Held: Friday, 6 November 2015

Back to Outline 36 - Randomized (Unpredictable) Drawing. On to Outline 38 - Vectors.

Summary

We consider pairs, the basic data type used to build lists and other structures in Scheme. We also consider why it is useful to understand about pairs.

Related Pages

• Reading: Pairs and Pair Structures
• Lab: Pairs and Pair Structures
• EBoard 37 (Source 37)

Overview

• Representing lists: Pairs and cons cells.
• Why care about the underlying representation?

Administrivia

• Continue partners!
• Review sessions next week:
  • Monday, 8pm (Zachary)
  • Wednesday, 8pm (Caleb)
  • Thursday, 10am (SamR)
  • Thursday, 8pm (Erin)
• I've added MTWF at 1pm to my office hours.
• It's almost preregistration time. Should you continue in CS? I hope many of you will.
  • Are you enjoying CS? (And why are you enjoying CS?)
  • Can you spend the time on CS?
  • Are you likely to succeed? (I'd be happy to discuss this with you.)
• I will get exams back to you by Monday of week 2 of prereg, so that you can think about likely success.
• Friday PSA

Upcoming Work

• Reading for Monday: Vectors
• Today's Lab writeup (due before class Tuesday)
  • Email to grader-151-01@cs.grinnell.edu
  • Subject: CSC 151 Lab Writeup Class 37 (Your Names)
  • Exercises:

Extra Credit Opportunities

• If possible, send me these in advance.

Academic

• Any visit to the current show in the Faulconer gallery.
• Learning Deconstructed Workshop Series, Tuesdays at 11 in JRC 209.
• Any activity with Visiting Musician and Sound Artist Ellen Fullman. (Yes, that includes Drinks @ Lonnski's on Thursday.)
• Sexual Misconduct at Grinnell College: Results from the 2013 and 2015 Sexual Conduct Surveys. 1-3 pm, November 8, JRC 101.
• CS Table next Tuesday.
• CS Extras next Thursday.

Peer Support

• EC for ISO Food Bazaar. Nov. 15, 5-7 pm, Harris Gym/Concert Hall.
  • Sam will advance/reimburse the $10 if you feel that it's a financial stretch.
  • Tickets are on sale today, 11-1, 5-7.
• Alumni Game for Womens Basketball 1pm on Saturday.

Regular Peer Support

• Pals of PALS, pals@grinnell.edu, normally Saturday at 7:45 am (breakfast included; felins only), and Mondays at 4:45. Requires sign up in advance. MORE HELP NEEDED!
• Socrates Cafe', Saturdays, 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, 1 pm, Nov. 15, vs. Silver Lake.
• One-act Festival Dec. 5 & 6.

Cons Cells

• One set of interesting values in Scheme are the things that cons creates. We often call these cons cells or pairs
• The first value of a pair is a reference to the first parameter to cons.
  The second value is a reference to the second parameter.
• I'll also draw some pictures of these things in class.

Dotted Pairs

• Although we've used lists as the second parts of each cons cell, you can use other values. If the second part of a cons cell is not a list, you get what is called a dotted pair.
• The shorthand for a dotted pair is '(x . y).
• You'll also see the dot at the end of a long sequence of cons cells.
• Why is the dot there?
  
  • To distinguish things close to lists from lists.
• Why is it right before the last element, and not earlier?
  
  • Because we don't know until we hit the end that it's not a list.

Why Care?

• Why should we care about the underlying structure used for lists?
• Because it helps explain why Scheme does some things the way it does.
• Because it helps us understand the hidden costs associated with some operations.
• Because it helps us better understand recursion over lists.
• Because it provides background for future discussions.

Lab

• Any preliminary questions?
• Do the labthe lab.
• Be prepared to reflect.