Lists

Originally created by Dr. Griener

Instructions for students & labbies: Start with short group discussion. Students use DrScheme on the exercises at their own pace, while labbies wander among the students, answering questions, bringing the more important ones to the lab's attention. Example solutions are attached at the bottom of the page.

• Follow the design recipe, including using templates.
• Use the stepper on at least a few examples.
• It's likely you won't have time to do all the examples during lab. Students should look over the rest on their own.

Review: Definitions of Lists

• What is the data definition of a list of numbers?
• What are the constructors, selectors, and predicates for lists of numbers, and what are their contracts?
• What are some examples of lists of numbers?
• What is the template for functions on lists of numbers?

Practice Exercises

Functions on Lists of Numbers

All of these examples use the same input type: a list of numbers. So, don't forget to reuse what you can to save yourself some time. Use your data examples (i.e., sample lists of numbers) for each of the function examples (i.e., what the function does on each of those inputs). Copy-and-paste the type's template as your starting point for each function.

1. Develop a program count-positive which takes a list of numbers and returns a count of how many are positive, i.e., greater than zero.
   Use the stepper on count-positive applied to a list of length 2. How many times is your function called (counting both the initial call and recursive calls)?
2. Develop a program map-add which takes a number and a list of numbers and returns a list of numbers, where the resulting list is constructed by adding the given number to each element in the list. E.g.,

      > (map-add 3 (cons 1 (cons 4 (cons 8 empty))))
      (cons 4 (cons 7 (cons 11 empty)))
   

   Later in the course, we'll generalize this idea of mapping some operation over a list to generate a new list of the results.
3. Develop a program filter-positive which takes a list of numbers and returns a list of all those numbers which are positive. E.g.,

      > (filter-positive (cons 1 (cons -5 (cons 3 empty))))
      (cons 1 (cons 3 empty))
   

   Later in the course, we'll generalize this idea of filtering a list by some test condition.

Functions on Structures

We've seen lists of symbols and lists of numbers; you can of course have a list with elements of any given type...including other structures like posn-s. This just uses ideas you've already learned, but in new contexts.

1. Define the type of lists of posn-s.
2. Write the templates of lists of posn-s. (NB: There are two non-primitive types here, one inductive the other a structure. So this may require two templates or one, depending on if you believe in structure templates.)
3. Write the function positive-quadrant? , which takes in a single posn and returns true only when its components are both positive.
4. Develop count-positive-quadrant , which takes a list of posn-s and returns a count of those in the upper-right quadrant.
5. Develop filter-positive-quadrant , which takes a list of posn-s and returns a list of those in the upper-right quadrant.

Optional: Variants of the List Definition

1. Make a data definition for non-empty lists of numbers. Hint: The base case should not be empty, since that is not a non-empty list of numbers! What is a description of the shortest non-empty list of numbers?
2. Develop a program which takes a non-empty list of numbers and returns the average (a.k.a, arithmetic mean) of all the numbers.
3. Develop a program which takes a standard, potentially empty list of numbers and returns the average of all the numbers. For this example, arbitrarily define the average of an empty list to be false.

Here are two reasonable definitions for non-empty lists.

• Solution: comp211-lab01.ss (https:wikiriceedudisplayCSWIKI211lab01comp211lab01ss)

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