Overview

• Preliminaries.
  • Admin.
  • Upcoming Work.
  • Extra Credit.
  • Questions.
• Recursion lab
• Preconditions lab

Preliminaries

Admin

• Continue partners.
• We'll do the recursion lab for the first half of class and the preconditions lab for the second half of class.

Reminders

• Office hours this week
  • See http://rebelsky.youcanbook.me.
  • I should be gone next Monday and Tuesday :-(
• Tutor hours
  • Sunday, 3-5 p.m.
  • Sunday-Thursday, 7-10 p.m.
• Weekly review sessions:
  • Wednesday at 8pm in the CS Commons with Evan.
  • Thursday review session this week with Sam.
  • Thursday at 8pm in the CS Commons with Alex.

Upcoming Work:

• Reading for Friday?
• Exam 2 due NEXT TUESDAY.
• Quiz Friday: Recursion basics.
  • You should be able to read and write procedures like those from the recursion basics lab.
• Lab Writeup/Recursion Lab: Exercise _
  • Send email titled CSC 151 Lab Writeup 26 (Your Names)
  • Do not include the underscores.
  • Send to CSC151-02-grader@grinnell.edu
  • Due before class on Monday.
• Lab Writeup/Preconditions Lab: Exercise _
  • Send email titled CSC 151 Lab Writeup 27 (Your Names)
  • Do not include the underscores.
  • Send to CSC151-02-grader@grinnell.edu
  • Due before class on Monday.

Extra Credit

• Send your reports to rebelsky@grinnell.edu with subject "CSC 151 Extra Credit". (Do not include the quotation marks.)
• Send opportunities to me before class with subject "CSC 151 EXTRA CREDIT OPPORTUNITY!"

Academic / Artistic

• Friday at 4:00 pm. in Burling: Decolonizing Digital Humanities

Peer

• The Tempest. This weekend. 7:30 Friday and Saturday, 2:00 Sunday. Tickets in Bucksbaum box office.
• Salseros Friday at 8:30 pm in Garnder. Food! The JS non-brothers, sighing deeply.
• 8:00-10:00 show with various cool jazz-funk-pop at PC. No free food.
• Sunday, 6:00-8:00 AAA Potluck in JRC 209. Free ingredients!

Regular Peer

• Social Dance Workshop Tuesdays 7:00-8:00 in Bucksbaum Dance Studio
• Pun club Saturdays at 4pm in Younker
• Electronic Potpourri on KDIC Fridays at Five
• Space Odyssey KDIC Fridays at Six
• Bollywood, Friday, 7:30-8:30, Younker

Misc

Far in the Future

• Lords of the Flies, April.

No Extra Credit, But Still Good

• Indoor track nationals.

Questions

Recursion Lab

(define list-length
  (lambda (lst)
    (if (null? lst)
        0
        (+ 1 (list-length (cdr lst))))))

(list-length '(1 2 3))
; Is it null? No
=> (+ 1 (list-length (cdr '(1 2 3))))
=> (+ 1 (list-length '(2 3)))
; list-length is innermost operation.  
; is '(2 3) null?  NO!  So we replace the inner list-length
; with the alternate
=> (+ 1 (+ 1 (list-length (cdr '(2 3)))))
=> (+ 1 (+ 1 (list-length '(3))))
; list-length is innermost operation.  
; is '(3) null?  NO!  So we replace the inner list-length
=> (+ 1 (+ 1 (+ 1 (list-length (cdr '(3))))))
=> (+ 1 (+ 1 (+ 1 (list-length '()))))
; list-length is innermost operation.  
; is '() null?  YES!  Replace with 0
=> (+ 1 (+ 1 (+ 1 0)))
=> (+ 1 (+ 1 1))
=> (+ 1 2)
=> 3

Can we think about the base case for product?

> (*)
1

Okay, why? Because 1 is the multiplicative identify. That means,

> (product '())
1

What's the writeup?

Writeup 26: 6ad, Recursion Basics Lab

Preconditions Lab

Writeup 27: 3h, Verifying Preconditions Lab