• Same Partners!

Overview

• Preliminaries.
  • Admin.
  • Upcoming Work.
  • Extra Credit.
  • Questions.
• Quick notes on named let.
• Quiz!
• Lab.

Preliminaries

Admin

• Note: Every eboard should remain available and linked.
• Friday PSA.
  • Remember that the Romans were paragons of moderate behavior.
  • Alternately: Remember that Rome fell.
  • Please take care of yourselves and your colleagues.

Upcoming Work

• Lab writeup: Exercise 5. (Posted during quiz.) http://bit.ly/151-2015S-lab35
• Reading for Monday: Pairs and Pair Structures
• HW7 due Tuesday. Choose your own partners.

Extra Credit Opportunities

Academic

• Scholars Convocation Wednesday: Alison Bechdel.
• CS Table today: Therac 25.

Peer Support (Morning Section)

• Julia's radio show, "The Hot Box". Wednesday night/Thursday morning 1:00-2:00 a.m.
• KY thinks she's funny Monday from 9-10.
• Host an admitted student! Talk to JMU for information.

Miscellaneous

Other Good Things (no extra credit)

Questions

Quick notes on named let

Situation: Writing a helper procedure using what Sam calls the "column model"

• I want to tally all the numbers in a list.

  tally   remaining
  

• Example: Tally the numbers in the list (list 1 'two "three" 4.0)

  tally   remaining
  -----   ---------
  0       (list 1 'two "three" 4.0) ; first thing is a number
    add 1   throw away the 1
  1       (list 'two "three" 4.0) ; first thing is a symbol
    keep    throw away the 'two
  1       (list "three" 4.0) ; first thing is a symbol
  

• Generalize tally remaining ----- --------- 0 lst

• Turn into code

  (define tally-numbers
    (lambda (lst)
      (letrec ([kernel (lambda (tally remaining)
                         (cond
                           [(null? remaining)
                            tally]
                           ...))])
         (kernel 0 lst))))
  

• This looks significantly different than what we wrote in designing the solution. our solution tied the column names to the initial values; this separates them. As language designers, we can build something better

  (define tally-numbers
    (lambda (lst)
      (let kernel ([tally 0]
                   [remaining lst])
         (cond 
           [(null? remaining)
            tally]
           ...))))
  

Quiz

• Feel free to set up the lab while waiting for your partner.
• Warning! Central campus at noonish may be disturbing.
• A note on the extra credit. Think about the spiral example.

Lab

• Make sure to use a relatively small brush. For example,

  (context-set-brush! "2. Hardness 100" 2)