of
+        [] -> [(1, x)]
+        (hd@(hd_n, hd_a) : tl) ->
+          if x == hd_a
+            then (hd_n + 1, hd_a) : tl
+            else (1, x) : hd : tl
+    )
+    []
+
+encodeModified :: (Eq a) => [a] -> [ListItem a]
+encodeModified = map getListItem <$> encode

Problems 11-20/problem_12.hs

@@ -0,0 +1,8 @@
+data ListItem a = Single a | Multiple Int a deriving (Read)
+
+fromListItem :: ListItem a -> [a]
+fromListItem (Single x) = [x]
+fromListItem (Multiple n x) = replicate n x
+
+decodeModified :: [ListItem a] -> [a]
+decodeModified = concatMap fromListItem

Problems 11-20/problem_13.hs

@@ -0,0 +1,21 @@
+-- This is the same solution as in Problem 11
+data ListItem a = Single a | Multiple Int a deriving (Show)
+
+getListItem :: (Int, a) -> ListItem a
+getListItem (1, x) = Single x
+getListItem (n, x) = Multiple n x
+
+encode :: (Eq a) => [a] -> [(Int, a)]
+encode =
+  foldr
+    ( \x acc -> case acc of
+        [] -> [(1, x)]
+        (hd@(hd_n, hd_a) : tl) ->
+          if x == hd_a
+            then (hd_n + 1, hd_a) : tl
+            else (1, x) : hd : tl
+    )
+    []
+
+encodeModified :: (Eq a) => [a] -> [ListItem a]
+encodeModified = map getListItem <$> encode

Problems 11-20/problem_14.hs

@@ -0,0 +1,2 @@
+dupli :: [a] -> [a]
+dupli x = concat $ [replicate 2] <*> x

Problems 11-20/problem_15.hs

@@ -0,0 +1,2 @@
+repli :: [a] -> Int -> [a]
+repli x n = concat $ [replicate n] <*> x

Problems 11-20/problem_16.hs

@@ -0,0 +1,2 @@
+dropEvery :: [a] -> Int -> [a]
+dropEvery l n = map snd . filter ((0 /=) . (`mod` n) . fst) $ zip [1 ..] l

Problems 11-20/problem_17.hs

@@ -0,0 +1,5 @@
+split :: [a] -> Int -> ([a], [a])
+split x 0 = ([], x)
+split (hd : tl) n = (hd : next_hd, next_tl)
+  where
+    (next_hd, next_tl) = split tl (n - 1)

Problems 11-20/problem_18.hs

@@ -0,0 +1,4 @@
+slice :: [a] -> Int -> Int -> [a]
+slice [] _ _ = error "Invalid indices"
+slice l 1 n = take n l
+slice (_ : tl) m n = slice tl (m - 1) (n - 1)

Problems 11-20/problem_19.hs

@@ -0,0 +1,5 @@
+rotate :: [a] -> Int -> [a]
+rotate l n =
+  if n >= 0
+    then (\(x, y) -> y ++ x) $ splitAt n l
+    else rotate l (length l + n)

Problems 11-20/problem_20.hs

@@ -0,0 +1,5 @@
+removeAt :: Int -> [a] -> (a, [a])
+removeAt n ls = (last l, take m l ++ r)
+  where
+    (l, r) = splitAt n ls
+    m = length l - 1