Held: Monday, 2 December 2013
Back to Outline 48 - Merge Sort.
On to Outline 50 - Objects in Scheme.
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
Overview
- File basics: Data persistence and beyond.
- Ports.
- Basic file operations.
Administrivia
- I will reserve time for questions on The Project.
- I will reserve time for questions on Exam 3.
- People who missed the merge sort class are expected to do
lab writeup 29: Problems 6 and extra 1.
- Upcoming extra credit opportunities:
- Learning from Alumni Thursday: Erik Hanson (in person)
- CS Extras Thursday: Multiple Models for Mediascripting (Manuella et al.)
- CS Table Friday: TBD.
- Swim meet Friday/Saturday.
- Any self-care week activity.
- One Grinnell rally on December 4 at 4pm (unless you are taking photos).
- And yes, I've sent a note to Dean Arora about the scheduling.
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
