tasks a-s

Task/Self-describing-numbers/Haskell/self-describing-numbers-1.hs

@@ -0,0 +1,16 @@
+import Data.Char
+
+count :: Int -> [Int] -> Int
+count x = length . filter (x ==)
+
+isSelfDescribing :: Integer -> Bool
+isSelfDescribing n =
+    nu == f where
+            nu = map digitToInt (show n)
+            f = map (\a -> count a nu) [0 .. ((length nu)-1)]
+
+main = do
+    let tests = [1210, 2020, 21200, 3211000,
+                 42101000, 521001000, 6210001000]
+    print $ map isSelfDescribing tests
+    print $ filter isSelfDescribing [0 .. 4000000]

Task/Self-describing-numbers/Haskell/self-describing-numbers-2.hs

@@ -0,0 +1,21 @@
+import Data.Char (intToDigit)
+import Control.Monad (replicateM, forM_)
+
+count :: Int -> [Int] -> Int
+count x = length . filter (x ==)
+
+-- all the combinations of n digits of base n
+-- a base-n number are represented as a list of ints, one per digit
+allBaseNNumsOfLength :: Int -> [[Int]]
+allBaseNNumsOfLength n = replicateM n [0..n-1]
+
+isSelfDescribing :: [Int] -> Bool
+isSelfDescribing num =
+  all (\(i,x) -> x == count i num) $ zip [0..] num
+
+-- translate it back into an integer in base-10
+decimalize :: [Int] -> Int
+decimalize = read . map intToDigit
+
+main = forM_ [1..7] $
+  print . map decimalize . filter isSelfDescribing . allBaseNNumsOfLength