; Vectors
; - another datatype that can store multiple objects in order
; - have a lot of similarities to lists, but also some important differences
; - vectors look like like lists but have # in front
; - also you cannot change the size of a vector, so you cannot cons or cdr
;   a vector
; - vectors also have an important advantage, which we we may not see...
;    because of the fixed way that they are stored, vector-ref is a lot quicker
;    than list-ref
;  - the way list-ref is implemented by DrScheme, is it walks through the list
;    and stops when it have found the item it wants
;   - in contrast, vector-ref can jump right to the item it wants....

; before we get started, it is a good idea to do

(print-vector-length #f)
; keeps Dr. Scheme from using shorthand to represent some vectors

; vector commands.... what are three ways of creating the vector #(1 1 1)
(make-vector 3 1) ; what happens if I type (make-vector 3), it returns #(0 0 0)
(vector 1 1 1)
'#(1 1 1)  ; you may not be able to modify this in other versions of scheme

; my-list->vector, changes the input list into a vector
(define my-list->vector
  (lambda (ls)
    (apply vector ls)))

; it is a bit trickier to change a vector into a list, here is the version from the
; reading
(define our-vector->list
  (lambda (vec)
    ; remaining is the number of items left to copy
    (let kernel ((remaining (vector-length vec))
                 (result null))
      (if (zero? remaining)
          result
          (let ((position (- remaining 1)))
            (kernel position (cons (vector-ref vec position) result)))))))

;; now let's write vector-string-append, which takes in a vector of strings
;; and returns the result of string-appending all the strings together into
;; one long string
;(define vector-string-append
;  (lambda (vec)
;    (apply string-append (vector->list vec))))

; let's write it without vector->list
(define vector-string-append
  (lambda (vec)
    ; remaining is the number of items left to copy
    (let kernel ((remaining (vector-length vec))
                 (result ""))
      (if (zero? remaining)
          result
          (let ((position (- remaining 1)))
            (kernel position (string-append (vector-ref vec position) result)))))))

; so now 
(vector-string-append (vector "abc" "def" "ghi"))
; returns "abcdefghi"