list 如何从球拍语言中给定索引处的列表中获取项目？
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How do I get an item from a list at a given index in racket language?
提问by lu1s
I'm trying to get an item from a list at a given index for a loop statement.
我正在尝试从循环语句的给定索引处的列表中获取项目。
(define decision-tree-learning
(lambda (examples attribs default)
(cond
[(empty? examples) default]
[(same-classification? examples) (caar examples)] ; returns the classification
[else (lambda ()
(let ((best (choose-attribute attributes examples))
(tree (make-tree best))
(m (majority-value examples))
(i 0)
(countdown (length best)) ; starts at lengths and will decrease by 1
(let loop()
(let example-sub ; here, totally stuck now
; more stuff
(set! countdown (- countdown 1))
; more stuff
)))))])))
In this case, bestis the list and I need to get its value at the countdownindex. Could you help me on that?
在这种情况下，best是列表，我需要在countdown索引处获取它的值。你能帮我吗？
回答by soegaard
回答by Gil Matzov
Or build this yourself:
或者自己构建：
(define my-list-ref
(lambda (lst place)
(if (= place 0)
(car lst)
(my-list-ref (cdr lst) (- place 1)))))
but if you want to check if the list is done and don't worry by error yo can do this as well:
但是如果你想检查列表是否完成并且不要担心错误，你也可以这样做：
(define my-list-ref
(lambda (lst place)
(if (null? lst)
'()
(if (= place 0)
(car lst)
(my-list-ref (cdr lst) (- place 1))))))
