多列表评估错误
我是一名 CommonLisp 菜鸟，有一个问题。我有下面这两个功能。
辅助函数：
(defun make-rests (positions rhythm)
"now make those positions negative numbers for rests"
(let ((resultant-rhythm rhythm))
(dolist (i positions resultant-rhythm)
(setf (nth i resultant-rhythm) (* (nth i resultant-rhythm) -1)))))
和主要函数：
(defun test-return-rhythms (rhythms)
(let ((positions '((0 1) (0)))
(result nil))
(dolist (x positions (reverse result))
(push (make-rests x rhythms) result))))
当我运行 (test-return-rhythms '(1/4 1/8)) 时，它的计算结果为： ((1/4 -1 /8) (1/4 -1/8))
但是，我期望： (test-return-rhythms '(1/4 1/8)) 评估为： ((-1/4 -1/8) (-1/4 1/8))。
我做错了什么？
I'm a CommonLisp noob with a question. I have these two functions below.
A helper function:
(defun make-rests (positions rhythm)
"now make those positions negative numbers for rests"
(let ((resultant-rhythm rhythm))
(dolist (i positions resultant-rhythm)
(setf (nth i resultant-rhythm) (* (nth i resultant-rhythm) -1)))))
And a main function:
(defun test-return-rhythms (rhythms)
(let ((positions '((0 1) (0)))
(result nil))
(dolist (x positions (reverse result))
(push (make-rests x rhythms) result))))
When I run (test-return-rhythms '(1/4 1/8)), it evaluates to: ((1/4 -1/8) (1/4 -1/8))
However, I expected: (test-return-rhythms '(1/4 1/8)) to evaluate to: ((-1/4 -1/8) (-1/4 1/8)).
What am I doing wrong?
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评论（1）
您的
make-rests实现具有破坏性。因此，如果您运行测试，第二次迭代将看到
(-1/4 -1/8)和(make-rests '(0) '(-1/4 - 1/8))返回(1/4 -1/8)。在make-rests中使用let不会复制列表，它只是创建一个引用它的新绑定。在let中使用copy-list，或者首先编写一个非破坏性版本：Your implementation of
make-restsis destructive.So, if you run your test, the second iteration will see
(-1/4 -1/8), and(make-rests '(0) '(-1/4 -1/8))returns(1/4 -1/8). Your use ofletinmake-restsdoes not copy the list, it just creates a new binding that references it. Usecopy-listin yourlet, or write a non-destructive version in the first place: