Overview

• Preliminaries.
  • Admin.
  • Upcoming Work.
  • Extra Credit.
  • Questions.
• Some incorrect implementations of iota.
• Lab.

Preliminaries

Admin

• New partners.
• More not-quite food at the back of the room.
• More stuff before lab today
• Review sessions this week
  • Wednesday 8pm (Caleb)
  • Thursday 10am (SamR)
  • Thursday 8pm (Erin)
• Notes on lab 30 are now available.
• Named let is confusing. It will make more sense as time goes on.

Upcoming Work

• Reading for Friday: Turtle Graphics
• Quiz Friday: Recursion, Recursion, and more Recursion (but no letrec or named let).
• HW 6 due next Tuesday.
  • May be a few bugs in part A.
  • More examples may appear for part B.
• Today's Lab writeup (due before class Monday)
  • Email to grader-151-01@cs.grinnell.edu
  • Subject: CSC 151 Lab Writeup Class 32 (Your Names)
  • Exercises: 4ab

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.
• Grinnell Town Hall Wednesday at noon: Self-Gov.
• Conversation about Study Abroad at AIT in Hungary: Wednesday at 4:00 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!
• R.L. Stephens II events Thursday and Friday.
• Expressing Yourself Inside and Outside the Classroom: A Faculty Panel. Friday, noon, JRC 209. (With Prof. Katya, SamR, Prof. Praitis, Prof. Willig Onwuachi, and Prof. Byrd)
• Saturday at 3pm: Harry Potter and the Methods of Rationality discussion group. http://hpmore.com.
• CS Table Tuesday: Why people support and bully online.
• Sexual Misconduct at Grinnell College: Results from the 2013 and 2015 Sexual Conduct Surveys. 1-3 pm, November 8, JRC 101.

Peer Support

• Thursday at 7pm, Womens BBall (scrimmage vs MCC). Sam will send out email once he hears more.
• 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.
• Socrates Cafe', Younker, 2pm.
• Pun Club, Saturdays, 4pm, Way over Younker.

Upcoming Peer Support

• November 14, Orchestra Concert, Sebring Lewis, 7:30 p.m.
• Women's Basketball Opening Game, 1pm, Nov. 15, vs. Silver Lake.
• One-act Festival Dec. 5 & 6.

Other Good Stuff to Do

• Women's Soccer Saturday
• Men's Soccer Saturday
• Halloween event Saturday
• Think good thoughts about EG on her Saturday birthday

Questions

Some Incorrect Implementations of iota

A typical start

(define my-iota
  (lambda (n)
    (if (zero? n)
        null
        (cons (- n 1) (my-iota (- n 1))))))

Problem. It's backwards! Here's an incorrect approach to fixing the problem.

(define my-iota
  (lambda (n)
    (if (zero? n)
        null
        (cons (- n 1) (reverse (my-iota (- n 1)))))))

What happens?

(my-iota 1) => '(0)
(my-iota 2) => (cons 1 (reverse '(0))) => '(1 0)
(my-iota 3) => (cons 2 (reverse '(1 0))) => '(2 0 1)
(my-iota 3) => (cons 3 (reverse '(2 0 1))) => '(3 1 0 2)

Maybe reverse is in the wrong place

(define my-iota
  (lambda (n)
    (if (zero? n)
        null
        (reverse (cons (- n 1) (my-iota (- n 1)))))))

(my-iota 1) => '(0)
(my-iota 2) => (reverse (cons 1 '(0))) => (reverse '(1 0)) => '(0 1)
(my-iota 3) => (reverse (cons 2 '(0 1))) => (reverse '(2 0 1)) => '(1 0 2)

Lab

You can use all and any to check a predicate on every member of a list

    > (all irgb? (list "a" "b" "c"))
    #f

    > (all irgb? (list (irgb 1 1 1)))
    #t

let vs letrec (aka Sam is evil)

(define colonel
  (lambda (vals largest-so-far)
    0))

(define largest
  (lambda (vals)
    (let ([colonel (lambda (vals largest-so-far)
                     (if (null? vals)
                         largest-so-far
                         (colonel (cdr vals) 
                                  (max (car vals) largest-so-far))))])
      (colonel (cdr vals) (car vals)))))

Exercise: 4 a/b