Overview

• Preliminaries.
  • Admin.
  • Upcoming Work.
  • Extra Credit.
  • Questions.
• More Discussion of Recursion.
• Lab.

Preliminaries

Admin

• Continue partners.
• I brought another food-like substance.
• New method for calling on students.
• Still some pre-lab stuff this week.
• Review sessions this week
  • Wednesday 8pm (Caleb)
  • Thursday 10am (SamR)
  • Thursday 8pm (Erin)

Upcoming Work

• Reading for Wednesday: Naming Local Procedures
• Quiz Friday: Recursion, Recursion, and more Recursion
• HW 6 is ready.
  • We'll look at a few issues.
  • Pick your own partners (or work alone)!
• Today's Lab writeup (due before class Friday)
  • Email to grader-151-01@cs.grinnell.edu
  • Subject: CSC 151 Lab Writeup Class 31 (Your Names)
  • Exercises: 3

Extra Credit Opportunities

• If possible, send me these in advance.

Academic

• Any visit to the current show in the Faulconer gallery.
• Any of the Grinnell Prize Week Activities this week.
• CS Table TODAY: Hear about the Grace Hopper Celebration of Women in Computing.
• Grinnell Town Hall Wednesday at noon: Self-Gov.
• Conversation about Study Abroad at AIT in Hungary: Wednesday at 4:15 in 3821.
• Convocation Thursday at 11: Contesting Muhammad: Contermporary Controversies in Historical Perspective
• CS Extras Thursday at 4:15pm in 3821: Algorithms for Assembling the Tree of Life.
• What's the Deal with the Digital Liberal Arts? Thursday at 4:15 in the D-Lab. With Autumnal snacks!
• Sexual Misconduct at Grinnell College: Results from the 2013 and 2015 Sexual Conduct Surveys. 1-3 pm, November 8, JRC 101.
• R.L. Stephens II events Thursday and Friday.

Peer Support

• YZ on "Rainbow Ramsey Theory" Tuesday at 11 a.m. 2517.
• Thursday at 7pm, Womens BBall (scrimmage)
• Spark Tank, Wednesday, 7pm in Roberts Theatre in Bucksbaum.
• Pals of PALS, pals@grinnell.edu, normally Saturday at 7:45 am (breakfast included), Sunday at 4:45, and Mondays at 4:45. Requires sign up in advance.
• Pun Club, Saturdays, 4pm, Way over Younker.

Upcoming Peer Support

• Women's Basketball Opening Game, 1pm, Nov. 15, vs. Silver Lake.
• One-act Festival Dec. 5 & 6.

Good Things to Do, No EC

• 8:30 p.m. tonight in Loose Lounge: Campus Conversation on Healthy and Unhealthy Relationship Dynamics

Questions

What are you doing with our questions on lab 30?

Questions have started to arrive. I am sending answers when I can.

I am gathering all of the questions and answers in a document that I will release in class on Wednesday. Make sure to get your questions in before then.

It helps to try the lab.

What should we do if we don't finish a lab or lab writeup in class?

Required: Finish the writeup.

Preferred (but not required): Finish the lab.

Preferred (but not required): With your partner.

Key ideas from pre-break lab

A few minutes with partner: Consider the following code.

;;; Parameters:
;;;   colors, a non-empty list of IRGB colors
(define irgb-brightest
  (lambda (colors)
    (cond
      [(null? (cdr colors))
       (car colors)]
      [(>= (irgb-brightness (car colors)) 
           (irgb-brightness (irgb-brightest (cdr colors))))
       (car colors)]
      [else
       (irgb-brightest (cdr colors))])))

• What's wrong with this code?
• What are three ways we can fix it?

What should this code do?

• Picks the brightest color in a list of colors.
• Does it accomplish the task: Yes!

What's wrong?

• It's really inefficient
• 1 element: 1 call
• 2 elements: 3 calls
• 3 elements: 7 calls
• 4 elements: 15 calls
• k elements: 2^k - 1 calls. Ouch!

How would you figure out that your code was this inefficient?

• Could add a display procedure to see how many steps are taken.
• We'll also learn other techniques.

How to fix?

• "Make sure that there's only one recursive call per element in the list."

Technique one: Use a recursive kernel

(define irgb-brightest
  (lambda (colors)
    (irgb-brightest-kernel (cdr colors) (car colors))))

 (define irgb-brightest-kernel
   (lambda (colors brightest-so-far)
     (cond 
       [(null? colors)
        brightest-so-far]
       [(>= (irgb-brightness (car colors))
            (irgb-brightness brightest-so-far))
        (irgb-brightest-kernel (cdr colors) (car colors))]
       [else
        (irgb-brightest-kernel (cdr colors) brightest-so-far)])))

Technique two: When you have repeated code, use let to name the repeated code.

(define irgb-brightest
  (lambda (colors)
    (if (null? (cdr colors))
        (car colors)
        (let ([brightest-in-rest (irgb-brightest (cdr colors))])
          (if (>= (irgb-brightness (car colors)) 
                  (irgb-brightness brightest-in-rest))
              (car colors)
              brightest-in-rest)))))

Technique three: Try to avoid the second recursive call by using another helper procedure.

Lab

Writeup: Exercise 3