Held: Wednesday, 4 March 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
• Morning EBoard 26 (Source a.m. 26)
• Afternoon EBoard 26 (Source p.m. 26)

Overview

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

Administrivia

• New partners.
• No daytime review sessions on Thursday. Evening review session at 8:00.
• Office hours again next week.
  • http://rebelsky.youcanbook.me

Upcoming Work

• Exam 2.
  • Prologue due Sunday night
  • Due 10:30 p.m. Thursday the 12th.
  • One error discovered already. Keep finding more!
• Reading for Friday:
  • Characters and Strings
• Friday's quiz: Pictures!
  • No, you don't have to get everyone. About 85% correct will earn you full credit.
  • Doing better than me will also earn you extra credit.

Extra Credit Opportunities

Academic

• David Berliner lecture Thursday at 7:00 p.m. in Harris.
• CS Table Friday - Read cartoons!
• Town Hall on Town Halls, noon or 7:30 p.m. on March 11.
• Post-break: April 1 Scholars Convocation on Implicit Bias.

Peer Support (Morning Section)

• Julia's radio show, "The Hot Box". Wednesday night/Thursday morning 1:00-2:00 a.m.
• Orchestra Saturday at 2 p.m. in Sebring-Lewis: Musicals, Show Tunes, and Soloists.

Peer Support (Afternoon Section)

• Belly Dancing Club Wednesday, 7-8 p.m. in Bucksbaum Dance Studio
• Panel at 4:15 Wednesday for National Womens' Week on Women in the Global Economy.
• Men's Tennis Saturday at 9:00 and 4:30 in the Bear Field House.
• Tour Flanagan Theatre to see how it has been transformed for Nobody Got Hurt

Miscellaneous (Extra Credit)

• Fill out the American College Health Association's National College Health Assessment survey. (Yes, your response can be "I filled out the survey.")

Other Good Things (No Extra Credit)

• Panel at 4:15 Wednesday for National Womens' Week on Women in the Global Economy.
• Orchestra Saturday at 2 p.m. in Sebring-Lewis: Musicals, Show Tunes, and Soloists.
• Belly Dancing Club Wednesday, 7-8 p.m.
• Tour Flanagan Theatre to see how it has been transformed for Nobody Gets Hurt.

Too Many Tallies

Here are some of the ways that students wrote irgb-tally-dark. Which do you prefer?

(define irgb-tally-dark
  (lambda (lst)
    (cond
      [(null? lst) 0]
      [(irgb-dark? (car lst))
       (+ 1 (irgb-tally-dark (cdr lst)))]
      [else
       (irgb-tally-dark (cdr lst))])))

(define irgb-tally-dark
  (lambda (lst)
    (if (null? lst)
        0
        (if (irgb-dark? (car lst))
            (+ 1 (irgb-tally-dark (cdr lst)))
            (irgb-tally-dark (cdr lst))))))

(define irgb-tally-dark
  (lambda (lst)
    (if (null? lst)
        0
        (+ (if (irgb-dark? (car lst)) 1 0)
           (irgb-tally-dark (cdr lst))))))

(define irgb-tally-dark
  (lambda (lst)
    (- (length lst) (length (irgb-filter-out-dark lst)))))

(define irgb-tally-dark
  (lambda (lst)
    (length (irgb-select-dark lst))))

(define irgb-select-dark
  (lambda (lst)
    (cond
      [(null? lst) null]
      [(irgb-dark? (car lst))
       (cons (car lst)
             (irgb-select-dark (cdr lst)))]
      [else
       (irgb-select-dark (cdr lst))])))

(define irgb-tally-dark
  (lambda (lst)
    (sum (map (lambda (val) (if (irgb-dark? val) 1 0))))))

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