combination of pair subsets from a list in lisp

问题

How to create all possible pairs subsets from a list in conman lisp. For example the list A contain four elements

list A= ("A" "B" "C" "D")  

the expected output is as follows:

(("A","B"),("A","C"), ("A","D"),("B","C"),("B","D"), ("C","D"))

Could someone please help me out to generate these subsets. Thanks a lot

回答1:

Read up on mapcar et al:

(defparameter a (list 1 2 3 4))
(mapcon (lambda (tail)
          (mapcar (lambda (x)
                    (cons (car tail) x))
                  (cdr tail)))
        a)
==> ((1 . 2) (1 . 3) (1 . 4) (2 . 3) (2 . 4) (3 . 4))

来源：https://stackoverflow.com/questions/28942009/combination-of-pair-subsets-from-a-list-in-lisp