Overview

• Preliminaries.
  • Admin.
  • Upcoming Work.
  • Extra Credit.
  • Questions.
• Lists, revisited.
• Lab.

Preliminaries

Admin

• New partners!
• Office hours
  • Normal (MThF 1:45-3:15; walking 1:15-1:45)
  • In the Grill 8:00-9:30 Wed and Fri
  • In my office 8:00-9:30 Thu
• Review sessions Wednesday and Thursday at 8pm (led by Evan)

Upcoming Work

• Writeup (due Fri): Problem 4, http://bit.ly/151-2014F-w23
• HW 5 due tonight.
• Exam 2 will be distributed tomorrow.
• Reading for Wednesday:
  • Recursion Basics
  • We will spend at least two days on this topic, so don't worry if it seems hard.

Cool Things Coming to Campus

• Two versions of Anna Christie!
• Live Streaming of Verdi's Macbeth, Saturday at 11:55 a.m.
• Pop-up Exhibit, Thursday October 9th, all day in BCA 132

Extra Credit Opportunities

Academic

• Grinnell Prize events this week (particularly events related to the prize winners whose project involves Web tools)
• Artist Talk, Wednesday October 8th at 4:15pm in BCA 152 Rewriting the Application Programming Interface (API) for Art Making
• BCC Faculty Chat, Tuesday, October 14, 7pm

Peer Support

• Football (Saturday at 1pm)
• Swimming October 18: Alumni Weekend
• Karan's radio show 11pm Thursday nights on KDIC
• Evan's radio show 5pm Friday nights on KDIC
• Donna's radio show Sunday midnight on KDIC
• The College's version of Anna Christie, Oct. 9-12 (SB plays Marthy)
• Men's Tennis (???)
• Women's Tennis (???)

Questions

• Note that I tend to respond more quickly to email questions than questions posted to the form.

Could you explain Problem 2 on Assignment 5?

Please explain Euclidean distance

Requires whiteboard. (Sam will try to transcribe.)

Lists, revisited

• Original model: Homogeneous collections
  • Make them with list, make-list, iota
  • Build new lists from old with map
  • Join lists with append
  • reverse - obvious
  • list-take and list-drop - build a new list by taking the first N elements or dropping the first N elements
• New model: Lists are collections that may be heterogenous and that you might decompose
  • car - get the first element
  • cdr - get all but the first element
  • cadr - get the second element
  • cons - given a value and a list, build a new list by shoving a value on the front
  • list-ref - given a list and a number, extract the particular value from the list
• We can now work with individual elements, rather than doing the same thing to every value

Is there a way to put an element at the end of a list?

No built-in one, but you can probably figure out some strategy.

    (append lst (list val))

    (reverse (cons val (reverse lst)))

How do you get the last element in a list?

    (list-ref lst (+ -1 (length lst)))

    (car (reverse lst))

Lab

How do we refer to the empty list?

Any of the following

    null
    '()
    (list)
    (make-list 0 "anything")

I prefer null.

Why do the indices start with 0 rather than 1?

Computer scientists like to start counting with 0.

It lets you think of (list-ref lst i) as "take the cdr i times, then take the car"

What would lead you to name three lists one-two, won-too, and want-to?

Lack of sleep.

Congratulations, you are the last class that will ever have to deal with these stupid names.

Why did we have that exercise?

To reinforce that "if you ssee a period, it's not a list"

What do you think of the following code to find element 5 using cdr and car?

    ((o car cdr cdr cdr cdr cdr) letters)

Creative.

What do you think of the following for rectangularize?

    (define rectangularize 
      (lambda (drawing)
        (list 'drawing 'rectangle
              (list-ref drawing 2)
              (list-ref drawing 3)
              (list-ref drawing 4)
              (list-ref drawing 5)
              (list-ref drawing 6)
              (list-ref drawing 7))))

My, that's a lot of work.