Held: Friday, 22 April 2016

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

• Continue partners!
• I will continue to bring you food (or food-like substances) until I am caught up on grading. (You may be getting food until you graduate from Grinnell.)
  • Today: Fruit!
• Note that there's a grading rubric for the project. Pay attention to it as you work on your proposal and project.

Reminders

• Office hours this week
  • See http://rebelsky.youcanbook.me.
  • Ask me about other available times.
• Tutor hours
  • Sunday, 3-5 p.m.
  • Sunday-Thursday, 7-10 p.m.
• Review Sessions
  • Wednesday at 8pm in CS commons with Zachary
  • Thursday at 10 am with SamR
  • Thursday at 8pm in CS commons with Alex

Upcoming Work:

• No reading for Monday!
  • Monday will be time for you to work with your group on finishing the project proposal.
• Lab Writeup:
  • Send email titled CSC 151 Lab Writeup 44 (Your Names)
  • Do not include the underscores.
  • Send to CSC151-02-grader@grinnell.edu
  • Due before class on Tuesday.
• Project proposals due Monday night.

Extra Credit

• Send your reports to rebelsky@grinnell.edu with subject "CSC 151 Extra Credit". (Do not include the quotation marks.)
• Send opportunities to me before class with subject "CSC 151 EXTRA CREDIT OPPORTUNITY!"

Academic / Artistic

• Tonight at 7pm: Sharon Day, Environmental and Social Justice from the Lens of a Two Spirit Ojiwe Woman, ARC 302
• Any of the fora on the report from the Residential Learning Task Force.
  • Tuesday, April 26, 11:00 a.m. - 12:00 noon, JRC 101
  • Tuesday, April 26, 6:00 p.m. - 7:00 pm, JRC 209
• Scholars' Convocation: Dutch Global Horizons, Thursday, April 28, 11 am, JRC 101
• Damon Williams: Bigger Than The Cops, April 28, 4pm or 5:15 pm

Peer

• Lords of the Flies, 7:30 pm April 22nd, 2:00 pm April 23rd, 2:00 9pm April 24th. (Sunday is the only one with tickets left.)
• Second Annual Contra into Spring Dance, April 22nd, Main Quad, 7:30 p.m.
• The strangly titled "Soul Innoculation" Friday in Herrick at 7pm
• Drag, Saturday. Bring money to give to the performers to give to charity.
• Rushdie's East/West, May 6/7 at 7:30 p.m.

Regular Peer

• Social Dance Workshop Tuesdays 7:00-8:00 in Bucksbaum Dance Studio
• Post-break ExCo on British Politics Wednesdays at 8:00 in JRC 203.
  Just show up; you don't need to sign up.
• Pun club Saturdays at 4pm in Younker
• Electronic Potpourri on KDIC Fridays at Five
• Space Odyssey KDIC Fridays at Six
• Bollywood, Fridays, 7:30-8:30, Younker
• Effective Altruism club, 2:30-3:30 Sundays in JRC 226.

Misc

• Town hall on Substance Use, Sunday, April 24, 1 p.m. and 7 p.m.

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.