合并方案

Question

试图在计划合并两个列表，并运行到#void错误与我的代码

(define x '(1 2 3 4 5 6 7 8 15 10)) 
    (define z '(11 12 13)) 

    (define merge 
     (lambda (list1 list2) 
      (cond [(null? list1) list2] ;if first list is empty return second list 
        [(null? list2) list1] ;if second list is empty return first list 
        [(if (< (car list1) (car list2)) 
         (cons (car list1) (merge (cdr list1) list2)))] ;if 1st item in 1st list is less than 1st item in second list (cons 1st item from 1st list with merge call) 
        [(if (> (car list1) (car list2)) 
         (cons (car list2) (merge (cdr list2) list1)))] ;if 1st item in 1st list is greater than 1st item in second list (cons 1st item in the 2nd list with merge call) 
    ))) 

输出在合并结束了无效

 > (merge x z) 
     (1 2 3 4 5 6 7 8 . #<void>) 
     > 

Answer 1

你有if陈述你的cond，并且缺少else条款。

(define x '(1 2 3 4 5 6 7 8 15 10)) 
(define z '(11 12 13)) 

(define merge 
    (lambda (list1 list2) 
     (cond [(null? list1) list2] ;if first list is empty return second list 
       [(null? list2) list1] ;if second list is empty return first list 
       [(< (car list1) (car list2)) 
        (cons (car list1) (merge (cdr list1) list2))] ;if 1st item in 1st list is less than 1st item in second list (cons 1st item from 1st list with merge call) 
       [(> (car list1) (car list2)) 
        (cons (car list2) (merge (cdr list2) list1))] ;if 1st item in 1st list is greater than 1st item in second list (cons 1st item in the 2nd list with merge call) 
       [else (cons (car list1) (cons (car list2) (merge (cdr list1) (cdr list2))))] 
))) 

看出：

> (merge x z) 
'(1 2 3 4 5 6 7 8 11 12 13 15 10) 

的#<void>不见了。