new: Solved problems 21 to 28

Problems 21-28/problem_21.hs

@@ -0,0 +1,4 @@
+insertAt :: a -> [a] -> Int -> [a]
+insertAt x ls n = left ++ [x] ++ right
+  where
+    (left, right) = splitAt (n - 1) ls

Problems 21-28/problem_22.hs

@@ -0,0 +1,2 @@
+range :: Int -> Int -> [Int]
+range m n = if m > n then [] else m : range (m + 1) n

Problems 21-28/problem_23.hs

@@ -0,0 +1,7 @@
+import Data.List
+import System.Random
+
+rndSelect :: [a] -> Int -> IO [a]
+rndSelect ls n = do
+  gen <- getStdGen
+  return $ take n [ls !! m | m <- nub $ randomRs (0, length ls - 1) gen]

Problems 21-28/problem_24.hs

@@ -0,0 +1,7 @@
+import Data.List
+import System.Random
+
+diffSelect :: Int -> Int -> IO [Int]
+diffSelect n m = do
+  gen <- getStdGen
+  return $ take n [[1 .. m] !! x | x <- nub $ randomRs (0, m - 1) gen]

Problems 21-28/problem_25.hs

@@ -0,0 +1,7 @@
+import Data.List
+import System.Random
+
+rndPermu :: [a] -> IO [a]
+rndPermu ls = do
+  gen <- getStdGen
+  return $ take (length ls) [ls !! m | m <- nub $ randomRs (0, length ls - 1) gen]

Problems 21-28/problem_26.hs

@@ -0,0 +1,10 @@
+combinations :: Int -> [a] -> [[a]]
+combinations 0 _ = []
+combinations _ [] = []
+combinations n (hd : tl) = with ++ without
+  where
+    with =
+      if n == 1
+        then [[hd]]
+        else [(hd :)] <*> combinations (n - 1) tl
+    without = combinations n tl

Problems 21-28/problem_27.hs

@@ -0,0 +1,11 @@
+import Data.List
+
+group' :: [Int] -> [a] -> [[[a]]]
+group' ln ls = map ((([([fst] <*>)] <*>) . groupBy grouper . sortBy sorter) . zip ls) (nub (permutations placers))
+  where
+    placers = concatMap (uncurry replicate) (zip ln [0 ..])
+    sorter (_, r1) (_, r2) = r1 `compare` r2
+    grouper (_, r1) (_, r2) = r1 == r2
+
+group3 :: [a] -> [[[a]]]
+group3 = group' [2, 3, 4]

Problems 21-28/problem_28.hs

@@ -0,0 +1,9 @@
+import Data.List
+
+lsort :: [[a]] -> [[a]]
+lsort = sortBy (\x y -> length x `compare` length y)
+
+lfsort :: [[a]] -> [[a]]
+lfsort ls = sortBy (\x y -> count x `compare` count y) ls
+  where
+    count x = length . filter (length x ==) $ map length ls