Fundamentals of Computer Science I (CSC-151.02 2000F)


Input ports

When a Scheme program is designed to work with large volumes of data, it is often more convenient for the user to prepare its input in one or more separate files, using an appropriate tool (such as a text editor or a statistical package), than to type the data in as the program is running. The Scheme program itself finds the files containing the data and reads them, without user intervention.

To provide for this possibility, each of Scheme's input procedures can be provided with an extra argument that specifies the input port through which the data will be read in. In theory, any kind of a device that supplies data on demand can be on the other side of the input port, and some implementations of Scheme provide several ways of creating them. However, we'll consider only the default input port, through which data typed at the keyboard are transmitted to a Scheme program interactively, and file input ports, through which Scheme programs read data stored in files.

When DrScheme or MzScheme starts up, it automatically creates the default input port and connects the keyboard to it. This is the input port on which the read procedure normally operates. When the user exits from Scheme, this port is closed as part of the cleanup process.

To read data from a file, however, the programmer must explicitly open an input port and connect that file to it. There is a built-in Scheme procedure to do this: open-input-file takes one argument, a string, and returns an input port to which the file named by the string is connected. For instance, the call

(open-input-file "/home/rebelsky/Web/Courses/CS151/2000F/Examples/hi.dat")

returns an input port to which the named file is connected.

Constructing the input port does you no good unless you give it a name, so open-input-file is almost always invoked within some binding construction, such as a definition or a let-expression:

(define source (open-input-file "..."))

The hi.dat file is a text file that contains one line, consisting of the cheerful greeting Hi there!. One can now access the contents of the file by calling Scheme's build-in input procedures, but giving them the input port source as an argument.

Reading one character at a time

An input port can be used as an argument to two primitive input procedures: read-char, which reads in (and returns) one character from the file on the other side of the input port, and peek-char, which looks through the input port to see what the next character in the file is, and returns that character, but does not actually read it in from the file. The difference is that you can peek at the next character as often as you like, and it remains accessible through the input port, but once you read in a character there is no way to ``un-read'' it -- the port advances inexorably to the next character in the file.

For example, using the source input port that we defined above:

> (read-char source)
> (peek-char source)
> (peek-char source)
> (read-char source)
> (read-char source)
> (read-char source)

Notice that the peek-char procedure peeks through the port to see what the next available character of the file is, and returns the character it sees. The read-char procedure pulls that character in through the port and returns it, leaving the port open with the following character accessible through it.

Finding the end of a file

Scheme automatically provides a sentinel for every file input port it opens. The sentinel is a special value known as the end-of-file object. It is returned by any of the three input procedures when there is nothing left to be read from the file. MzScheme prints the end-of-file object as #<eof>. To continue the preceding example:

> (peek-char source)
> (read-char source)
> (read-char source)

The end-of-file object is not a character, and there is no standard Scheme name for the end-of-file object, but there is a primitive predicate eof-object? that detects it:

> (eof-object? (read source))

As an example of the use of read-char, here's the definition of a procedure called read-line, which reads in characters through a given input port until it reaches the end of the file or encounters a #\newline character, then returns a string containing all of the characters that it has read in:

;;; Read one line of input from a source and return that line
;;; as a string.
;;; Pre: The source is open for reading. [Unverified]
;;; Post: Has read characters from the source (thereby affecting
;;;       future calls to read-char and peek-char)
;;; Returns: A string
(define read-line
  (lambda (source)
    (list->string (read-line-of-chars source))))

;;; Read one line of input from a source and return that line
;;; as a list of characters.
;;; Pre: The source is open for reading. [Unverified]
;;; Post: Has read characters from the source (thereby affecting
;;;       future calls to read-char and peek-char)
;;; Returns: A list of characters.  If we're at the end of the file
;;;          or the line is empty, returns the empty list.
(define read-line-of-chars
  (lambda (source)
    (let ((next (read-char source)))
      (if (or (eof-object? next)
              (char=? next #\newline))
          (cons next (read-line-of-chars source))))))

When all of the data have been read from a file, the programmer should explicitly close the input port by invoking the close-input-port procedure, giving it the input port as an argument. Close-input-port is invoked only for its side effect.

> (close-input-port source)

For example, here's a simple program that prints the first line of a file.

(define firstline
  (lambda (fname)
    (let ((source (open-input-file fname)))
      (display "The first line of ")
      (display fname)
      (display (read-line source))
      (close-input-port source))))

File recursion

It is also possible to use a one-argument form of the read procedure, which pulls a complete Scheme datum through a given input port instead of just one character. It too leaves the port open, with the next character accessible through it.

