io - 用 lisp 将列表写入文件

Question

(defun foo (mylist)
  (with-open-file (str "out.txt"
                       :direction :output
                       :if-exists :append
                       :if-does-not-exist :create)
    (if mylist
        (progn
          (format str (car mylist))
          (foo (cdr mylist))))))

我有两个问题。第一个是我不能用这个表达式 (format str (car mylist)) 写一个列表的元素，其次它会产生另一个错误，如下所示。

             already points to file
             "out.txt", opening the file again for :OUTPUT may produce
             unexpected results
             Open the file anyway

Answer 1

错误

此错误记录在 the manual 中.您正在递归调用 foo 并且每次调用都会再次打开文件。这可能会产生非常糟糕的后果。

重新打开

您可以通过将递归调用移到 with-open-file 之外来修复它:

(defun write-list-to-file-reopen (mylist destination)
  (when mylist
    (with-open-file (str destination
                         :direction :output
                         :if-exists :append
                         :if-does-not-exist :create)
      (prin1 (pop mylist) str)
      (terpri str))
    (write-list-to-file-reopen mylist)))

但这效率很低，因为 open是一个相对昂贵的操作。

更好的解决方案是迭代

(defun write-list-to-file-iterate (mylist destination)
  (with-open-file (str destination
                       :direction :output
                       :if-exists :append
                       :if-does-not-exist :create)
    (dolist (el mylist)
      (format str "~S~%" el))))

或者，如果您需要使用递归，

(defun write-list-to-file-recursion (mylist destination)
  (with-open-file (str destination
                       :direction :output
                       :if-exists :append
                       :if-does-not-exist :create)
    (labels ((write-list (list)
               (when list
                 (prin1 (pop list) str)
                 (terpri str)
                 (write-list list))))
      (write-list mylist))))

格式

函数format是相对的重量很重，用于交互式 pretty-print ，你可能更喜欢使用更简单的函数，如 write .

你的问题是因为你的(format str x)应该是(format str "~S~%"x):你忘了 format string .