Class 53: File Basics

Held: Monday, 7 December 2009

Summary: We consider files, a technique for structuring information that permits the information to persist across invocations of Scheme. Files also let our Scheme programs share information with other programs.

Related Pages:

• EBoard.
• Lab: Files.
• Reading: Files.

Notes:

• Reminder: Attendance is expected this week!
• EC checklist distributed via email. Let me know if I missed anything.
• I will reserve time at the start of class for questions on the exam.

Overview:

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

Why Use Files?

• As I hope you've figured out by now, it is possible (although not necessarily easy) to use Scheme to do anything you can do on the computer.
• Two similar things that you often want to do are to save data to files and to recover data from files.
• Why?
  • So that data can last a long time.
  • So that you can deal with more data than you can easily enter by hand.
  • So that you can write a word processor.
  • ...
• As you might guess, you can do both activites with Scheme.

Ports

• Rather than dealing directly with files, Scheme adds a layer of abstraction called a port.
• Each port is associated with something that can be used for input or output.
  • That thing can be a file.
  • That thing can also be the keyboard (for input), the screen (for output), or a network connection.
• Why do we have ports?
  • So that the process of writing anywhere (or reading anywhere) is the same; our code doesn't need to change.
  • So that we can read from the same file more than once simultaneously and not get lost about where we are in the file.
• In effect, an input port has two parts:
  • a link to the file that it is associated
  • a cursor that indicates where in the file the next read should take place.
• To create a port that corresponds to a file that you want to read from, use (open-input-file file-name).
• To create a port that corresponds to a file that you want to write to, use (open-output-file file-name).
• You can read from input ports with (read port)
• You can write to ports with
  • (newline port)
  • (write value port)
  • (display value port)
• When you're done with an input port, use (close-input-port port)
• When you're done with an output port, use (close-output-port port)
• What does read do when there's nothing left in the file? It returns a special value (which DrScheme displays as #<eof>).
• You can tell that that value indicates the end of the file with eof-object?

Characters

• You can read a character at a time (rather than a value at a time) using (read-char port).
• Why is this useful? It lets you be more general than the basic read procedure.
• You can also look at the next character that you're about to read using (peek-char port)
• Why is this useful? Sometimes you want to read differently based on what you see next.
  • If you see a semicolon, read the whole line as a string and ignore it.
  • If you see anything else, read it as a Scheme value
• When read-char encounters the end of the file, it returns the same special value as read

Lab

• Do the lab on files.