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Summary:
Files permit you to save values between invocations of programs and to
provide information to programs without typing the information interactively.
In this reading, we explore key ideas in the use of files within Scheme.
Contents:
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.
Similarly, 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.
Note that files therefore let us store values between invocation of Scheme
programs (and other programs). This permanence is another benefit of
using files.
Here we consider the techniques used in Scheme to read data from files
and to write data to files.
Scheme provides two basic operations for reading from files, read
and read-char. The read procedure reads the
next Scheme value from the given file and returns it as a Scheme value.
The read-char procedure reads the next character from the file
and returns that character.
For example, if the file began with 23512 11 13,
the first two values returned by the read procedure would
be the integers 23512 and 11, while the first
two values returned by the read-char procedure would
be #\2 and #\3.
These procedures take as their argument an 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 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/CS151/2007S/Examples/sample.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 either named explicitly (e.g.,
with define or some variant of let)) or used as the parameter to
procedure call that expects a port.
(define source
(open-input-file "..."))
or
(let ((source (open-input-file "...")))
...)
or
(define helper
(lambda (source)
...))
(helper (open-input-file "..."))
When you're done with a port, you should make sure to close it again with
close-input-port. To finish the examples above,
; Prepare to read from a file.
(define source
(open-input-file "..."))
; Read some parts of the file.
...
; We're done, so clean up.
(close-input-port source)
or
(let ((source (open-input-file "..."))) ; Prepare to read from a file
; Read some or all of the file.
...
; We're done, so clean up.
(close-input-port source))
As the example above suggests, an input port is often used as an argument
to read-char, which reads in (and returns) one character
from the file on the other side of the input port. It can also be used as
parameter to 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.
The file /home/rebelsky/Courses/CS151/2007S/Examples/hi.dat
text file that contains one line, consisting of the cheerful greeting
Hi there!. Let us see what happens when we read from
this file using read-char.
> (define source
(open-input-file "/home/rebelsky/Web/Courses/CS151/2001S/Examples/hi.dat"))
> (read-char source)
#\H
> (peek-char source)
#\i
> (peek-char source)
#\i
> (read-char source)
#\i
> (read-char source)
#\space
> (close-input-port 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.
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. DrScheme prints the
end-of-file object as #<eof>. To continue the
preceding example,
> (define source
(open-input-file "/home/rebelsky/Web/Courses/CS151/2007S/Examples/hi.dat"))
> (read-char source)
#\H
> (read-char source)
#\i
> (read-char source)
#\space
> (read-char source)
#\T
> (read-char source)
#\h
> (read-char source)
#\e
> (read-char source)
#\r
> (read-char source)
#\e
> (read-char source)
#\!
> (read-char source)
#\newline
> (peek-char source)
#<eof>
> (read-char source)
#<eof>
> (read-char source)
#<eof>
> (close-input-port 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-char 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
encounters a #\newline character, then returns a string
containing all of the characters that it has read in:
;;; Procedure:
;;; read-line
;;; Parameters:
;;; source, an input port
;;; Purpose:
;;; Read one line of input from a source and return that line
;;; as a string.
;;; Produces:
;;; line, a string
;;; Preconditions:
;;; The source is open for reading. [Unverified]
;;; Postconditions:
;;; Has read characters from the source (thereby affecting
;;; future calls to read-char and peek-char).
;;; line represents the characters in the file from the
;;; "current" point at the time read-line was called
;;; until the first end-of-line or end-of-file character.
;;; line does not contain a newline.
(define read-line
(lambda (source)
; Read all the characters remaining on the line and
; then convert them to a string.
(list->string (read-line-of-chars source))))
;;; Procedure:
;;; read-line-of-chars
;;; Parameters:
;;; source, an input port
;;; Purpose:
;;; Read one line of input from a source and return that line
;;; as a list of characters.
;;; Produces:
;;; chars, a list of characters.
