Class 45: Files in Scheme

Held:

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.*

Preliminaries

Overview

• File basics
• Data persistence
• Ports
• Basic file operations

Related Pages

• Eboard 45 (Source)

• Reading: Files Reading

• Lab: Files in Scheme

Updates

News / Etc.

• Sit with project partners!
• Welcome to any prospectives who are here.
• We’ll try to split class between lab work and project work.

Upcoming Work

• Lab writeup: Exercise 4 (it’s class 45)
  • Title your email CSC 151.01 Writeup for Class 45 (YOUR FULL NAMES).
• Reading for Tuesday: Analyzing Procedures
• Project Proposals due TONIGHT (sketches due Tuesday).
  • Make sure to describe the underlying design goals.
  • Make sure to indicate the three techniques you will use.
  • Make sure to explain how you know you will get 1000 different images.

Extra credit (Academic/Artistic)

• CS Table Tuesday, noon: Algorithmic [sic] bias
• CS Extras Thursday, 4:15pm: Gadfly

Extra credit (Peer)

• AAA hosts Kit Yan performing Queer Heartache, Wednesday, Harris, 7pm.
• Next home baseball game: May 7 (Senior Day)

Extra credit (Misc)

• May 4 town hall on belonging. 11am, May 4, JRC 101

Other good things to do

• Dag Field Day, Saturday, April 29, noon-5pm, in the Club Athletic Field.

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.