[lisp] scheme学习2

Posted 2020-10-09 wilderness

1.在scheme中，为了效率，对序对的操作 cons car 和cdr是内部实现的，这里是scheme实现， 其中cons用到了闭包

 1 (define (cons a b)
 2     (define (dispatch m)
 3         (cond ((= m 1) a)
 4             ((= m 2) b)
 5             (else (error "index out of range")))
 6     )
 7     dispatch
 8 )
 9 
10 (define (car d) (d 1))
11 (define (cdr d) (d 2))

 2.计算列表长度和叶子节点数目

 1 (define (count_leaf x)
 2     (cond ((null? x) 0)
 3           ((not(pair? x)) 1)
 4           (else (+
 5             (count_leaf (car x))
 6             (count_leaf (cdr x))
 7             ))
 8           )
 9 )
10 
11 
12 (define (length items)
13     (define (length_iter count x)
14             (if (null? x)
15                 count
16                 (length_iter (+ 1 count) (cdr x))
17         )
18     )
19 
20     (length_iter 0 items)
21 )

 3.Honor规则计算多项式（SICP P80 2.34）

(define (accumulate op inital seq)
        (if (null? seq)
            inital
            (op (car seq) (accumulate op inital (cdr seq)))
            )
)


(define (honor_eval x coeff_seq)
        (accumulate
            (lambda (this_coeff highter_terms) 
                (
                    + this_coeff
                      (* x highter_terms)
                )
            )
            0
            coeff_seq
        )
)