CSC 151.01, Class 46: Analyzing Procedures

Overview

• Preliminaries
  • Notes and news
  • Upcoming work
  • Extra credit
  • Questions
• A quick overview
• Lab
• Debrief

News / Etc.

• New partners! (Keep these partners tomorrow.)
• We have the evil Friday at 2:00 p.m. final exam time. But I think I can argue that we can use the Thursday 9:00 a.m. final exam time. https://www.grinnell.edu/about/offices-services/registrar/calendars/exams You could also take it during the other section’s time, Friday at 9:00 a.m.

Upcoming Work

• Lab writeup: TBD.
• Project sketches due NOW
• Reading for Wednesday: Association Lists
• Quiz Friday.
• Projects due next Tuesday.

Extra credit (Academic/Artistic)

• CS Table TODAY, noon: Algorithm [sic] accountability
• CS Extras Thursday, 4:15pm: Project Gadfly

Extra credit (Peer)

• AAA hosts Kit Yan performing Queer Heartache, Wednesday, Harris, 7pm.
• Ooh and Ah at the Water Bottle dress in Dance Ensemble show, Staying with the Trouble. Tickets are available in Bucksbaum box office for Thu, Fri, Sat at 7:30 and Sunday at 2:00.
• LINK, Saturday, 1-4pm. Selling food. Eat!
• Drill Team Show, May 5, at Triple V stables (alternately, W+V stables); the other side of the golf course. Free rides (cars, not pony) to the event!
• Next home baseball game: May 7 (Senior Day)

Extra credit (Misc)

• May 4 town hall on belonging. 11am, May 4, JRC 101
• Raising of Peace Rock today at 11am.

Other good things to do

• Dag Field Day, Saturday, April 29, noon-5pm, in the Club Athletic Field.
• Raging Cow Atlatl event, Saturday, near Softball field.
• Careers in Finance symposium.
• Titular head (?); Tickets today at noon at Bucksbaum.

Questions

Will you open the shades?

Thanks!

A quick overview

• We don’t always think carefully about the “cost” of a procedure. The number of steps; the number of calls. For example, until we looked carefully at the implementation of lists, length seemed “cheap”.
• Amazingly, even knowing those costs, we sometimes mis-estimate.
• To help learn better, we should annotate our programs so that they count calls, and then look at those counts.
• We have magic counters.

1. Create a counter.

   (define cons-counter (counter-new "cons"))
   

2. Write an alternative to the base procedure that updates the counter.

   (define $cons
     (lambda (a b)
    (counter-count! cons-counter)
    (cons a b)))
   

3. Update our procedures that use that procedure to use the new version

(define not-reverse
  (lambda (lst)
    (if (null? lst)
        null
        (cons (car lst) (not-reverse (cdr lst))))))

(define not-reverse
  (lambda (lst)
    (if (null? lst)
        null
        ($cons (car lst) (not-reverse (cdr lst))))))

Question: Could we rewrite length so that it stops when it hits

too many cons cells?

Yes. What a good idea. Next exam.

Lab

How many calls to list-append will there be if we write

(list-append lst 'a)?

(length lst) + 1.

It recurses through the list until the end, plus one more call

What happens in terms of append when we call (list-reverse-1 (0 1 2 3 4 5 6))?

(append (reverse (1 2 3 4 5 6)) (list 0))

Seven calls to append

To compute (reverse (1 2 3 4 5 6)), we have to (append (reverse (2 3 4 5 6)) (list 1))

Six calls to append

To compute (reverse (2 3 4 5 6)), (append (3 4 5 6) (list 2)).

Five calls

…

List of length seven, 1 + 2 + 3 + 4 + 5 + 6 + 7 = 28

Can we generalize? (Will you tell us the story about Gauss?)

Writeup

Debrief