Class 34: Files

Held: Friday, 31 October 2003

Summary: Today we consider how to make Scheme read values and characters from files and write values to other files.

Related Pages:

• EBoard.
• Sample Code.
• Reading on Files.
• Lab on Files.

Notes:

• No reading for Monday!
• Are there questions on the exam?

Overview:

• Reflection.
• Files and ports.
• Reading values.
• Reading characters and lines.
• Lab.

Reflection on Input and Output

• What did you learn yesterday?
• Why should we care about input and output?
• Your reasons:
  • ...
• Here's one of mine: Once you know the I/O procedures, you can start using files.

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.
• 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)
• For example, here's some code that reads one value from the file data

  (let* (; Prepare to read from a file
         (source (open-input-file "data")) 
         ; Read one value
         (value (read source)))
    ; We're done, so clean up.
    (close-input-port source)
    ; Return the value read
    value)
  

• 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?

Reading All the Values in A File

• Let's consider an example in which we read all the numbers in a file and add them up.
• How do we approach the problem?
• It seems like a case for repetition, so we'll need to ask ourselves our favorite questions about recursion.
• How do we break up the problem?
  • Read the first number.
  • Read the remaining numbers.
• What's the base case?
  • No numbers remain.
• What value do we return in that case?
  • 0, the identity for addition.
• How do we write the recursive call?
  • We'll figure that out later.
• What do we do if the recursive call works correctly?
  • We've read one number. We've summed all the remaining numbers. So ... just add 'em together.
• Okay, now we're ready to worry about some details.
• We don't read from files, we read from ports. Hence, we'll add a husk that converts the file name to a port.
• See the reading for more information.
• We'll also add some error checking.

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
• Let's consider an example in which we read a line of text from a file.
• See the reading for more details.

Lab

• Do the lab on files.