CSC151 2007S, Class 33: File Basics

* I will continue to reserve time at the start of class for discussion of campus issues.

Admin:
* Happy April Fool's Day!
* I distributed notes on exam 2 via email.  Please let me know if you did not get those notes.
* Are there questions on Assignment 7?
* Reading for tomorrow: Pixel Maps.

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

==Focus: Files==

* What is a file?
  * Something saved on your computer
  * "Permanently" stored (at least until your hard drive dies)
  * And that lots of programs can access
* Simple distinction of files:
  * Text files - Contain lots and lots of characters; easy to read
  * Everything else

* Since files are useful, we should be able to process them in Scheme
* To process files, we need
  * Internal representation
  * Basic procedures
  * Standard Scheme techniques

* Can we just refer to files by name?
  (read-from-file "/home/rebelsky/sample-file.txt")
  (read-from-file "/home/rebelsky/sample-file.txt")
* Difficult to decide when read-from-file reads the *next* value and
  when it reads *the first* value
* Scheme's solution: ports
  * Link to a named file
  * PLUS a position in the file
* (read port) reads STARTING AT THE POSITION, and advances
* When you read beyond the last value, read needs to say "Wait, there's
  nothing left"
  * Crash and burn
    "Error: Attempted to read beyond the end of a file."
    Or worse, "Success"
  * Return arbitrary value [BAD]
  * Return special "Sorry, you've hit the end of the file" value [Scheme's choice]

You can also read characters, rather than whole values   
read-char
* Also returns the same special value for end of file

Also peek-char, which reads, BUT DOES NOT ADVANCE THE POSITION