Added List and Let List assignment.

list-and-let-list/main.rkt

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+;; The first three lines of this file were inserted by DrRacket. They record metadata
+;; about the language level of this file in a form that our tools can easily process.
+#reader(lib "htdp-intermediate-reader.ss" "lang")((modname main) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #f #t none #f () #f)))
+; [Number] l -> Number
+; Greatest common denominator of a list of positive integers.
+(define (gcdl l)
+  (local
+    [(define ln (length l))]
+    (cond [(= 0 ln) (error "No denominator common among nothing.")]
+          [(= 1 ln) (first l)]
+          [(= 2 ln) (gcd (first l) (second l))]
+          [else (gcdl (cons (gcd (first l) (second l)) (rest (rest l))))])))
+
+(check-error (gcdl '()))
+(check-expect (gcdl '(1)) 1)
+(check-expect (gcdl '(1 2 3)) 1)
+(check-expect (gcdl '(6 18 12)) 6)
+(check-expect (gcdl '(100 50 2)) 2)
+
+; [T] l -> [[T]]
+; Groups equivalent consecutive elements of a list into their own list.
+(define (pack l)
+  (local [; [[T]] l -> T
+          ; Gets the first element of the first element of l.
+          (define (first-twice l) (first (first l))) ; This could also be replaced with caar().
+          
+          ; [[T]] l -> [[T]]
+          ; Repeat twice the first element of the first element of l.
+          (define (twice-first l) (cons (cons (first-twice l) (first l)) (rest l)))
+
+          ; [T] l, [[T]] m -> [[T]]
+          ; Inserts first element of list l in new first list in list of lists m.
+          (define (enlist l m) (cons (list (first l)) m))
+          
+          ; [T] l, [[T]] a -> [[T]]
+          ; Groups all consecutive elements equivalent to the first element of the first element of a.
+          ; What's being accumulated: a reversed packed list.
+          ; How it's initialized: empty.
+          ; How it's it updated: cons to beginning.
+          ; How it's used in the final answer: reverse.
+          (define (consequivalent l a)
+            (cond [(empty? l) a]
+                  [(empty? a)
+                   (consequivalent (rest l) (enlist l empty))]
+                  [(equal? (first l) (first-twice a))
+                   (consequivalent (rest l) (twice-first a))]
+                  [else
+                   (consequivalent (rest l) (enlist l a))]))]
+    (reverse (consequivalent l '()))))
+
+(check-expect (pack '()) '())
+(check-expect (pack '(1 2 3)) '((1) (2) (3)))
+(check-expect (pack '(1 1 1 2 3 3 3 4 4 2 2 5)) '(( 1 1 1) (2) (3 3 3) (4 4) (2 2) (5)))
+
+; Number start, Number lower, Number upper, [Number] l -> Boolean
+; Determines if an object that begins at the starting location and then moves delta by delta ever ever steps outside of the bounds (inclusive).
+(define (out-of-bounds? start lower upper l)
+  (or (< start lower) (> start upper)
+      (and (not (empty? l)) (out-of-bounds? (+ start (first l)) lower upper (rest l)))))
+
+(check-expect (out-of-bounds? 0 -1 2 '(0 -2 3 -4 6)) #t)
+(check-expect (out-of-bounds? 0 -1 1 '(1)) #f)
+(check-expect (out-of-bounds? 0 -1 1 '()) #f)
+(check-expect (out-of-bounds? 0 1 2 '()) #t)
+(check-expect (out-of-bounds? 0 0 0 '()) #f)

list-and-let-list/main.rkt~

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+;; The first three lines of this file were inserted by DrRacket. They record metadata
+;; about the language level of this file in a form that our tools can easily process.
+#reader(lib "htdp-beginner-reader.ss" "lang")((modname main) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #f #t none #f () #f)))

welcome-back-to-dr-racket/main.rkt~

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+;; The first three lines of this file were inserted by DrRacket. They record metadata
+;; about the language level of this file in a form that our tools can easily process.
+#reader(lib "htdp-beginner-reader.ss" "lang")((modname main) (read-case-sensitive #t) (teachpacks ()) (htdp-settings #(#t constructor repeating-decimal #f #t none #f () #f)))
+; List of Number -> String
+; Determines whether a list of 3 numbers is ascending or descending. Throws error if neither.
+(define (scending-how l)
+  (cond [(< (first l) (second l) (third l)) "ascending"]
+        [(> (first l) (second l) (third l)) "descending"]
+        [else (error "too complex")]))
+
+(check-expect (scending-how (list 1 2 3)) "ascending")
+(check-expect (scending-how (list 5 4 3)) "descending")
+(check-error (scending-how (list 1 0 3)))
+
+
+; Number, Number -> Number
+; Finds two positive numbers' greatest common denominator.
+(define (gcd2 a b)
+  (cond
+    [(= a 0) b]
+    [(= b 0) a]
+    [(= 0 (+ (modulo a 2) (modulo b 2))) (* 2 (gcd2 (/ a 2) (/ b 2)))]
+    [(= 0 (modulo b 2)) (gcd2 a (/ b 2))]
+    [(= 0 (modulo a 2)) (gcd2 (/ a 2) b)]
+    [else (if (<= a b) (gcd2 a (- b a)) (gcd2 b (- a b)))]))
+
+(check-expect (gcd2 0 0) 0)
+(check-expect (gcd2 100 0) 100)
+(check-expect (gcd2 10 2) 2)
+(check-expect (gcd2 10 10) 10)
+(check-expect (gcd2 14 7) 7)
+(check-expect (gcd2 288 64) 32)