问题描述
我想在特定条件下从列表列表中获得一些列表.例如:
I would like to have some lists from a list of lists under a certain condition. For example:
`((a w) (e w))`
来自
`((a w) (e w) (i u) (o u))`
条件是在第二个位置有 'w.
where the condition is to have 'w in the second position.
我的代码
(filter (andmap (equal? (cdr lst) 'w)) 'lst)
与
(define lst '((a w) (e w) (i u) (o u)))
不工作.我做错了什么?
is not working. What am I doing wrong?
推荐答案
您不应该引用列表:'lst 与 lst 不同.要访问第二个元素，请使用 second(如果未定义，请改用 cadr).使用cdr，您可以获得列表的rest(这是另一个列表)，而不是它的第二个元素.哦，filter 需要一个 lambda 作为它的第一个参数，而不是 andmap.这就是我的意思:
You should not quote the list: 'lst is not the same as lst. And to access the second element, use second (if that's not defined, use cadr instead). With cdr you get the rest of the list (which is another list), not its second element. Oh, and filter expects a lambda as its first argument, not andmap. This is what I mean:
(define lst '((a w) (e w) (i u) (o u)))
(filter (lambda (sl) (equal? (second sl) 'w))
lst)
=> '((a w) (e w))
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