Parse through list in Scheme and get the sum of all the numbers

Question

So I need to write a recursive solution that can go through the directory and subdirectory of a list and add up the file sizes. I have zero experience in scheme and my professor isn't getting back to me. I've tried a few different attempts but they all end in errors. If anyone could help me out it'd be greatly appreciated because I don't have much time left to figure out this assignment.

(define disk '("D" "main"
(
    ("F" "file1.txt" (30))
    ("F" "file2.txt" (11))

    ("D" "sub1"
    (
        ( "F" "file1.txt" (1234))
        ( "F" "file2.txt" (2345))
        ( "F" "file3.txt" (3456))
    )
    )
    ("D" "sub2"
    (
        ( "F" "file1.txt" (1234))
        ( "F" "file2.txt" (2345))
        ( "F" "file3.txt" (3456))
    )
    )
)
)
)

(define (getEnd n)
    (let ((n 1)))
    (let ((m 0)))
    (member 3 '(disk.n))
    (+ m (getEnd (+ n 1)))

)

When I run the code I always get this error:

          Ill-formed special form: (let (...))

I've tried using the let statement outside of the function but it still doesn't work because I have no clue how scheme works. I've reached out to my classmates and they can't seem to figure it out either.

Any help would be appreciated, thank you!

Answer 1

A let is a local bound variable that exist during the body or the form. That is:

(let ((n 1))           ; bind one variable 1
  (display (+ n n))    ; use n for something
  )                    ; end of let. `n` no longer exist.

Since you have tag java, C dialect, it's the same as this:

{
  int n = 1;
  System.out.println(n + n); // use n for something
}
// end of block. n no longer exist

In your code you have no expressions using the binding and if it was allowed it would have been dead code, like this:

{
  int n = 1;
}
// end of block. n no longer exist

The code (member 3 '(disk.n)) is always #f since '(disk.n) is a list with one symbol, disk.n and (equal? 3 'disk.n) ; ==> #f (equal? 3 'disk.n) ; ==> #f

The last line (+ m (getEnd (+ n 1)) does an unconditional recursion by increasing n . It does not sum any numeric values from the structure.

questions

It seems you can check if a list is a file or directory by looking at the first element is either "F" or "D" Thus you could make:

(define (file? lst)
  ; todo: implement
  )

(file? '("F" "file1.txt" (1234))) ; ==> #t
(file? '("D" ())                  ; ==> #f

You can get a list of files by checking the third element of a directory:

(define (directory-elements dir)
  ; todo: implement
  )

(directory-elements '("D" "D" (("F" "F1" (1234)) ("F" "F2" (2345)))))
; ==> (("F" "F1" (1234)) ("F" "F2" (2345)))

You can make a procedure that gets the size of a file:

(define (file-size file)
  ; todo: implement
  )

(file-size '("F" "F1" (1234)))
; ==> 1234

Now you can make your target procedure using the ones above:

(define (size file-or-dir)
  (if (file? file-or-dir)
      (file-size file-or-dir)
      (directory-elements-size (directory-elements file-or-dir)))

(size '("F" "F1" (1234))) ; ==> 1234
(size '("D" "D" (("F" "F1" (1234)) ("F" "F2" (2345))))) ; ==> 3579

You are missing directory-elements-size . That can be made with recursion:

(define (directory-elements-size lst)
  (if (null? lst)
      <???>                                  ; size of an empty directory
      (+ (size <????>)                       ; add the size of first file/dir
         (directory-elements-size <????>)))) ; with the size of the rest of the elements

There you go. You should have more than enough to get you going.