loops - 在 lisp 中求和斐波那契数

Question

我刚刚开始学习 Lisp，并且正在解决一些 Project Euler 问题。我被困在你将偶数斐波那契数加起来低于最大数的那个上。我尝试过的事情如下。我读过 this post但我仍然不确定为什么我的两种方法都不起作用!

CL-USER> (defun sum-even-fibs (max)
           (do ((curr 0 next)
                (next 1 (+ curr next))
                (sum  0 (if (evenp curr) (+ sum curr))))
               ((> max curr) sum)))
SUM-EVEN-FIBS
CL-USER> (sum-even-fibs 10)
0


CL-USER> (defun sum-even-fibs (max)
           (let ((sum 0))
             (do ((curr 0 next)
                  (next 1 (+ curr next)))
                 ((> max curr))
               (if (evenp curr) 
                   (setq sum (+ sum curr))))
             (format t "~d" sum)))
SUM-EVEN-FIBS
CL-USER> (sum-even-fibs 10)
0
NIL

Answer 1

arbautjc 答案的稍微“更好”的版本:

(loop for a = 0 then b and b = 1 then (+ a b) 
      while (< a 4000000) 
      when (evenp a) sum a)