CSC151, Class 51: Records

Overview:
* Grouping data
* About records
* Implementing records in Scheme
* An example
* Lab

Notes:
* In-class exam on Friday
  + Optional; Try it and decide after fifteen minutes to stay
    or go.
  + Closed book.  Closed computer.
  + You may bring one page of notes.
* No, you don't have to write a report on your project.
  Hi Alex!
* Questions on take-home exam 3?
* Extra credit for today's Math talk.
* Sorry about "in jokes".

> (less-than? 'a 4)
BUG: a is not a valid thing to compare

> (less-than? "a" 4)
#f  ; same as (less-than? "a" "4")a

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Observation: We regularly group data items together
  Cartoon Hero and his/her sidekick
  ("Asterix" "Obelix")
  ("Fred Flinstone" "Barney Rubble")
  Buy/Sell/Free Board
  (transacation name email category comment price)
  Phone directory
  (last-name first-name department phone-number)
  Nile.com

Disadvantages of this strategy 
* It may be hard to remember what goes where
* It may be hard to remember the type of each thing

Once you've decided on what goes in a data item, you should
provide procedures to fill in and get the values

(define get-hero
  (lambda (hero-and-sidekick)
     (car hero-and-sidekick)))

(define bsf-price
  (lambda (bsf)
    (list-ref bsf 5)))

BSF decided to represent their six-element thingys as lists.
A vector would be better.  

Changing their code could be hard.  Some of the times they
use list-ref, they need to change it to vector-ref.  Some of
the times they use car, they need to change it to vector-ref.

If they had started by writing and using bsf-price and company, 
they'd only have to update those procedures.

Grouping together "this is how I want to represent my data" and
"these are the procedures for manipulating those data" is a
good thing.

Vectors also allow you to change values.
* Second moral.
then update all of our bsf-xxx procedures and everything else
would still work.

Defining a simple record

; Design: A course grade entry is an N-tuple with
;   Department
;   Number
;   Section
;   Prof
;   Grade (numeric)
; Vector of five elements
(define grade-entry-get-department
  (lambda (grade-entry)
    (vector-ref grade-entry 0)))
(define grade-entry-set-department!
  (lambda (grade-entry new-department)
    (if (not (string? new-department))
        (error 'grade-entry-set-department
               "Departments are strings, you bozo!")
        (if (not (member new-department 
                   (list "CSC" "MAT" "ECN" "MUS" ...)))
           (error 'grade-entry-set-department
               (string-append "Sorry, we offer no courses in "
                              department))
                  
        (else
          (vector-set! grade-entry 0 new-department)))))

> (grade-entry-set-department! dans-cs-course 123)
BUG Bozo
> (grade-entry-set-department! dans-cs-course "BUS")
Sorry, we offer no course in BUS

Do the lab!  This means you, too, Alex.

BREAKING NEWS!  STRATEGIC PLANNING COMMITTEE MEETS TOMORROW,
2-6 p.m in Chrystal Basement to select PLANNING DIRECTION

1) Merit scholarships for highly talented students
(probably increasing number of international students)

2) "A Fine Liberal Arts College in a Prairie Setting"  (No
major changes from but some fine tuning of the status quo).

3) Connections with Iowa--Becoming the school of choice
for Iowans

4) Focusing on student needs, desires, and quality of life

5) Focusing on faculty needs, desires, and quality of life

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Bye Alex.  Get better.  Same to all of those who missed class
because you're sick.