清单的方案和 [英] Scheme sum of list

问题描述

首先，这是家庭作业，但我只是在寻找有关如何执行此操作的提示或伪代码.

First off, this is homework, but I am simply looking for a hint or pseudocode on how to do this.

我需要使用递归对列表中的所有项目进行汇总.但是，如果遇到列表中的非数字内容，则需要返回空集.这是我的尝试:

I need to sum all the items in the list, using recursion. However, it needs to return the empty set if it encounters something in the list that is not a number. Here is my attempt:

(DEFINE sum-list
  (LAMBDA (lst)
    (IF (OR (NULL? lst) (NOT (NUMBER? (CAR lst))))
      '()
      (+
        (CAR lst)
        (sum-list (CDR lst))
      )
    )
  )
)

此操作失败，因为它无法将空集添加到其他内容.通常，如果它不是数字，我只会返回0并继续处理列表.

This fails because it can't add the empty set to something else. Normally I would just return 0 if its not a number and keep processing the list.

推荐答案

我愿意这样做:

(define (mysum lst)
  (let loop ((lst lst) (accum 0))
    (cond
      ((empty? lst) accum)
      ((not (number? (car lst))) '())
      (else (loop (cdr lst) (+ accum (car lst)))))))

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