Reading and writing from file in Scheme

Question

I am attempting to read and write a matrix from file "data.txt". The matrix is lists with strings inside of them. When I am writing I want to write from the begining an override the data. Basically I delete the file every time. I need bether solusion for this. May main problem is that after a couple readings and writhings of the file corrupts.

system error: Access is denied.; errno=5

My code:

;reading file returning matix of strings
(define (file-reader file-name)
  (define pointer (open-input-file file-name))
  (define (helper line)
    (cond
      ((equal? line eof) '())
      ((cons (list line) (helper (read-line pointer))))))
  (list-matr (helper (read-line pointer)))
  )

;converting matrix of string to matrix of lists with strings inside
(define (list-matr str-matr)
  (define (helper str-matr line-num)
    (cond
      ((null? str-matr) '())
      ((= line-num 1) (cons (map (lambda (x) (string-append x "?")) (string-split (caar str-matr) "? ")) (helper (cdr str-matr) (+ line-num 1))))
      ((cons (string-split (caar str-matr) " ") (helper (cdr str-matr) (+ line-num 1))))))
  (helper str-matr 1))


;saving in file
(define (writer file-name questions answers)
  (cond
    ((file-exists? file-name) (delete-file file-name)))

  (write-to-file file-name (string-append (string-join questions) "\n"))
  (define (helper cur-l ans)
    (cond
      ((null? ans))
      ((helper (write-to-file file-name (string-append (string-join (car ans)) "\n")) (cdr ans)))))
  (helper '() answers)
  )


(define (write-to-file path string)
  (call-with-output-file path #:exists 'append
    (lambda (newline)
      (display string newline))))

Commands for calling the functions.

(file-reader "data.txt")

(writer "data.txt" questions answers)

I think the problem coming from that I don't close the files, but I can't figure out where to put the command for that.

If my code is very bad you can give me other examples for reading and writing matrix from file.

Thank you.

Answer 1

You are correct that the file will corrupt - it's never properly closed.

Without overwriting the file each time, you will need something outside of the normal R5RS/R7RS-small specification, and I'm not aware off the top of my head of any (final) SRFI that allows random file access. That said, many/most Scheme implementations provide some form of low-level I/O interface. The disadvantage of such is that you will have to track the structure very carefully so as to overwrite or add only the correct amount, which will probably be more work than rewriting the entire file.

I would recommend restructuring this completely. First, the call-with-output-file/with-output-to-file procedures will automatically overwrite the output file unless flagged otherwise (in most implementations - though the specifications state that the behaviour is undefined). They will also automatically close the file upon completion. Similar behaviour for the call-with-input-file/with-input-from-file procedures.

You can probably simplify everything by something like the following:

; reader 
; this could be further simplified by replacing the cons call with 
; (cons (<parse-procedure> l) r), to parse the input at the same time
(define (matrix-read filename)
  (with-input-from-file filename (lambda ()
    (let loop ((l   (read-line))
               (r   '()))
      (if (eof-object? l)
          (reverse r)
          (loop (read-line) (cons l r))))))

; I don't understand the input/output format...

; writer
(define (matrix-write filename data)
  (with-output-to-file filename (lambda ()
    (for-each 
      (lambda (l)
        ; again, I don't know the actual structure outside of a list
        (display l)
        (newline))
      data))))