same-parity: 不借助 辅助函数的 特殊 “超纲” 写法

same-parity.scm

; 基础方法
(define (same-parity x . l)
    (same-parity-adpater x l)
)

(define (same-parity-adpater x l)
    (if (null? l)
        (cons x l)
        (let ((car-item (car l)))
            (if (even? (+ car-item x))
                (cons x (same-parity-adpater car-item (cdr l)))
                (same-parity-adpater x (cdr l))
            )
        )
    )
)

#|
> (same-parity 1 3 5 6 7)
|(same-parity 1 3 5 6 7)
|(same-parity-adpater 1 (3 5 6 7))
| (same-parity-adpater 3 (5 6 7))
| |(same-parity-adpater 5 (6 7))
| |(same-parity-adpater 5 (7))
| | (same-parity-adpater 7 ())
| | (7)
| |(5 7)
| (3 5 7)
|(1 3 5 7)
(1 3 5 7)

> (same-parity 2 3 4 5 6 7)
|(same-parity 2 3 4 5 6 7)
|(same-parity-adpater 2 (3 4 5 6 7))
|(same-parity-adpater 2 (4 5 6 7))
| (same-parity-adpater 4 (5 6 7))
| (same-parity-adpater 4 (6 7))
| |(same-parity-adpater 6 (7))
| |(same-parity-adpater 6 ())
| |(6)
| (4 6)
|(2 4 6)
(2 4 6)

|#

; 更进一步: 有了 apply, 就可以 直接将 list 作为参数传递了.  
; ref: https://stackoverflow.com/questions/26749297/how-do-you-use-dotted-tail-notation-correctly-in-this-algorithm
(apply same-parity 1 (list 2 3 4))

(define (same-parity x . l)
    (if (null? l)
        (cons x l)
        (let ((car-item (car l)))
            (if (even? (+ car-item x))
                (cons x (apply same-parity car-item (cdr l)))
                (apply same-parity x (cdr l))
            )
        )
    )
)

#|
> (same-parity 1 3 5 6 7)
|(same-parity 1 3 5 6 7)
| (same-parity 3 5 6 7)
| |(same-parity 5 6 7)
| |(same-parity 5 7)
| | (same-parity 7)
| | (7)
| |(5 7)
| (3 5 7)
|(1 3 5 7)
(1 3 5 7)

> (same-parity 2 3 4 5 6 7)
|(same-parity 2 3 4 5 6 7)
|(same-parity 2 4 5 6 7)
| (same-parity 4 5 6 7)
| (same-parity 4 6 7)
| |(same-parity 6 7)
| |(same-parity 6)
| |(6)
| (4 6)
|(2 4 6)
(2 4 6)
|#

; 更进一步 实现 filter. 参考 下一个题目之前的 背景介绍中 的 map 实现
(define (filter proc items)
    (if (null? items)
        items
        (if (proc (car items))
            (cons (car items) (filter proc (cdr items)))
            (filter proc (cdr items))
        )
    )
)

(define (same-parity x . l)
    (cons x (filter (lambda (item) (even? (+ item x))) l))
)

#|
> (same-parity 1 3 5 6 7)
|(same-parity 1 3 5 6 7)
| (filter #<procedure> (3 5 6 7))
| |(filter #<procedure> (5 6 7))
| | (filter #<procedure> (6 7))
| | (filter #<procedure> (7))
| | |(filter #<procedure> ())
| | |()
| | (7)
| |(5 7)
| (3 5 7)
|(1 3 5 7)
(1 3 5 7)

> (same-parity 2 3 4 5 6 7)
|(same-parity 2 3 4 5 6 7)
| (filter #<procedure> (3 4 5 6 7))
| (filter #<procedure> (4 5 6 7))
| |(filter #<procedure> (5 6 7))
| |(filter #<procedure> (6 7))
| | (filter #<procedure> (7))
| | (filter #<procedure> ())
| | ()
| |(6)
| (4 6)
|(2 4 6)
(2 4 6)
|#