Held: Monday, 12 October 2015

Back to Outline 25 - Recursion Basics, Continued. On to Outline 27 - Characters and Strings.

Summary

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

Related Pages

• Reading: Verifying Preconditions
• Lab: Verifying Preconditions
• EBoard 26 (Source 26)

Overview

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

Administrivia

• New partners!
• Review sessions this week
  • Monday 8pm (Zachary)
  • Wednesday 8pm (???)
  • Thursday 8pm (???)
• I will be AFK (Away from Keyboard) starting about noon on Tuesday. If you have exam questions, ask them before then.
• I sent out notes on the prologues from exam 2. If you don't regularly read your email, you should check for those notes.
• I'll demo the first problem on quiz 6.
• I'll demo an "interesting" use of recursion.

Upcoming Work

• Reading for Tuesday: Characters and Strings
• Exam 2
  
  • Exam due in electronic form Tuesday night
  • Exam due in hardcopy Wednesday class time
  • Epilogue due Wednesday night
• Today's Lab writeup (due before class Wednesday)
  • Email to grader-151-01@cs.grinnell.edu
  • Subject: CSC 151 Lab Writeup Class 26 (Your Names)
  • Exercises:

Extra Credit Opportunities

• If possible, send me these in advance.

Academic

• Any visit to the current show in the Faulconer gallery
• CS Table Tuesday, noon, White Dining Room, THERAC 25 (C)

Peer Support

• Pals of PALS, pals@grinnell.edu, normally Saturday at 7:45 am (breakfast included), Sunday at 4:45, and Mondays at 4:45. Requires sign up in advance.
• Pun Club, Saturdays, 4pm, Way over Younker.
• Study Abroad Session, Monday the 12th 7-8pm in JRC 101. Hear about Study Abroad Experiences and think about Study Abroad.

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