Questions

• Questions about Quiz 1?
• Questions about Assignment 2?

Setup

• make a directory labs/L07 inside your ~/cs2613 git repository
• All of your work from today should be committed in that directory (and pushed before you leave).

Introducing foldr

• Form a group of 2 or 3.

• Work together to solve FICS Exercise 27

• Contracts were introduced in The Design Recipe

• Here is an implementation of foldr, with the name changed so as not to conflict with the builtin. It is often easier to infer a type from a (simple) implementation.

(define (Foldr combine base lst)
  (cond
    [(empty? lst) base]
    [else (combine
            (first lst)
            (Foldr combine base (rest lst)))]))

Guess the function

This part of the lab is based on FICS Exercise 29

Fill in the two instances of ... in the following template with a builtin function so that the tests pass. Try some other values for mylist, mylist1, mylist2 to help you guess.

#lang htdp/isl+
(define mylist '(1 2 buckle my shoe))
(check-expect (foldr cons empty mylist) (... mylist))
(define mylist1 '(3 4 5))
(define mylist2 '(1 2))
(check-expect (foldr cons mylist1 mylist2) (... mylist2 mylist1))

• Do these uses of foldr fit your contract above? If not, how can you fix your contract.

Filter in terms of foldr

On your own, complete FICS Exercise 28

• You will have to rename the function to Filter to avoid colliding with the builtin filter.

• You will need to use the language htdp/isl+ or "Intermediate Student with Lambda"

• Your solution should pass the following test cases. odd? is a builtin function.

• The previous part (Ex. 29) may give you some ideas about how to do this part.

(check-expect (Filter (lambda (x) #f) '(1 2 3)) '())

(check-expect (Filter odd? '(0 1 2 3 4 5 6 7 8 9))
              (list 1 3 5 7 9))

Before next lab