Held: Friday, 1 May 2015

Back to Outline 50 - Merge Sort. On to Outline 52 - Project Assessment: Images.

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

• Morning EBoard 51 (Source a.m. 51)
• Afternoon EBoard 51 (Source p.m. 51)

Overview

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

Administrivia

• Next week has no new material.
  
  • On Monday and Tuesday, we will go over your projects.
  • On Wednesday, we will review what we've learned (or might have learned).
  • On Thursday, we will use review sessions to go over the sample final.
  • On Friday, you will evaluate the course, we will talk about the final, and I'll talk about the course.
• Don't forget to grab a yellow card.
• Friday PSA.

Upcoming Work

• Exam 4 prologue due tonight!
• Exam 4 due in electronic form on Monday night.
• Exam 4 epilogue due Monday night.
• Extensions are available.
• No more readings!

Extra Credit Opportunities

Academic

• CS Table today, Last CS Table of the Year: Planning for next year

Peer Support (Morning Section)

• KY's radio show, "We Think We're Funny", 9-10pm Mondays
• Julia's radio show, "The Hot Box". Wednesday night/Thursday morning 1:00-2:00 a.m.
• Baseball, Saturday, Noon and 2:30, Sunday 10:00 and 12:30.
• Cheer for Nick R in the triathalon bike section.
• Latin American Music Ensemble Friday in Sebring-Lewis at 7:30pm
• Quinceañera at a date and time still to be determined?

Peer Support (Afternoon Section)

• Baseball, Saturday, Noon and 2:30, Sunday 10:00 and 12:30.
• Triathalon on Saturday
  • Participate!
  • Cheer! is acceptable too
• Quinceañera at a date and time still to be determined.

Miscellaneous

Other Good Things (no extra credit)

• Symphonic band Saturday at 2pm

Notes on Exam 4

We'll do an example of binary search on the animals tree. (We'll use the animals, rather than the letters.)

We'll do an example of the advanced flattening algorithm on a generated tree. Yes, you may take a photo after class. You should probably take notes, too.

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.