Here's another example of how to use Scheme's facilities for input from a file. The sum-of-file procedure takes one argument, a string that names a file full of numbers; the procedure opens that file, reads in the numbers it contains one by one, adds each one in turn to a running total, closes the file, and returns the total.

(define sum-of-file
  (lambda (file-name)
    (letrec ((sof-helper
              (lambda (source)
                (let ((nextval (read source)))
                  (cond ((eof-object? nextval) 0)
                        ((number? nextval) 
                         (+ nextval (sof-helper source)))
                           (close-input-port source)
                           (error "The file contains a non-number."))))))))
      (let* ((source (open-input-file file-name))
             (result (sof-helper source)))
        (close-input-port source)

In the base case of the recursion, there are no numbers in the file, and the call to the read procedure immediately returns the end-of-file object. The helper returns 0. The main function closes the file and returns the 0.

If the value of (read source) is a number, it is added to the value of a recursive call to sof-helper, which is the sum of all the subsequent numbers in the file.

If sof-helper discovers a non-number in the file whose contents it is adding up, then one of its preconditions has been violated, and it closes the file and reports the error.

Creating New Files

When a Scheme program generates a lot of output, it is often more convenient to have it store the output in one or more files, instead of displaying it in the window that the interactive interface is using. Other programs can recover the results from such files if further processing is needed.

To provide for this possibility, each of Scheme's output procedures can be provided with an extra argument that specifies the output port through which the data will be written. As before, we'll consider only the default output port -- the interaction box, under DrScheme -- and file output ports, through which Scheme programs write data to files.

If you followed the discussion of input ports, there are few surprises about output ports. The default output port is created when the Scheme interactive interface starts up and closed when it shuts down; in between, Scheme uses this port for most calls to write, display, and newline. To write data to a file instead, the programmer must explicitly invoke open-output-file, which returns a file output port; once this output port is given a name, it can be used as an extra argument to any of the output procedures, with the effect that the values will be written to the file rather than to the interaction window. When no more output is to be written to the file, the programmer must explicitly close the port by invoking close-output-port.

As an example, here's a procedure that takes two arguments -- the first a string that names the output file to be created, the second a positive integer -- and writes the exact divisors of the positive integer into the specified output file:

(define store-divisors
  (lambda (file-name dividend)
    (let ((target (open-output-file file-name)))
      (let kernel ((trial-divisor 1))
        (if (< dividend trial-divisor)
            (close-output-port target)
              (if (zero? (remainder dividend trial-divisor))
                    (write trial-divisor target)
                    (newline target)))
              (kernel (+ trial-divisor 1))))))))

Not-so-surprisingly, Scheme doesn't let you call open-output-file using a file that already exists. To enable the programmer to test the precondition for open-output-file, DrScheme supplies a file-exists? predicate, which takes a string as argument and returns #t if it is the name of an existing file and #f if it is not. It also supplies a delete-file procedure that takes a string as argument and tries to annihilate the file that it names (if there is such a file). Neither of these procedures is standard, however, so other Scheme implementations do not always provide them.

Writing one character at a time

Besides write, display, and newline, Scheme provides a primitive procedure write-char that is used to create an output file one character at a time. It takes two arguments, the character to be written and the output port through which it is to be sent.

The following procedure uses write-char to write a given string to a file through a given output port, converting any upper-case letters to lower case en route:

(define display-string-in-lower-case
  (lambda (str out)
    (let ((len (string-length str)))
      (let kernel ((written 0))
        (if (< written len)
              (write-char (char-downcase (string-ref str written)) out)
              (kernel (+ written 1))))))))

Miscellaneous facilities

Scheme provides the type predicate input-port?, which can be applied to any object to determine whether it is an input port, and the analogous predicate output-port?.

The current-input-port procedure, which takes no arguments, returns the default input port, in case you want to give it a name, pass it as an argument to a procedure that expects a port, and so on. Similarly, the current-output-port procedure takes no arguments and returns the default output port.

It is a bad idea to attempt to close the default ports. The best thing that can happen is that whatever implementation of Scheme you're using will ignore the attempt or report it as an error.

Disclaimer Often, these pages were created "on the fly" with little, if any, proofreading. Any or all of the information on the pages may be incorrect. Please contact me if you notice errors.

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