---
title: Eboard 16  Preconditions, revisited
number: 16
section: eboards
held: 2017-09-29
---
CSC 151.03, Class 16:  Preconditions, revisited
===============================================

_Overview_

* Preliminaries
    * Notes and news
    * Upcoming work
    * Extra credit
    * Friday PSA
    * Questions
* Quiz
* Lab

### News / Etc.

* New partners.  
* Please make sure to return your computer cards to the jar.
* I was sorry to see so few of you at the Wilson Okello convo.  It was
  one of the better convocations I've seen.

### Upcoming Work

* [Writeup for class 15](../writeups/writeup15) due TONIGHT at 10:30 p.m.
    * Exercise 5 
    * To: <csc151-03-grader@grinnell.edu>
    * Subject: CSC 151.03 Writeup 15 (YOUR NAMES)
* [Writeup for class 16](../writeups/writeup16) due Monday at 10:30 p.m.
    * Exercise 2
    * To: <csc151-03-grader@grinnell.edu>
    * Subject: CSC 151.03 Writeup 16 (YOUR NAMES)
* [Assignment 5](../assignments/assignment05) due Tuesday at 10:30 p.m.
* Reading for Mondays's class: [Recursion basics](../readings/recursion-basics)

### Extra credit (Academic/Artistic)

* Any of the many Grinnell Prize events.
* CS Table, Tuesday: Tapia.

### Extra credit (Peer)

* ISO Fundraiser for Rohingya crisi in Myanmar, Friday, 5:30 p.m. 
  JRC 209.  (You need not donate to attend.) (Henna tatoos.)

### Other good things

* Women's Tennis vs. Cornell, Noon, Sunday (high-school courts)
* Volleyball vs. Knox Tuesday at 7:00 p.m.
* Volleyball vs. Beloit, next Friday at 7:00 p.m.
* Men's Soccer Wednesday at 4:30 vs. Cornell

### Friday PSA

* Think about your choices, in advance.
* Self gov (individually and collectively)
* Consent is absolutely positively necessary.  But you should strive
  for more than consent.

### Questions

What is the difference between a Boolean and a predicate?
  : Booleans are basic values, like integers, reals, and strings.  The two Boolean values are `#t` and `#f`.  The design of Scheme is such that anything that's not a Boolean is treated as "truish" and equivalent to `#t` in conditioanls.
  : Predicates are procedures that return Boolean values.
  : We put question marks after both predicates and Boolean values to highlight that they are or return true/false.

Can you help with problem 1 on the homework?
  : Sure.  Let's look at what [the problem](../assignments/assignment05) says.

```
(define explore-lengths
  (lambda (fname)
    (let* ([words (file->words fname)]
           [num-words (length words)]
           ; Given a (length frequency) pair, scale the
           ; frequency by the number of words.
           [scale-frequency (lambda (oh)
                              (list "hi" (/ 2 num-words)))])
      (map scale-frequency
           (tally-all (map (section string-ref <> 0) words))))))
```

I'm not sure what to put as the postconditions for `explore-lengths`.
  : Yeah, that's a hard one.  You can't be as precise, but say something
    about the number of bars you see and such.
  : Kind of like the purpose, but perhaps more detail.  Here, the purpose
    is "make a histogram of word frequencies" and the postcondition
    probably explains what each of the two potentially puzzling terms
    mean ("histogram" "frequency")

For problem 1, what should I do about word lengths that aren't included?  I feel like it distorts the results.
  : Ideally, you'd come up with a clever solution.  But it's a rare
    enough case that I do not require you to come up with a solution.

Suppose I have data for a stacked histogram.  How do I figure out the
number of entries in the legend?
  : You have '(("a" (1 2 3)) ("b" ()) ("c" (6 1 3 2)) ("d" (5 1 4)))
  : I can use `(map cadr data)` to extract the lists
  : And so on and so forth
    ```
    > (map cadr data)
    '((1 2 3) () (6 1 3 2) (5 1 4))
    > (map length (map cadr data))
    '(3 0 4 3)
    > (reduce max (map length (map cadr data)))
    4
    > (define len (reduce max (map length (map cadr data))))
    ```
  : We'll suppose that I have a predefined list of legends and colors.
    I can use `(take legends len)` and `(take colors len)`

Can you extend that example?
  : Sure
    ```
    > (define all-labels (list "twas" "brillig" "and" "the" "slithy" "toves" "did" "gyre"))
    > (define all-colors (list "red" "orange" "yellow" "green" "blue" "indigo" "violet" "black" "white" "gray" "purple" "pink"))
    > (define data '(("a" (1 20 8)) ("b" (10 10)) ("c" (3 4 5 6)) ("d" (2))))
    > (define len (reduce max (map length (map cadr data))))
    > (plot (stacked-histogram data
                               #:colors (take colors len)
                               #:labels (take legends len)))
    > (define data '(("a" (1 8)) ("x" ()) ("b" (5 5 5 5 5 5)) ("c" (3 4 5 6)) ("d" (2))))
    > (define len (reduce max (map length (map cadr data))))
    > (plot (stacked-histogram data
                               #:colors (take colors len)
                               #:labels (take legends len)))
    ````

Quiz
----

Lab
---

How do I define city?
  :
    ```
    (define city?
      (lambda (val)
        (and (list? val)
             ...
             (string? (list-ref val 0))
             (real? (list-ref val 1))
             ...)))
    ```

Are there other inputs I should try?
  : `(city? (list "50112" 52 -90))`

Do I have to verify that the zip code is five digits?
  : No.

Debrief
-------