CSC 151.03, Class 24: Randomness and simulation

Overview

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

News / Etc.

• More snacks
• Introductory videos

Upcoming Work

• Quiz Friday!
  • Recursion
  • letrec
  • Named let
• Writeup for class 23 due TONIGHT at 10:30 p.m.
  • Exercises 4d and 5d.
  • To: csc151-03-grader@grinnell.edu
  • Subject: CSC 151.03 Writeup 23 (YOUR NAMES)
• Writeup for class 24 due Friday at 10:30 p.m.
  • Exercises TBD.
  • To: csc151-03-grader@grinnell.edu
  • Subject: CSC 151.03 Writeup 24 (YOUR NAMES)
• Assignment 6 distributed.
  • Although the assignment says that you have assigned pairs, I will permit you to choose your own partners (or to work in individuals or trios).
• Read Pairs for Friday’s class.

Extra credit (Academic/Artistic)

• Saving Brinton, TONIGHT at 7pm in the Strand.
• Gates Lecture, Wednesday at 7:00 p.m. in JRC 101. Professor Sylvester Johnson, Director of the Humanities, Virginia Tech, talk will examine the twentieth-century roots of contemporary national security responses targeting American Muslims as state enemies.
• Convocation Thursday (11 am in JRC 101).
• Random escape room thingy. See signs and advertisements.
• Protest BOT workshop, Friday 4pm in Burling 1st.

Extra credit (Peer)

• Women’s Soccer vs. Cornell today at 4 p.m.
• Advocates Meet and Greet (with ice cream) Thursday 7pm-9pm in JRC 209.
• Submit to the Grinnell Review (grinnellreview.com)
• Pioneer Weekend in about two weeks: A one weekend innovation challenge. Registration at https://grinnell.co1.qualtrics.com/jfe/form/SV_0oKEvGlV5twGczr

Extra credit (Misc)

Other good things

• Grinnell Singers Sunday at 2 p.m. in Sebring Lewis.
• Star Wars Music Talk Friday at 4pm.
• Swimming and Diving meet Saturday.
• SGA hosts tailgate at 11:30 on Saturday near the grassy knoll.
• Football Saturday.

Questions

Lab

Problem one should look something like the following

(define count-odd-rolls
  (lambda (n)
    (cond
      [(zero? n)
       0]
      [(odd? (roll-a-die))
       ...]
      [else
       ...])))

There are two primary approaches to problem 2, which has the problem of evaluating the same expression twice. (That’s usually bad. In this case, it breaks the program.)

• You could use a let to name the result of the expression. That turns out to be more subtle than you might expect.
• You could write a procedure, seven-or-eleven? that, given an integer input, returns #t exactly when that input is 7 or 11.

Hints on 6

6a: Use map and reduce.

6b: Generate a random number between 0 and 100, inclusive, then check if it’s less than 24, less than 38, …

6c: You should know a pattern for generating lists of values.

6d: No additional help should be necessary.

Debrief

Yes, it is intentional that we’ve asked you to look at two different forms of tallying.

Reminder: Your notes should probably have “Ways to tally” and “Ways to generate lists recursively” and other patterns you’ve been observing.

(define tally-odd
  (lambda (n)
    (cond
      [(zero? n)
       0]
      [(odd? (roll-a-die))
       (+ 1 (tally-odd (- n 1)))]
      [else
       (tally-odd (- n 1))])))

This can be translated fairly straightforwardly to the following.

(define seven-or-eleven?
  (lambda (val)
    (or (= val 7) (= val 11))))

(define tally-7-11
  (lambda (n)
    (cond
      [(zero? n)
       0]
      [(seven-or-eleven? (pair-a-dice))
       (+ 1 (tally-7-11 (- n 1)))]
      [else
       (tally-7-11 (- n 1))])))