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(cons 1
(cons 2
(cons 3
(cons 4 nil))))
(define one-through-four (list 1 2 3 4))
one-through-four
(car one-through-four)
(cdr one-through-four)
(define (list-ref items n)
(if (= n 0)
(car items)
(list-ref (cdr items)
(- n 1))))
(list-ref one-through-four 3)
(define (length items)
(if (null? items)
0
(+ 1 (length (cdr items)))))
(define (length items)
(define (length-iter a count)
(if (null? a)
count
(length-iter (cdr a)
(+ 1 count))))
(length-iter items 0))
(define odds (list 1 3 5 7))
(define squares (list 1 4 9 16 25))
(define (append list1 list2)
(if (null? list1)
list2
(cons (car list1)
(append (cdr list1)
list2))))
(append odds squares)
;; Exercise2.17
(define (last-pair items)
(if (null? (cdr items))
(car items)
(last-pair (cdr items))))
(last-pair (list 1 2 3 4 9 23))
;; Exercise2.18
(define (reverse items)
(if (null? (cdr items))
items
(append (reverse (cdr items))
(cons (car items) nil))))
(list 4 3 2 1)
(reverse (list 1 2 3 4))
;; Exercise2.19 Counting change
(define (count-change amount)
(cc amount 5))
(define us-coins
(list 50 25 10 5 1))
(define uk-coins
(list 100 50 20 10 5 2 1 0.5))
(define (cc amount coin-values)
(cond ((= amount 0)
1)
((or (< amount 0)
(no-more? coin-values))
0)
(else
(+ (cc
amount
(except-first-denomination
coin-values))
(cc
(- amount
(first-denomination
coin-values))
coin-values)))))
(define (first-denomination coin-values)
(car coin-values))
(define (no-more? coin-values)
(null? coin-values))
(define (except-first-denomination coin-values)
(if (null? coin-values)
null?
(cdr coin-values)))
(cc 100 us-coins)
;; Exercise2.20
(define (same-parity x . y)
(define (find-same-parity parity z)
(if (null? z)
nil
(if (= parity 1)
(if (odd? (car z))
(cons (car z) (find-same-parity 1 (cdr z)))
(find-same-parity 1 (cdr z)))
(if (even? (car z))
(cons (car z) (find-same-parity 2 (cdr z)))
(find-same-parity 2 (cdr z))))))
(if (odd? x)
(cons x (find-same-parity 1 y))
(cons x (find-same-parity 2 y))))
(define (same-parity first . rest)
(define (same-parity-iter source dist remainder-val)
(if (null? source)
dist
(same-parity-iter (cdr source)
(if (= (remainder (car source) 2) remainder-val)
(append dist (list (car source)))
dist)
remainder-val)))
(same-parity-iter rest (list first) (remainder first 2)))
(same-parity 1 2 3 4 5 6 7)
(same-parity 2 3 4 5 6 7)
;; Exercise2.21
(define (map proc items)
(if (null? items)
nil
(cons (proc (car items))
(map proc (cdr items)))))
(define (squares items)
(map (lambda (x) (* x x)) items))
(squares (list 1 2 3 4))
;; Exercise2.22
(define (square x) (* x x))
(define (square-list items)
(define (iter things answer)
(if (null? things)
answer
(iter (cdr things)
(cons (square (car things))
answer))))
(iter items nil))
;; Modify one
(define (square-list items)
(define (iter things answer)
(if (null? things)
answer
(iter (cdr things)
(cons answer
(square
(car things))))))
(iter items nil))
(square-list (list 1 2 3 4 5))
(list 1 4 9 16 25)
;; The right one;
(cons 1 (cons 4 (cons 9 (cons 16 (cons 25 nil)))))
;; The wrong one
(cons (cons (cons (cons (cons nil 1) 4) 9) 16) 25)
;; Exercise2.23
;; why nil print '()
(define (for-each proc items)
(define (iter proc items item)
(if (null? item)
nil
(proc item))
(if (null? items)
nil
(iter proc (cdr items) (car items))))
(iter proc items nil))
(for-each
(lambda (x) (newline) (display x))
(list 57 321 88))
(cons (list 1 2) (list 3 4))
(cons (list 1 2) (list 3 4))
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