Held: Wednesday, 9 March 2016

Back to Outline 26 - Recursion Basics, Continued. On to Outline 28 - Recursion with Helper Procedures.

Summary

We revisit preconditions. We then consider programming techniques for ensuring that preconditions are met.

Related Pages

• Reading: Verifying Preconditions
• Lab: Verifying Preconditions
• EBoard 27 (Source 27)

Overview

• Verifying preconditions.
• The error procedure.
• Husk and Kernel programming.

Administrivia

• Continue partners.
• We'll do the recursion lab for the first half of class and the preconditions lab for the second half of class.

Reminders

• Office hours this week
  • See http://rebelsky.youcanbook.me.
  • I should be gone next Monday and Tuesday :-(
• Tutor hours
  • Sunday, 3-5 p.m.
  • Sunday-Thursday, 7-10 p.m.
• Weekly review sessions:
  • Wednesday at 8pm in the CS Commons with
  • Thursday review session this week with Sam.
  • Thursday at 8pm in the CS Commons with

Upcoming Work:

• Reading for Wednesday * Verifying Preconditions
• Exam 2 due NEXT TUESDAY.
• Quiz Friday: Recursion basics.
• Lab Writeup: Exercise _
  • Send email titled CSC 151 Lab Writeup 26 (Your Names)
  • Do not include the underscores.
  • Send to CSC151-02-grader@grinnell.edu
  • Due before class on Monday.

Extra Credit

• Send your reports to rebelsky@grinnell.edu with subject "CSC 151 Extra Credit". (Do not include the quotation marks.)
• Send opportunities to me before class with subject "CSC 151 EXTRA CREDIT OPPORTUNITY!"

Academic / Artistic

• Friday at 4:00 pm. in Burling: Decolonizing Digital Humanities

Peer

• The Tempest. This weekend. 7:30 Friday and Saturday, 2:00 Sunday. Tickets in Bucksbaum box office.
• Salseros Friday at 8:30 pm in Garnder. Food! The JS non-brothers, sighing deeply.
• 8:00-10:00 show with various cool jazz-funk-pop at PC. No free food.
• Sunday, 6:00-8:00 AAA Potluck in JRC 209. Free ingredients!

Regular Peer

• Social Dance Workshop Tuesdays 7:00-8:00 in Bucksbaum Dance Studio
• Pun club Saturdays at 4pm in Younker
• Electronic Potpourri on KDIC Fridays at Five
• Space Odyssey KDIC Fridays at Six

Misc

Far in the Future

• Lords of the Flies, April.

No Extra Credit, But Still Good

• Indoor track nationals.

Preconditions

• Remember: Documentation is a contract, be careful.
• Even if you don't write documentation (or don't write careful documentation), you should reflect on your expectations for parameters.
• We call those expectations the preconditions of the procedure.
• In some cases, you may want to explicitly test to ensure that the preconditions are met.
  • Testing lets you provide more useful error messages.
  • Testing prevents dangerous results from being returned.

Reporting Failure

• When a precondition fails, you want to stop computation immediately and report the error.
• In Scheme, you can do so with error or throw
  • The one to use depends on the version of Scheme you are using.
  • Our Scheme interpreter uses error

Husk and Kernel Programming

• Particularly for recursive procedures, it is inefficient to check preconditions at every recursive call
  • If the preconditions were met for the first call, they should be met for every subsequent call.
• Hence, programmers tend to use what I refer to as "Iowa's Great Contribution to Programming": The Husk-and-Kernel approach
  • The husk checks the preconditions and, if all preconditions are met, calls the kernel.
  • The kernel does the real work.
• Corn serves as the metaphor: The husk protects the kernel, and the kernel is the valuable part.
  • And no, Husk-and-Kernel programming was not invented in Iowa.

Lab