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
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
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
(define source (open-input-file "..."))
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.
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
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
> (read-char source) #\H > (peek-char source) #\i > (peek-char source) #\i > (read-char source) #\i > (read-char source) #\! > (read-char source) #\newline
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.
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
> (peek-char source) #<eof> > (read-char source) #<eof> > (read-char source) #<eof>
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)) #t
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
#\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)) null (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
Close-input-port is invoked only for its side
> (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) (newline) (display (read-line source)) (newline) (close-input-port source))))
It is also possible to use a one-argument form of the
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
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))) (else (begin (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) result))))
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.
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.
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 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) (begin (if (zero? (remainder dividend trial-divisor)) (begin (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
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
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.
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) (begin (write-char (char-downcase (string-ref str written)) out) (kernel (+ written 1))))))))
Scheme provides the type predicate
can be applied to any object to determine whether it is an input port,
and the analogous predicate
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.
This page may be found at http://www.cs.grinnell.edu/~rebelsky/Courses/CS151/2000F/Readings/files.html
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