The Global Engineer’s Notebook

Chapter 1 Exercises

1. Integer, Integer, Integer, Integer, Double, Integer, Integer, Interger, Integer, Double, Integer, Integer, Integer, Integer, Integer
2. I see all values bound to variables
3. Based on WC.hs in the chapter:

   -- file: ch01/WC_ex3.hs
   -- Notes:
   --  the words function splits a list of characters on a space
   main = interact wordCount
       where wordCount input = show (length (words(input))) ++ "\n"

4. Based on WC.hs in the chapter:

   -- file: ch01/WC_ex2.hs
   -- Notes:
   --  a string is just a list of characters
   main = interact wordCount
       where wordCount input = show (length input) ++ "\n"

1. Bool;  ([[Char]],Char) which is a tuple containing a list of strings and a character; [(Bool, [[Char]])] which is a list of tuples containing a bool and a list of strings

Chapter 2 End-of-chapter Exercises

1. Return an element from a list; return a list

2. --file: lastButOne.hs (Exercise 2)
   lastButOne xs = if length (xs) > 2
                   then lastButOne(tail(xs))
                   else head(xs)

3. If the list length is greater than one, the above solution works.  If the list has one element, it incorrectly returns the only element in the list.  If the list is empty, an exception is thrown:

   *** Exception: Prelude.head: empty list

1. fromList (Cons x xs) = x : fromList xs
   fromList Nil         = []

2. data Tree a = Node a (Maybe (Tree a)) (Maybe (Tree a))

Chapter 3 End-of-chapter Exercises

1. --file: ch03/ex1.hs
   numElem (x:xs) = 1 + numElem(xs)
   numElem []     = 0

2. --file: ch03/ex2.hs
   numElem :: [a] -> Int
   numElem (x:xs) = 1 + numElem(xs)
   numElem []     = 0

3. --file: ch03/ex3.hs
   computeMean x = sum (x) / fromIntegral( length x)

4. --file: ch03/ex4.hs
   palindrome :: [a] -> [a]
   palindrome (x:xs)  = [x] ++ palindrome xs ++ [x]
   palindrome []      = []

5. It is not pretty, but it works. The “Eq a=>” addition to the type line was required to get the == operator work. As of writing this solution, I do not know what this does…

   --file: ch03/ex5.hs
   ispalindrome :: Eq a => [a] -> Bool
   ispalindrome [] = True
   ispalindrome x  = if sizeSafe && firstElem == lastElem && ispalindrome midElem
                     then True
                     else False
         where sizeSafe  = if length x `mod` 2 == 0
                           then True
                           else False
               firstElem = head x
               lastElem  = head( reverse ( tail x ) )
               midElem   = drop 1 (reverse (drop 1 (reverse x)))

1. import Data.Maybe
   
   safeHead :: [a] -> Maybe a
   safeHead [] = Nothing
   safeHead xs = Just (head xs)
   
   safeTail :: [a] -> Maybe [a]
   safeTail [] = Nothing
   safeTail xs = Just (tail xs)
   
   safeLast :: [a] -> Maybe a
   safeLast [] = Nothing
   safeLast xs = Just (last xs)
   
   safeInit :: [a] -> Maybe [a]
   safeInit []     = Nothing
   safeInit (x:[]) = Just []
   safeInit (x:xs) = Just (x : init xs)