如何在Common Lisp中将依赖循环变量与`loop`宏一起使用

Question

Consider the following snippet of C code; the actual values for M and the content of arr don't actually matter.

int arr[M] = {...};

for (int i=0; i<M; ++i) {
    for (int j=i+1; j<M; ++j) {
        /* do something */
    }
}

What would be the (idiomatic and efficient) Common Lisp analog using the loop macro (or if loop is not right tool, any other construct)?

I know that comparing array access and list access is not correct but could not come up with a better example.

One possibility I tried is the following:

(defvar l '(1 2 ...))

(loop :for n :on l :do
   (loop :for x :in (cdr n) :do
      ;; do something
      ))

But this seems rather clunky.

Please note, that other similar questions deal with ranges not with lists.

Answer 1

Not sure what you actually want to compute, but I don't think it is clunky.

Example:

CL-USER 6 > (loop for (head . tail) on '(1 2 3 4)
                  do (loop for item in tail
                           do (print (list head item))))

(1 2) 
(1 3) 
(1 4) 
(2 3) 
(2 4) 
(3 4) 
NIL

If you want to keep pairs based on some test:

CL-USER 36 > (defun mapcan-pairs (fn list)
               (loop for (head . tail) on list
                     nconc (loop for item in tail
                                 nconc (funcall fn head item))))
MAPCAN-PAIRS

CL-USER 37 > (defun keep-pairs (test list)
               (mapcan-pairs (lambda (a b)
                               (when (funcall test a b)
                                 (list (cons a b))))
                             list))
KEEP-PAIRS

CL-USER 38 >  (keep-pairs (lambda (a b)
                             (= 13 (+ a b)))
                          '(1 2 3 7 1 4 5 6 3 5 10 15 3))
((3 . 10) (7 . 6) (3 . 10) (10 . 3))