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#lang scheme
;; PROGRAM NAME: hw02.rkt
;; DESCRIPTION: Homework 2 for CSI 310
;; AUTHOR: Ben Burwell
;; HISTORY: Turned in 2013-01-29
;; NOTES: When this program runs, a series of expected and
;; actual results will print out. Compare them to
;; ensure the code functions properly.
;; ============ PROCEDURE ============
;; NAME: invert
;;
;; DESC:
;; takes a list of 2-lists and inverts each of the lists
;; hence, the list
;; ((2 1) (4 3) (6 5))
;; will become
;; ((1 2) (3 4) (5 6))
(define invert
(λ (lst)
(cond
[ (null? lst) null ]
[ (not (list? lst)) (display "Invalid Parameters") ]
[ (null? (cdr lst)) (list (invert-reverse (car lst))) ]
[ else (append (list (invert-reverse (car lst))) (invert (cdr lst))) ] )))
;; a helper function that reverses a 2-list
;; PRECONDITION: lst is a list of the form (a b)
;; POSTCONDITION: lst is a list of the form (b a)
;; ERROR: lst is not a list with 2 elements
(define invert-reverse
(λ (lst)
(cond
[ (null? lst) null ]
[ (not (list? lst)) (display "Invalid Parameters") ]
[ (not (equal? (length lst) 2)) (display "Invalid Parameters") ]
[ else (list (cadr lst) (car lst)) ] )))
;; ============ TEST CODE ============
(newline)
(display "Testing (invert) =========================================")
(newline)
(display "Expected output: () ")
(invert '())
(display "Expected output: ((1 2)) ")
(invert '((2 1)))
(display "Expected output: ((1 2) (3 4) (5 6)) ")
(invert '((2 1) (4 3) (6 5)))
;; ============ PROCEDURE ============
;; NAME: vector-index
;;
;; DESC:
;; returns the zero-based index of the first
;; occurence of a parameter in a vector, or
;; -1 if there is no occurrence.
(define vector-index
(λ (needle haystack)
(cond
[ (null? needle) (display "Invalid Parameters") ]
[ (vector? haystack) (list-index needle (vector->list haystack)) ]
[ else (display "Invalid Parameters") ] )))
;; a helper function
;; PRECONDITION: haystack is a non-nested list (e.g. the result of vector->list)
(define list-index
(λ (needle haystack)
(if (null? haystack)
-1
(if (equal? (car haystack) needle)
0
(if (equal? (list-index needle (cdr haystack)) -1)
-1
(+ 1 (list-index needle (cdr haystack))))))))
;; ============ TEST CODE ============
(newline)
(display "Testing (vector-index) ===================================")
(newline)
(display "Expected output: 2 ")
(vector-index 3 #(1 2 3 4 5 6 7 8 9))
(display "Expected output: -1 ")
(vector-index 4 #(1 2 3))
(display "Expected output: -1 ")
(vector-index 3 #())
;; ============ PROCEDURE ============
;; NAME: count-occurrences
;;
;; DESC:
;; counts the occurrences of needle in haystack
(define count-occurrences
(λ (needle haystack)
(cond
[ (null? needle) (display "Error: nothing to search for") ]
[ (null? haystack) 0 ]
[ (list? (car haystack)) (+ (count-occurrences needle (car haystack)) (count-occurrences needle (cdr haystack))) ]
[ (equal? (car haystack) needle) (+ 1 (count-occurrences needle (cdr haystack))) ]
[ else (count-occurrences needle (cdr haystack)) ] )))
;; ============ TEST CODE ============
(newline)
(display "Testing (count-occurrences) ==============================")
(newline)
(display "Expected output: 10 ")
(count-occurrences 'a '(a b a c d (((((((((a))))))))) e f a b a a (d e) (a) c (a (a (a)))))
(display "Expected output: 0 ")
(count-occurrences 'a '())
(display "Expected output: 1 ")
(count-occurrences 'a '(a))
;; ============ PROCEDURE ============
;; NAME: compose
;;
;; DESC:
;; takes 1, 2, or 3 procedures and composes them, as specified by the equation:
;; (compose f g h) = (compose f (compose g h))
(define compose
(λ funcs
(cond
[ (equal? (length funcs) 1) (car funcs) ]
[ (equal? (length funcs) 2) (λ (x) ((car funcs) ((cadr funcs) x))) ]
[ (equal? (length funcs) 3) (λ (x) ((car funcs) ((cadr funcs) ((caddr funcs) x)))) ]
[ else (display "Invalid parameters") ] )))
;; ============ TEST CODE ============
(newline)
(display "Testing (compose) ========================================")
(newline)
(display "Expected output: 1 ")
((compose car cdr cdr) '(0 2 1 3))
(display "Expected output: 1 ")
((compose car) '(1 2 3 4))
(display "Expected output: 1 ")
((compose car cdr) '(0 1 2 3))
(display "Expected output: 1 ")
((compose - -) 1)
;; =========== END OF FILE ===========
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