2025-12-14 Short Review

Five questions. Nothing tricky. Less than 40 minutes.

Remove all the repetitions. Keep numbers in the order you find them.

 norep :: [Int] -> [Int]
 norep [1,1,1,2,1,2,2,2,3,4,3] == [1,2,3,4]

Count the repetitions.

 cr :: [String] -> [(String,Int)]
 cr ["a","b","b","b","a"] == [("a",2),("b",3)]

Given a string and a list of letters, replace every letter not in the list of letters with a underscore '_'.
```
 cross :: String -> [Char] -> String
 cross "Wombat" "ao" == "_o__a_"
```
Find the total y coordinates of every pair for which the selection function returns true when run on x.
```
 cwan :: (String->Bool) -> [(String,Int)] -> Int
```
(a) use recursion (b) use some higher order function (map, filter, fold, lambda) in addition to some built-in function(s)

Is this an efficient way to add up the corresponding product of the first n term in two lists? Comment/fix.

 f n xs ys = helper 0
     where helper k
         | k < n     = xs!!k * ys!!k + helper (k+1)
         | otherwise = 0

Next year I’m going to teach >>= with Maybe

 ff n
   | even n = Just (n*5)
   | otherwise = Nothing
 [Just 5, Nothing, Just 8] >>= ff == [Just 40]

Anyway, you know enough to write the function >>= now. Try it.