Held: Monday, 6 April 2015

Back to Outline 35 - Geometric Art Through Numeric Recursion. On to Outline 37 - Trees.

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
• Morning EBoard 36 (Source a.m. 36)
• Afternoon EBoard 36 (Source p.m. 36)

Overview

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

Administrivia

• New Partners!
• 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.

Upcoming Work

• Lab writeup (hardcopy): Exercises 1 and 3
• Reading for Tuesday: Trees
• HW7 due Tuesday at 10:30 p.m..

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.
• How an admitted student this coming Sunday night! Contact JMU for info.

Peer Support (Afternoon Section)

• EE (and SGAVPAA and IT) talk Tuesday at noon in JRC 101.
• Wednesday at 4:30 in ???. Psychology Alumni Job Talk Series, Alum '##, a Senior Research Analyst from ACT.
• How an admitted student this coming Sunday night! Contact JMU for info.
• Many things the following weeks.

Miscellaneous

Nothing at present

Other Good Things (no extra credit)

• Student research seminar this week.

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.