CSC151 2009F, Class 53: File Basics

Admin:
* Reminder: Attendance is expected this week!
  * Tomorrow: What is CS? Revisited
  * Wednesday: What did we learn this semester
  * Friday: Evaluate the class.  Thoughts on final.  Final thoughts.  
    Return exam.  Tenative grades.
* EC for
  * Marriage Equality Forum, Tonight at 7pm in JRC 101
* EC checklist distributed via email.  
  * Let me know if I missed anything.
  * Get it to me soon!
* I will reserve time at the start of class for questions on the exam.

Overview:
* Exam Questions.
* File basics: Data persistence and beyond.
* Ports.
* Basic file operations.
* Lab.

Exam Questions
* How do I swap the elements in a vector at positions x and y
  * This won't work
    (vector-set! vec x (vector-ref vec y))
    (vector-set! vec y (vector-ref vec x))
  * Why not?  Think about it.
  * So, we need to remember the values
    (let ((valx (vector-ref vec x))
          (valy (vector-ref vec y)))
      ...)
* Can I build a new vector when reversing?
  * No.  Your goal is to reverse in place.
* Can you give me a hint as to how to write lookup-in-dictionary using
  binary-search
(define lookup-in-dictionary
  (lambda (word dictionary)
    (let ((result (binary-search _____)))
      ...)))

Today's topic: Files
* A file is a collection of data, stored somewhere on the computer.
* Files persist between invocations of a program.
* Files can be shared between multiple programs.
* Most languages include a way to work with files.

* We can think of files two ways:
  * Just the name (a string)
    "/home/rebelsky/Desktop/stalkernet.jpg"
  * As a data type that we're working with
    * Also has a sense of where in the file we are. (cursor)
    * A file plus a cursor is a "port"

Operations:
* From file name to port
  (open-input-file FILENAME) returns a port
  (open-output-file FILENAME) returns a port
* Indicate that we're done
  (close-input-port PORT)
  (close-output-port PORT)
* Reading from input ports
  (read-char PORT)
  (read PORT)
* Writing to output ports
  (display VALUE PORT)
  (newline PORT)
  (write PORT)
  (write-char PORT)

> (display "Wake up Sam!" PORT)
Puts 
  Wake up Sam!
into the output file
> (write "Wake up Sam!" PORT)
Puts
  "Wake up Sam!"
into the output file

> (write (cool-proc param1) PORT)
Writes the result of calling cool-proc on param1