New partners! * Grab a card. * Read it, figure out where you go. * Put the card on the plate so that Evan doesn't have to collect it.

Overview

• Preliminaries.
  • Admin.
  • Upcoming Work.
  • Extra Credit.
• Questions/Ideas.
• Four kinds of repetition.
• Lab.
• Debrief. (If time.)

Preliminaries

Admin

• Extended/modified office hours continue this week and next.
  • Survey: Should I switch to online signup for office hours? (Choices are "Please have online signup" and "Please keep physical signup". You may also choose not to vote.)
• I've been answering a variety of prereg questions. Feel free to continue asking such questions.
• I apologize for missing much of class yesterday. It was an emergency.
• I hear from some of the tutors that some of you are letting your partners carry the weight in your groups. Doing so violates self-gov.
• Review sessions
  • Tonight at 8pm with Alex
  • Tomorrow at 10am with Sam
  • Tomorrow at 8pm with Ajuna
• A great quote from Made to Stick: "Trying to solve a problem beore being taught the solution leads to better learning, even when errors are made in the attempt."

Upcoming Work

• Lab writeup: Problems 2 and 4a. http://bit.ly/151-2014F-w36
• Reading for Friday: Geometric Art
• Exam 3, distributed today, due next Tuesday.
  • Required prologue due Friday night. http://bit.ly/151-2014F-exam3pro
  • Required epilogue due next Wednesday night. http://bit.ly/151-2014F-exam3epi
• Quiz Friday
  • Turtles
  • Iteration
  • Pairs and pair structures
  • You may bring a sheet of notes

Cool Upcoming Events on Campus

• Peace studies talk Monday Nov. 10 at 4:15 p.m. in JRC 101 "With All the World’s Violence, Where is Peace?"

Extra Credit Opportunities

Academic

• CS Extra Thursday: Study-Abroad Opportunities with a CS Component 4:30 p.m., Science 3821.
  • It's never too early to think about studying abroad!
• CS Table Friday: Open Data. 12:10-12:50, Day PDR.

Peer Support

• 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
• Swimming on Saturday (1pm) against Luther.
• The gaming club is hosting a Smash Brothers tournament on Saturday (Loose at 11am ...) (And thats "Loose", not "Lose")
• The Go Club is having its first meeting Sunday 1pm, JRC 203
  • Aimed for complete beginners.

Questions/Recent Ideas/Etc.

When do you use basic recursion and when do you use helper recursion?

Husk-and-kernel does one kind of helper recursion, but I think you mean the question more generally.

No general answer (at least not one that I know of). It's mostly which way makes it easier to think about solving the particular problem.

If I find it "natural" to have one or more extra values during your recursion, I start with helper.

I often end up writing whichever feels easier, and then go back and reflect on whether I might improve.

Looking at/remembering similar kinds of problems helps.

How do we write listp?

A list is either the empty list (i.e., null) or a pair whose cdr is a list.

    (define listp?
      (lambda (val)
        (or (null? val)
            (and (pair? val)
                 (listp? (cdr val))))))

Is the null list a pair?

No, it is the only list that is not a pair.

Four Kinds of Repetition

What's the relationship between the four kinds of somewhat general repetition? (for-each, map, repeat, recursion)`

• Take two minutes to talk to your partner about this question.
  • Identify similarites between pairs (or groups)
  • Identify differences between pairs (or groups)
• Be prepared to share.
• Things you might think about
  • Input types
  • Output types
  • Side effects vs. pure

Things we came up with

• Both for-each and map apply a function to every element in a list
  • map: Goal is to produce a new list
  • for-each: Goal is to have lots of side effects
• repeat does one procedure over and over again
  • With recursion, you do something over and over again
  • Repeat is a fixed number of times
  • recursion uses some "base case test", may be less predictable
• for-each and map do the application once to each element,
  • repeat does it multiple times to the same element
  • recursion could do anything (once, multiple times, no times, ...)
• (repeat 3 proc val) means do it three times
  • while recursion means doing one single function
• Input is a list
  • map, recursion, for-each (not repeat)
• Input is a number (or other non-list)
  • repeat, recursion (not for-each or map)
• Goal is side effects, rather than computing a value
  • repeat or for-each (or maybe recursion, but not map)
• Goal is to produce a list
  • map - if the result is the same length, and fits the model
  • recurision - otherwise

Lab

Debrief