Re: Homework question: LOOP

Kenny Tilton wrote:

(defun prior-sib-if (self list &optional (test-fn #'true-that))
   "Find nearest preceding sibling passing TEST-FN"
   (labels ((check-priors (sibs)
              (if (eql self (first sibs))
                  nil
                (or (check-priors (rest sibs))
                  (when (funcall test-fn (first sibs))
                    (first sibs))))))
     (check-priors list)))

Peter Seibel wrote:

Ah, I missed that bit in the maze of twisty, recursive passages, all
alike. How about this bit of double loop delight:
 
  (defun prior-sib-if (self list &optional (test-fn #'true-that))
    "Find nearest preceding sibling passing TEST-FN"
    (loop with candidates = nil
        for node in list
        until (eql node self) do (push node candidates)
        finally (return (loop for c in candidates when (funcall test-fn c) retur
n c))))

Gauche Scheme

(use srfi-1) ;; take-while

(define (prior-sib-if self the-list test-fn)
  (find test-fn
    (reverse (take-while (^x (not (equal? self x))) the-list))))

gosh> (prior-sib-if 8 '(0 2 3 4 5 6 8 2 8) even?)
6
gosh> (prior-sib-if 8 '(0 2 3 4 5 6 8 2 8) odd?)
5

Another way:

(define (prior-sib-if self the-list test-fn)
  (let go ((lst the-list) (seen '()))
    (if (equal? self (car lst))
      (find test-fn seen)
      (go (cdr lst) (cons (car lst) seen)))))