Sorting list of pairs by the second element of the

问题:

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))

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

回答1:

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))

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