scheme - 方案中的特殊排列

Question

我如何以这种方式在方案中生成排列：排列的第一个元素正好在 1-2 之间，第二个 1-4 和第三个 1-3 ......而且数字可以在排列中出现不止一次

Answer 1

使用 的排列for/list，在 Racket 文档中进行了介绍。

(for/list ([i '(1 2)]
             #:when i
             [j '(1 2 3 4)]
             #:when j
             [k '(1 2 3)])
    (list i j k))

Answer 2

Here's how you'd generate all the permutations for a sequence of numbers in a given range:

(define (cartesian-product . lsts)
  (foldr (lambda (lst acc)
           (for*/list ((x (in-list lst))
                       (y (in-list acc)))
             (cons x y)))
         '(())
         lsts))

(cartesian-product (range 1 3) (range 1 5) (range 1 4))

=> '((1 1 1) (1 1 2) (1 1 3) (1 2 1) (1 2 2) (1 2 3) (1 3 1) (1 3 2)
     (1 3 3) (1 4 1) (1 4 2) (1 4 3) (2 1 1) (2 1 2) (2 1 3) (2 2 1)
     (2 2 2) (2 2 3) (2 3 1) (2 3 2) (2 3 3) (2 4 1) (2 4 2) (2 4 3))