CS 1102: Lab 3

Lab Motivation and Goals

• Use map and filter, Scheme's loops
• Write your own iterators over trees

You need to use Intermediate Language Level for this lab.

Exercises with Map and Filter

Assume that we want to develop an inventory database for an online CD store. For each CD, the database stores its title, artist, price, how many copies are in stock, and its category of music (such as rock, blues, or country). Write a data definition for CD inventories.

Write each of the following functions using filter and/or map, or argue why it cannot be written using these (in which case you do not need to write the function).

1. Write a function all-titles that consumes an inventory and produces a list of all titles in the inventory.

2. Write a function titles-in-stock that consumes an inventory and produces a list of all titles in the inventory with at least 1 copy in stock.

3. Write a function restock that consumes a CD title, number of new copies of that CD and an inventory and produces an inventory in which the named CD has the given number of additional copies (and all other CDs remain the same).

4. Write a function total-stock that consumes an inventory and produces the total number of copies of CDs that are in stock.

5. Write a function copies-in-stock that consumes a CD title and artist name and produces the number of copies of that item that are in stock. Return 0 if the named CD isn't in the inventory.

6. Write a function blues-sale that consumes an inventory and produces an inventory in which all blues CDs are discounted by 10\%.

7. Write a function carry-cd? that consumes a title and artist name and produces a boolean indicating whether that item is in the inventory (whether in or out of stock).

Exercises with Iterators on Trees

For these exercises, we will work with the following data definition for binary trees:

   ;; A bintree[alpha] is either
   ;;  - false, or
   ;;  - (make-node alpha bintree bintree)

   ;; the data definition for node appears above in that for bintree
   (define-struct node (item left right))

1. Write the template for programs over bintrees. You can check for false using boolean?

2. Write a program sign-tree that consumes a bintree[number] (i.e., a bintree where the items at each node are numbers) and returns a bintree[symbol] with the same structure as the input tree, but with each number replaced with one of the symbols 'positive, 'negative, or 'zero, as appropriate.

3. Write a program flip-tree that consumes a bintree[posn] and returns a bintree[posn]. The returned tree should have the same structure as the input tree, but each posn should have its x- and y-coordinates flipped [i.e., (make-posn 3 4) would become (make-posn 4 3)].

   Recall that (define-struct posn (x y)) is built into DrScheme.

4. The programs sign-tree and flip-tree have a similar structure. Write a function tree-map that takes a bintree and a function and applies the function to each node in the tree. Show how to implement sign-tree and flip-tree using tree-map. Be sure to include a correct contract for tree-map.

5. Write a program tree-andmap that consumes a function of type (alpha -> boolean) and a bintree[alpha] and returns a boolean indicating whether the given function returned true at every node. Can you implement tree-andmap using a simple call to tree-map? By "a simple call", I mean can you just fill in the ... in the code below to implement this function? If not, why not?

   (define (tree-andmap func abt)
          (tree-map ...))
   

   Note: to check whether a value is false, you can use

   (and (boolean? value) (not value))

   Using tree-andmap, implement pos-tree? that consumes a bintree[number] and determines whether all numbers in the tree are positive.

Return to the labs page