;;; Preconditions:
;;; The source is open for reading. [Unverified]
;;; Postconditions:
;;; Has read characters from the source (thereby affecting
;;; future calls to read-char and peek-char).
;;; chars represents the characters in the file from the
;;; "current" point at the time read-line was called
;;; until the first end-of-line or end-of-file character.
;;; chars does not contain a newline.
(define read-line-of-chars
(lambda (source)
; If we're at the end of the line or the end of the file,
; then there are no more characters, so return the empty list.
(cond
; If we're at the end of the file, there are no more characters,
; so return the empty list.
((eof-object? (peek-char source)) null)
; If we're at the end of the line, we're done with the line
; skip over the end-of-line character and return the empty list.
((char=? (peek-char source) #\newline) (read-char source) null)
; Otherwise, read the current character, read the remaining
; characters, and join them together.
(else (cons (read-char source) (read-line-of-chars source))))))
There are many things we can now do with these procedures.
For example, here's a simple procedure that takes a file name as
an argument and prints the first line of a file.
;;; Procedure:
;;; first-line
;;; Parameters:
;;; file-name, a string that names a file.
;;; Purpose:
;;; Reads and displays the first line of the file.
;;; Produces:
;;; Absolutely nothing.
;;; Preconditions:
;;; There is a file by the given name.
;;; It is possible to write to the standard output port.
;;; Postconditions:
;;; Does not affect the file.
;;; The first line of the named file has been written to
;;; the standard output.
(define first-line
(lambda (file-name)
(let ((source (open-input-file file-name)))
(display "The first line of ")
(display file-name)
(newline)
(display (read-line source))
(newline)
(close-input-port source))))
It is also possible to read from a file using the one-argument form of
the read procedure, which pulls a complete Scheme datum
(instead of just one character) through a given input port. It also
leaves the port open, with the next character or Scheme datum accessible
through it.
Consider, again, the file described above with the form
If we were to work with this file using read-char, we
would see a sequence of values like the following
> (define source
(open-input-file "/home/rebelsky/Web/Courses/CS151/2007S/Examples/sample.dat"))
> (read-char source)
#\2
> (read-char source)
#\3
> (read-char source)
#\5
> (read-char source)
#\1
> (read-char source)
#\2
> (read-char source)
#\space
> (read-char source)
#\1
> (read-char source)
#\1
> (read-char source)
#\space
> (read-char source)
#\1
> (read-char source)
#\3
> (read-char source)
#\newline
> (read-char source)
#<eof>
> (close-input-stream source)
If, however, we were to use read, we would see the following
sequence.
> (define source
(open-input-file "/home/rebelsky/Web/Courses/CS151/2007S/Examples/sample.dat"))
> (read source)
23512
> (read source)
11
> (read source)
13
> (read source)
#<eof>
> (close-input-stream source)
Whether you use read or read-char depends on
your particular application.
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.
;;; Procedure:
;;; sum-of-file
;;; Parameters:
;;; file-name, a string that names a file.
;;; Purpose:
;;; Sums the values in the given file.
;;; Produces:
;;; sum, a number.
;;; Preconditions:
;;; file-name names a file. [Unverified]
;;; That file contains only numbers. [Unverified]
;;; Postconditions:
;;; Returns a number.
;;; That number is the sum of all the numbers in the file.
;;; Does not affect the file.
(define sum-of-file
(lambda (file-name)
(let kernel ((source (open-input-file file-name))) ; named let
(let ((nextval (read source)))
(cond
; Are we at the end of the file?
; Then stop and return 0 for "no numbers read".
; Here, we're taking advantage of 0 being the arithmetic identity.
((eof-object? nextval) (close-input-port source) 0)
; Have we just read a number?
; If so, add it to the sum of the remaining numbers.
((number? nextval) (+ nextval (kernel source)))
; Hmmm ... not a number. Skip it.
