CSci 1301: Problem Set 4

Due: Monday, November 12th at 11:59pm

Please make sure that every function has a signature, description, and test cases (as needed).

Problem 1

Read Chapter 9 of the book, paying attention to the design recipe for lists (it summarizes and rephrases the same thing we were discussing in class).
Then do the following problems, including the check-expect tests, signatures, descriptions, and then the implementation:

1. Exercise 140 (5 points).
2. A function that takes a list of strings and produces the sum of their lengths (5 points).
3. A function that takes a list of positions and another position (let's call it X) and produces the sum of distances from each of the positions on the list to X (5 points). Hint: write and use a helper distance function (think of what its signature would be).

Problem 2

1. Exercise 169 (5 points)
2. Exercise 170 (5 points)
3. Write a function that takes a list of strings and returns a list of their length (3 points).
4. Write a function that takes a list of strings and returns a list of all strings on that list that don't start with a lower-case or upper-case "a" (3 points).

Problem 3

Read Section 11.3 in the textbook and do the following exercises:

1. Exercise 187 (4 points)
2. Exercise 188 (4 points)
3. Exercise 190 (write both prefixes and suffixes functions) (9 points). Make sure to start with check-expects and a signature. Note that the function needs a recursive helper function. Don't forget to write signatures before you write the any code!
   
   Clarification of the problem: There are two possible interpretations of "all prefixes":
   • An empty list of letters is a prefix of every list of letters. Then the base case is an empty list of letters, and your check-expects would include:

     
     	    (check-expect (prefixes (list "a" "b" "c")) (list empty (list "a") (list "a" "b") (list "a" "b" "c")))
     	    (check-expect (prefixes (list "a")) (list empty (list "a")))
     	    ;; base case:
     	    (check-expect (prefixes empty) (list empty))
     	

   • The smallest list of letters (and the smallest prefix) is a one-letter list. Then your base case is when the list of letters has exactly one element, and your check-expects include:

     
     	    (check-expect (prefixes (list "a" "b" "c")) (list (list "a") (list "a" "b") (list "a" "b" "c")))
     	    ;; base case:
     	    (check-expect (prefixes (list "a")) (list (list "a")))
     	

   Both approaches have their challenges, but can be solved by carefully thinking of a solution in terms of the recursive case and the first element.

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CSci 1301 course web site.