Subject: Re: force eof on an output stream
From: (Rob Warnock)
Date: Thu, 29 Jun 2006 22:52:10 -0500
Newsgroups: comp.lang.lisp
Message-ID: <>
Pascal Bourguignon  <> wrote:
| writes:
| > someone asked if it's possible to remove the first 300 lines of a file
| > without ever having two copies of the file or loading the entire thing
| > into memory.
| The best you can do, portably, is:
| (defun file-truncate-lines (file nlines)
|   (let ((copy (merge-pathnames (make-pathname :type "TRU") file nil)))
|     (with-open-file (in file)
|       (with-open-file (out copy :direction :output
|                            :if-does-not-exist :create
|                            :if-exists :error)
|         (loop :repeat nlines
|            :for line = (read-line in nil nil)
|            :while line
|            :do (format out "~A~%" line))))
|     (delete-file file)
|     (rename-file copy file)))

I think it was asking for "tail +300", not "head -300". That is,
the loop needs to look something like this:

          (loop for i from 0
                and line = (read-line in nil nil)
                while line
	    when (>= i nlines)
              do (format out "~a~%" line))

But READ-LINE conses, PEEK-CHAR & READ-CHAR not so much (at least,
not in implementations in which CHAR is an "immediate" type), so
when doing this sort of thing I tend to replace the ignored-value
READ-LINES as follows:

          (loop repeat nlines
		while (and (peek-char #\newline in nil nil)
	                   (read-char in)))
          (loop for line = (read-line in nil nil)
                while line do
	    (format out "~a~%" line))

If NLINES is 29000 on a 30000-line file, the latter version will be
a *lot* faster...  ;-}


Rob Warnock			<>
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