英文:
Selecting Elements from a List in Racket
问题
我在Racket中有一个函数，它接受一个列表，遍历列表，检查列表中的每个项是否满足条件，并打算输出满足条件的每个项的索引列表。我不能使用 "indexes-of"。
目前，我的设置如下：
(define (function lst)
(define output empty)
(for ([i (length lst)])
(cond
[(条件) (append output (list i))]
)
)
output
)
这会输出一个空列表。我（认为我）明白原因是什么；append 不会像在Python中那样将其参数附加到变量output上，而是将其参数连接为一个新列表。我不知道如何修复这个问题。再次强调，我正在尝试获取索引，因此重要的是保持它们不变。我已经验证了我的条件函数的正常工作；它确实正确识别每个项，问题在于将它们放入可返回的列表中。
英文:
I have a function in Racket which takes a list, iterates through it, checks each item in the list against a condition, and is intended to output a list of the index of every item for which this condition is true. I cannot use indexes-of.
Currently, my setup is:
(define (function lst)
(define output empty)
(for ([i (length lst)])
(cond
[(CONDITION) (append output (list i))]
)
)
output
)
This outputs an empty list. I (think I) understand the reason why; append doesn't actually append to the variable output (the way one would expect in, say, Python), rather concatenates its parameters as a new list. I do not know how to fix this. Again, I'm trying to fetch indexes, so it's important those remain intact.
I have verified my condition functions properly; this does correctly identify each item, it's getting it into a returnable list that's the issue at this juncture.
答案1
得分: 1
在Racket中（不同于常规Scheme），列表是不可变的，并且append在两者中始终返回一个新的列表。你正在使用的普通for推导式也不返回任何值。
有很多方法可以实现你想要的，而不使用indexes-of（尽管这是简单的方法）。例如，可以使用for/list与#:when子句，仅选择满足CONDITION返回true的元素：
(define (function lst)
(for/list ([elem (in-list lst)]
[i (in-naturals)]
#:when (CONDITION elem))
i))
英文:
Lists in Racket (Unlike regular Scheme) are immutable, and append in both always returns a fresh list anyways. The plain for comprehension you're using doesn't return any value, either.
There's quite a few ways to do what you want without indexes-of (Though that's the easy way). For example, using for/list with a #:when clause to select just the elements that CONDITION returns true for:
(define (function lst)
(for/list ([elem (in-list lst)]
[i (in-naturals)]
#:when (CONDITION elem))
i))
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