---
title: Eboard 24  Randomness and simulation
number: 24
section: eboards
held: 2017-10-25
---
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](../writeups/writeup23) 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](../writeups/writeup24) 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](../assignments/assignment06) 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](../readings/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))])))
```