按方案中对的第二个元素对对的排序列表 [英] Sorting list of pairs by the second element of the pairs in scheme

问题描述

我有一个计划中的过程，该过程为我提供了一对对的列表，我需要按该对的第二个元素对该列表进行降序排序.像这样:

I have procedure in scheme which give me a list of pairs and I need to sort descending this list by the second element of the pairs. Like this:

((1 . 1) (2 . 3) (3 . 2)) --> ((2 . 3) (3 . 2) (1 . 1))
((1 . 1) (x . 3) (2 . 1) (3 . 1)) --> ((x . 3) (1 . 1) (2 . 1) (3 . 1))
((1 . 3) (3 . 4) (2 . 2)) --> ((3 . 4) (1 . 3) (2 . 2))

我不知道该如何使用排序.

I have no idea how I should use sorting for this.

推荐答案

只需使用内置的

Just use the built-in sort procedure:

(define (sort-desc-by-second lst)
  (sort lst
        (lambda (x y) (> (cdr x) (cdr y)))))

(sort-desc-by-second '((1 . 1) (2 . 3) (3 . 2)))
=> '((2 . 3) (3 . 2) (1 . 1))

这里的技巧是将适当的比较过程作为第二个参数传递给sort.

The trick here is passing to sort an appropriate comparison procedure as the second parameter.

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