Held: Wednesday, 18 November 2015

Back to Outline 43 - Higher-Order Procedures, Revisited. On to Outline 45 - Pause for Breath.

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 44 (Source 44)

Overview

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

Administrivia

• New partners!
• Review sessions this week:
  • TONIGHT, 8pm (Caleb)
  • Thursday, 10am (SamR)
  • Thursday, 8pm (Erin)

Upcoming Work

• Projects: Next Tuesday at 10:00 a.m.
• No Reading for Friday!
• Today's Lab writeup (due Monday)
  • Email to grader-151-01@cs.grinnell.edu
  • Subject: CSC 151 Lab Writeup Class 44 (Your Names)
  • Exercises:

Extra Credit Opportunities

• If possible, send me these in advance.
• Note: You don't get extra credit for supporting yourself.

Academic

• Any visit to the current show in the Faulconer gallery.
• Learning Deconstructed Workshop Series, Tuesdays at 11 in JRC 209.
• Scholars' Convocation Thursday: Cosmic Secrets: Soviet Space Exploration, Censorship, and the Cold War.
• Metropolitan Opera Simulcast, November 21, 11:30 a.m. Lulu by Berg, Harris Cinema. (Warning! It's about four hours long.)
• Saturday Night's Silent Film + Live Score.
• Talk on the Future of Libraries, Monday, noon.

Peer Support

• Translations (a play). 7:30 p.m. Thursday through Saturday, 2:00 p.m. Sunday. Tickets at the Box Office.

Miscellaneous

• Noontime rally, Thursday, in support of the Missouri students and of our own students of color.

Regular Peer Support

• Pals of PALS, pals@grinnell.edu, normally Saturday at 7:45 am (breakfast included; felines only), and Mondays at 4:45. Requires sign up in advance. MORE HELP NEEDED!
• Socrates Cafe', Saturdays, Younker, 2pm.
• Pun Club, Saturdays, 4pm, Way over Younker.

Upcoming Peer Support

• Women's Basketball vs. Beloit, Nov. 28 at 3:00 p.m.
• Women's Basketball vs. St. Norbert, Dec. 5 at 3:00 p.m.
• One-act Festival Dec. 5 & 6.
• Women's Basketball vs. Carroll, Dec. 12 at 3:00 p.m.

Other Good Things (No Extra Credit)

• Jazz Band Wednesday.
• Latin-American Ensemble Thursday.
• Swim Meet Saturday.
• Diwali event Friday.
• Jingle Bell Holiday Friday.

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.