(else (kernel source)))))))
In the base case of the recursion, there are no numbers left in the
file, and the call to the read procedure immediately
returns the end-of-file object. The helper closes the file and
returns 0.
If the value of (read source) is a number, it is added to the
value of a recursive call to the helper, which is the sum of all
the subsequent numbers in the file.
If the helper discovers a non-number in the file whose contents it is
adding up, then we skip it. (Ideally, we would report an error, but
this seems safest given our knowledge to this point.)
To provide for the possibility of having Scheme create files and write
data to those files, each of Scheme's output procedures can be provided
with a parameter 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, you should encounter 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:
;;; Procedure:
;;; store-divisors
;;; Parameters:
;;; file-name, a string that names a file
;;; dividend, a natural number
;;; Purpose:
;;; Compute all the divisors of dividend and store them
;;; to the named file.
;;; Produces:
;;; Nothing. That is, it returns no values. It does
;;; create a file.
;;; Preconditions:
;;; It must be possible to open the desired output file.
;;; dividend must be a non-negative, exact, integer.
;;; Postconditions:
;;; The file with name file-name now contains many integers.
;;; All the values in that file evenly divide dividend.
(define store-divisors
(lambda (file-name dividend)
(store-divisors-helper (open-output-file file-name) 1 dividend)))
;;; Helper:
;;; store-divisors-helper
;;; Parameters:
;;; target, an output port
;;; trial-divisor, the smallest divisor we should try
;;; dividend, the number we're working with
;;; Purpose:
;;; Stores all divisors of dividend that are at least as
;;; large as trial-divisor to target.
;;; Produces:
;;; Nothing.
;;; Preconditions:
;;; It is possible to write to the target port.
;;; Both trial-divisor and dividend are natural numbers.
;;; Postconditions:
;;; All divisors of dividend that are at least as large as
;;; trial-divisor have been added to target.
;;; target is still open for writing
(define store-divisors-helper
(lambda (target trial-divisor dividend)
; We only continue to work when the trial-divisor is not
; larger than the dividend. Note that I'm using cond because
; cond permits multiple operations when the test succeeds.
(cond ((<= trial-divisor dividend)
; Okay, does the current trial-divisor evenly divide
; dividend?
(if (zero? (remainder dividend trial-divisor))
; It does! Write it to the file
(write-number target trial-divisor))
; Continue with any other potential divisors
(store-divisors-kernel target (+ 1 trial-divisor) dividend))
; If the trial divisor is bigger than the dividend, then we're
; done, so close the port and stop.
(else (close-output-port target)))))
(define write-number
(lambda (target value)
(write value target)
(newline target)))
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.
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.
Scheme provides the type predicate input-port?, which
can be applied to any object to determine whether it is an input port.
It also provides the analogous output-port? predicate.
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.
(close-input-port input-port)
- Close an open input port.
(close-output-port output-port)
- Close an open output port.
(current-input-port)
- Get the current input port, typically associated with the keyboard.
(current-output-port)
- Get the current output port, typically associated with the screen.
(delete-file filename)- Delete the specified file, if it exists.
(write value)- Print a more readable representation of the value on the screen.
(write value output-port)- Print a more readable representation of the value to the specified port.
(eof-object? value)- Determine if the given value is something returned by
read
to indicate the end of the file.
(file-exists? filename)- Determine whether the specified file exists.
(input-port? value)- Determine whether the given value is an input port.
(newline)
- Write a newline to the screen.
(newline port)- Write a newline to the specified port.
(open-input-file filename)- Open the specified file for reading. Returns an input port.
(open-output-file filename)- Open the specified file for writing. Returns an output port.
(output-port? value)- Determine whether the given value is an output port.
(peek-char input-port)- Determine the next character available on the specified port.
(read input-port)- Read the next value available on the specified port.
(read-char input-port)- Read the next character available on the specified port.
(write value)- Print the verbose representation of the value on the screen.
(write value output-port)- Print the verbose representation of the value to the specified port.
;
