---
title: Eboard 13  Local bindings
number: 13
section: eboards
held: 2019-02-22
link: true
---
CSC 151 2019S, Class 13:  Local bindings
========================================

_Overview_

* Preliminaries
    * Notes and news
    * Upcoming work
    * Extra credit
    * Friday PSA
    * Questions
* Quiz
* Lab
* Debrief (maybe)

Preliminaries
-------------

### News / Etc.

* Mentor sessions Wednesday 8-9 p.m., Thursday 8-9 p.m., Sunday 5-6 p.m.
* Warning!  It's Friday the 13th (class).  Bad luck is possible.
* Note: It is a useful habit to reflect back after each exercise and ask
  yourself "_What did the exercise designer want me to take from this
  exercise?_"
* Since it seemed unclear to some of you: Your are not finished when
  you do the lab writeup.  You should look at (and, if possible, do)
  all of the exercises in each lab.
* With four students gone, we may need to reshuffle a bit after the
  quiz.
* I *think* I've now set things up so that the formatted eboard auto-updates 
  every five minutes during class.

### Upcoming work

* There is no reading for Monday!
* [Assignment 5](../assignments/assignment05) due next Tuesday.
    * Your homework partner is your partner from Wednesday's class
* [Flash cards](../flashcards/flashcards06) due Wednesday at 8:00 p.m.
    * Covers Wednesday/Friday/Monday classes
* Quiz next Friday: Randomness and Local Bindings

### Extra Credit

#### Extra credit (Academic/Artistic)

* Iowa Flautists in Concert.  Saturday, 23 February 2019.
  3:45 p.m. Sebring-Lewis Hall.

#### Extra credit (Peer)

* Last Chance Swim and Dive, Saturday at about 10:30 ec.
* Men's BBall, Friday, 3:00 p.m. vs. LFC at St. Norbert.  (Hopefuly a
  Saturday game, too.)
* Indoor Track and Field, Friday and Saturday, at Monmouth.

#### Extra credit (Wellness, Regular)

* 30 Minutes of Mindfulness at SHACS every Monday 4:15-4:45
* Any organized exercise.  (See previous eboards for a list.)
* 60 minutes of some solitary self-care activities that are unrelated to
  academics or work.  Your email reflection must explain how the activity
  contributed to your wellness.
* 60 minutes of some shared self-care activity with friends. Your email
  reflection must explain how the activity contributed to your wellness.

#### Extra credit (Misc)

### Other good things 

### Friday PSA

Be happy.  Be good.  Be well.  Be true to yourself.

Consent is essential.

### Questions

Quiz
----

_If you find that you finish early, take some time to meditate quietly.
Alternately, you may go get a drink of water or use the rest room._

Lab
---

Remember to review the self-checks!  (More generally, remember to
*do* the self-checks!)

Debrief
-------

I can fantasize that we'll have time to debrief, can't I?

_What is the "moral" of the `define`-vs-`let` exercise?_

> Using `define` within another definition does not always behave as
  we expect.  
  
> More broadly, the semantics of `define` are a bit more complex than
  the semantics of `let`.

> Therefore, we recommend that you do *not* use `define` within another
  `define`.  (Use `let` or `let*` instead.)

> However, we must admit that the designers of Racket prefer nested 
  `define` expressions to `let`.

_What were the "morals" of exercise 5?_

> If you are defining values that are independent of the parameters
  to a procedure, the `(let ... (lambda ...))` form is more efficient.

> Sam lied!  Although Sam claimed that the definitions disappear when
  the let finishes, in point of fact, they remain available in the
  body of procedure, just nowhere else!

_How can we simulate that stupid nested `let` with a `map`?_

> How Sam and others might do 2b

        (map (o (section * 7 <>) (section expt 2 <>)) 
                (range 5))
        (map (lambda (x) (* 7 (expt 2 x))) 
             (range 5))

_Can we explore the `define` thing a bit more?_

        ; This doesn't work
        (define sample-w/define
          (lambda (x)
            (define x (+ x 1))
            (list x x x)))

        ; What do you expect for this?
        (define sample-w/define
          (lambda (x)
            (define y (+ x 1))
            (list y y y)))