CSC151.01 2015S, Class 36: Pairs and Pair Structures
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* New Partners!

_Overview_

* Preliminaries.
    * Admin.
    * Upcoming Work.
    * Extra Credit.
    * Questions.
* Lab.
* Debrief.

Preliminaries
-------------

### Admin

* Grades distributed via email.  
    * Yes, I take comments and corrections.
    * No, you can't now do the labs you missed.  
    * As some of you have discovered, I didn't catch all absences, but 
      I caught many, and absences do affect your grade.
    * I have promised a number of people that I will revisit their exam 2
      grade at the end of the semester and see how much of an impact
      dropping it has had.  I will do so for anyone who asks me during
      finals week.
    * HW grades coming.

### Upcoming Work

* Lab writeup (paper!): Exercises 1 and 3
* Reading for Tuesday: 
    [Trees](../readings/tree-reading.html)
* HW7 due Tuesday at 10:30 p.m.
* Exam 3 distributed Wednesday.

### Extra Credit Opportunities

#### Academic 

* Joe Palca talk Tuesday at 7:30 p.m. in Sebring-Lewis.
* Scholars Convocation Wednesday at noon: Alison Bechdel.
* CS Extras Thursday: ???
* CS Table Friday: ???

#### Peer Support (Morning Section)

* KY's radio show, "We Think We're Funny", 9-10pm Mondays.
  (Remember that Sam thinks he's funny, too, and KY is is from MN.)
* Julia's radio show, "The Hot Box".  Wednesday night/Thursday morning 
  1:00-2:00 a.m.  
* Host an admitted student this coming Sunday night (and the following
  weekend)!  Contact JMU for info.

#### Miscellaneous

_Nothing at present_

### Other Good Things (no extra credit)

* Student research seminar this week.

### Questions

Lab
---

* A call to cons creates a box; a call to null does not
* Draw cons boxes
* When you have a null, put a slash in the box where the null is referenced
* When you have a non-null, draw the value separately and draw an arrow
  TO it.
* Most calls to `list` create pairs.

Debrief
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* Lists of length N have a "spine" with N pairs.
* Why do we do this?
    * Because we will be building more complex structures.
    * Because we want to understand how things work.
    * Because it makes a difference when you start doing some
      stranger things (at least in theolder verseions of Scheme)