我有一个小程序，它可以根据输入是正还是负向前或向后打印三角形。它工作得很好，除了在每个循环结束时都会打印零。我试着改变循环的边界，但每次这样做都会影响三角形的大小。有什么解决办法吗？请在下面找到代码。
(defun triangle(k)
(cond ((or(not(integerp k)) (= k 0)) '("invalid number; please enter a positive or a negative integer"))
((< k -1)
(loop for x from k to -1 do
(loop for y from k to -1 do
(if (< y x)
(princ #Space)
(princ '*)
)
)
(terpri)
)
)
((or (= k 1) (= k -1)) '*)
(t(loop for x from 0 to k do
(loop for y from 0 to k do
(if (< x y)
(princ '*)
)
)
(terpri)
)
)
)
)
(defun triangle (k)
(cond ((or (not (integerp k))
(= k 0))
'("invalid number; please enter a positive or a negative integer"))
((< k -1)
(loop for x from k to -1 do
(loop for y from k to -1 do
(if (< y x)
(princ #Space)
(princ '*)))
(terpri)))
((or (= k 1) (= k -1))
'*)
(t
(loop for x from 0 to k do
(loop for y from 0 to k do
(if (< x y)
(princ '*)))
(terpri))))
(values))
示例：
CL-USER 8 > (triangle 3)
***
**
*
CL-USER 9 >
考虑到您的问题是关于循环的，我知道这是偏离主题的，但我建议将数据生成、操作和表示分离，这始终是一种很好的做法。您可以制作一个三角形生成器，然后根据需要将其反转。它可能看起来像这样，例如：
(defun mk-tri (n)
(when (plusp n)
(loop for i from 1 to n
collect (nconc (loop repeat i collect #*)
(loop repeat (- n i) collect #space)))))
(defun print-field (field)
(format t "~{~{~a~}~%~}" field))
(print-field (mk-tri 5))
;; *
;; **
;; ***
;; ****
;; *****
(defun invert-v (data)
(reverse data))
(print-field (invert-v (mk-tri 5)))
;; *****
;; ****
;; ***
;; **
;; *
(defun invert-h (data)
(mapcar #'reverse data))
(print-field (invert-h (mk-tri 5)))
;; *
;; **
;; ***
;; ****
;; *****
这将给你全新的自由度，就像你可以想象的这个用于图案绘制的小工具包：
(defun concat-v (d1 d2)
(append d1 d2))
(defun concat-h (d1 d2)
(mapcar #'append d1 d2))
(defun repeat-v (n data)
(reduce #'concat-v (loop repeat n collect data)))
(defun repeat-h (n data)
(reduce #'concat-h (loop repeat n collect data)))
(let* ((tri (mk-tri 5))
(tri2 (concat-v tri (invert-v tri)))
(rect (concat-h tri2 (invert-h tri2))))
(print-field (repeat-v 2 (repeat-h 3 rect))))
;; * ** ** *
;; ** **** **** **
;; *** ****** ****** ***
;; **** ******** ******** ****
;; ******************************
;; ******************************
;; **** ******** ******** ****
;; *** ****** ****** ***
;; ** **** **** **
;; * ** ** *
;; * ** ** *
;; ** **** **** **
;; *** ****** ****** ***
;; **** ******** ******** ****
;; ******************************
;; ******************************
;; **** ******** ******** ****
;; *** ****** ****** ***
;; ** **** **** **
;; * ** ** *