在球拍中创建列表列表

Question

I am new to Racket programming, and I am working on a problem where I am given a list of numbers, and I have to make a list of list, of different combinations of numbers.

Something like :

(combine (list 3 1 2))  => (list 
                           (list 31 32 33)
                           (list 21 22 3)
                           (list 11 12 13))

How do I achieve this in Racket? Thank You

Answer 1

Just play with iterators and comprehension to implement a cartesian product that returns lists of lists, and a bit of arithmetic to obtain the right results. Try this:

(for/list ((i '(3 2 1)))
  (for/list ((j '(1 2 3)))
    (+ (* 10 i) j)))

Or alternatively, using more standard constructs (available in student languages):

(map (lambda (i)
       (map (lambda (j)
              (+ (* 10 i) j))
            '(1 2 3)))
     '(3 2 1))

Either way, it works as expected:

=> '((31 32 33) (21 22 23) (11 12 13))