Laboratory: Files

a. Scan through this lab to determine what kinds of tasks you'll need to complete.

b. You should also scan through the reading on files in Scheme.

c. Open the reference on files in Scheme in a separate window.

d. Start the GIMP and MediaScript.

e. Add the procedures from the reading (reprinted at the end of this lab) to your definitions window.

;;; 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 one line of 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))))))

;;; 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)
      (display "' is")
      (newline)
      (display (read-line source))
      (newline)
      (close-input-port source))))

;;; 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)))))))

;;; Procedure:
;;;   integer-store-divisors
;;; Parameters:
;;;   dividend, a natural number
;;;   file-name, a string that names a file
;;; Purpose:
;;;   Compute all the divisors of dividend and store them
;;;   to the named file.
;;; Produces:
;;;   [None; Called for the side effect of creating 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 integer-store-divisors
  (lambda (dividend file-name)
    (integer-store-divisors-kernel dividend (open-output-file file-name) 1)))

;;; Helper:
;;;   integer-store-divisors-kernel
;;; Parameters:
;;;   dividend, the number we're working with
;;;   target, an output port
;;;   trial-divisor, the smallest divisor we should try
;;; 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 integer-store-divisors-kernel
  (lambda (dividend target trial-divisor)
    ; 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?
           (cond ((zero? (remainder dividend trial-divisor))
                  ; It does!  Write it to the file
                  (write trial-divisor target)
                  (newline target)))
           ; Continue with any other potential divisors
           (integer-store-divisors-kernel dividend target (+ 1 trial-divisor)))
          ; If the trial divisor is bigger than the dividend, then we're
          ; done, so close the port and stop.
          (else (close-output-port target)))))