从Scheme中的列表中删除元素 [英] Delete element from List in Scheme

问题描述

我有这种形式的清单

( (1 3) (2 2) (3 1) (4 5) (5 1)))

我想删除一个项目，例如(3 1)

and I want to delete an item let's say (3 1)

结果将是

( (1 3) (2 2) (4 5) (5 1)))

我写了这样的东西，我不知道为什么它不能正常运行.

I have written something like this and I do not know why it is not running correctly.

(define (deleteItem list item)
  (cond
    ((equal? item (car  list)) (cdr list))
    (cons (car  list)(deleteItem(cdr list) item))))

推荐答案

有一个内置函数，称为

There's a built-in function for this, it's called remove:

(define lst
  '((1 3) (2 2) (3 1) (4 5) (5 1)))

(remove '(3 1) lst)
=> '((1 3) (2 2) (4 5) (5 1))

...但是我想您需要从头开始实现它.有关您的代码的一些建议:

… But I guess you need to implement it from scratch. Some suggestions for your code:

• 您不应使用list作为参数名称，这将与内置函数发生冲突.让我们将其称为lst
• 大多数列表过程都缺少基本情况:如果列表为空会怎样?
• 在最后一种情况下，您还缺少else部分

• You should not use list as a parameter name, that'll clash with a built-in function. Let's call it lst instead
• You're missing the base case necessary form most list procedures: what happens if the list is empty?
• You're also missing the else part in the last condition

在完成上述所有修复后，该过程将起作用:

With all the above fixes in place, the procedure will work:

(define (deleteItem lst item)
  (cond ((null? lst)
         '())
        ((equal? item (car lst))
         (cdr lst))
        (else
         (cons (car lst) 
               (deleteItem (cdr lst) item)))))

(deleteItem lst '(3 1))
=> '((1 3) (2 2) (4 5) (5 1))

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