CSC151 2007S, Class 37: File Basics

Admin:
* It's your final chance for in-class questions on the exam.  
  * I'll do my best to be available via email tonight.
* EC: Today's Thursday Extra on Scripting Video with Scheme (4:30, 3821).
* Reading for tomorrow: Storing images

Overview:
* Exam questions.
* Data persistence.
* Basic file operations.
* Lab.

=Your Questions on the Exam=

==How do I approach problem 1?==

* Step by step, experimentally plus code analysis
* Break it into smaller problems

* You only need give the six P's for r; short comments about purpose would  
  be useful for c and s.

==How do I build the values for problem 3?==

* Build with list
  (define a-red-disc
    (list "disc" color.red 85 95 70))
* Those interested in doing extra work might define a disc.new procedure
  (define disc.new
    (lambda (color col row radius)
      (list "disc" color colo row radius)))

==How do I compare strings?==

* In particular, how do I ask something like "Is the first element of the
  list the string "disc"?"
(string=? "disc" (car lst))
(equal? "disc" (car lst))
NOT
(= "disc" (car lst))

==What does it mean when I get "illegal function"?==

Almost invariably, that you are using a non-procedure in the procedure
position.

(null lst)
(null null)$a
((+ 2 3) 4)a

How do I track these problems down?
LOOK AT EVERY OPEN PAREN AND CONSIDER THE FOLLOWING THING

==Why do I get the dreaded "Error: Wrong number of parameters" message?==

=My Question on the Exam=

Solved

=Files!=

* Need a way for data to persist
* It's reasonable for programmers, even novice programmers, to know how to
  create and read from files
* Key ideas in Scheme: 
  * Files are named by strings
    "sams-incredbily-important-but-misspelled-picture.img"
  * These file names may, but need not, include a full path name
    "/home/rebelsky/glimmer/samples/junk.txt"
  * If you don't give the full name, it assumes the file is in your
    home directory (if you're lucky)
  * But you don't work directly with files, instead, you work with
    "ports"  
    * File + Position
  * (open-input-file "filename")
  * (open-output-file "filename")
  * Need to be named
  * Need to be closed
  * (close-input-port port)
  * (close-output-port port)