implemented the statistics exercise

haskell/08-statistics/statistics.hs

@@ -1,17 +1,63 @@
-import Data.List (nub, sortOn)
+import Data.Function (on)
+import Data.List (groupBy, nub, sortOn)
 import System.IO
 
 -- separator -> Input -> Separated Input
--- split :: Char -> String -> [String]
+split :: Char -> String -> [String]
+split _ "" = []
+split sep str =
+  let (word, rest) = break (== sep) str
+   in word : case rest of
+        [] -> []
+        (_ : xs) -> split sep xs
 
--- data Entry =
+-- Data structure for a student entry
+data Entry = Entry
+  { firstName :: String,
+    lastName :: String,
+    subject :: String,
+    scores :: [Int]
+  }
+  deriving (Show)
 
--- averages :: [Entry] -> [(String, Float)]
+-- Parse CSV content into entries
+parse :: String -> [Entry]
+parse content =
+  let lns = lines content
+      (header : rows) = lns
+   in map parseRow rows
+  where
+    parseRow line =
+      let fields = split ',' line
+          [fn, ln, subj] = take 3 fields
+          pts = map read (drop 3 fields) :: [Int]
+       in Entry fn ln subj pts
 
--- content -> Entries
--- parse :: String -> [Entry]
+-- Calculate averages per subject
+averages :: [Entry] -> [(String, Float)]
+averages entries =
+  let grouped = groupBy ((==) `on` subject) (sortOn subject entries)
+      avgPerSubject grp =
+        let subj = subject (head grp)
+            allScores = concat [scores e | e <- grp]
+            avg = fromIntegral (sum allScores) / fromIntegral (length allScores)
+         in (subj, avg)
+   in map avgPerSubject grouped
+
+-- Find the best student (most accumulated points)
+bestStudent :: [Entry] -> (String, Int)
+bestStudent entries =
+  let best = maximum [(sum (scores e), firstName e ++ " " ++ lastName e) | e <- entries]
+   in (snd best, fst best)
 
 main :: IO ()
 main = do
   putStrLn "Enter a filename: "
-  putStrLn "..."
+  filename <- getLine
+  content <- readFile filename
+  let entries = parse content
+      (name, points) = bestStudent entries
+      avgs = averages entries
+  putStrLn $ "Best student: " ++ name ++ " (" ++ show points ++ " points)"
+  putStrLn "Average points per subject:"
+  mapM_ (\(subj, avg) -> putStrLn $ "  " ++ subj ++ ": " ++ show avg) avgs