Homework 2 Answers

Part I

1.
a) 72
b) 48
c) 72
d) (72 5 4)

2. 14

3.
(define (flatten tree)
  (cond ((null? tree) '())
        ((pair? tree) (append (flatten (car tree)) (flatten (cdr tree))))
        (else (list tree))
  )
)

Part II

1.
(define (fact n)
  (define (fact-helper product n)
    (if (= n 0) product
        (fact-helper (* n product) (- n 1))
    )
  )
  (if (< n 0) (error "Cannot find factorial of a negative number")
      (fact-helper 1 n)
  )
)

2.
(define (curry f x) (lambda (y) (f x y)))

3.
(define (filter f l)
  (if (null? l) '()
      (let ((head (car l))
            (tail (filter f (cdr l))))
           (if (f head) (cons head tail)
               tail
           )
      )
  )
)

4.
(define (positive-only) (filter (curry < 0)))