编程范式20 函数式编程

lisp 定义函数

(define sum-list (numlist)
    (if (null?num-list) 0
        (+(car num-list)
            (sum-list (cdr num-list))))

>(sum-list '(1 2 3 4 5))
15
>(sum-list '()
0

（define fib(n)
    (if(zero?n) 0
        (if (= n 1) 1
            (+ (fib ( - n 1))
                (fib (- n 2))))))

(define fib(n)
    (if (or (= n 0)
        (if (= n 1)) n
            (+ (fib ( - n 1))
                (fib (- n 2)))))

flattern

类似python里面的flattern

>(flatten '(1 (2 "3") "4" ((5))))
(1 2 "3" "4" 5)

(define flattern (sequence)
    (cond(((null?sequence) '()
        ((list?（car sequence))
            (append (flattern (car sequence)
                (flattern(cdr sequence))...)
        （else (cms (car sequence)
            (flattern(cdr sequence))...)

sort

判断一个列表是否为排好序的

>（sorted? '(1 2 2 4 7))
#t
>（sorted? '(1 8 2 4 7))
#f

（define sorted? (num-list)
    (or (< (length num-list) 2)
        (and (<= (car num-list)
                 (cadr num-list)
             (sorted? (cdr num-list))...)

lisp的list是链表

泛型sort

>（sorted? '(1 3 4 7) <=)
#t
>（sorted? '("a" "b" "d" "c") string<)
#f

(define sorted?（seq comp）
    (or(< (length seq) 2)
       (and (comp(car seq)
                  (cadr seq))
             (sorted?(cdr seq comp))))