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Q:   I have a list of strings: '("apple" "pear" "peach" "nectarine" "watermelon"). How can I replace "peach" with "plum" and maintain the same order of fruits?  [CAVEAT: The list may be very long and the nth position of the element to be replaced will not be known at the outset.]

BACKBROUND:

I am working on implementing wdired compatibility with a dired-mode listing of files created by calling:

(dired (directory-files-recursively "/path/to/directory" "" 'include-directories))

This creates a listing of absolute paths to files and directories. After modifying the wdired buffer, and pressing the command to finalize/commit the edits, here is what happens:

  1.  The files get renamed correctly using a revised version of wdired-get-filename, thanks to the assistance of @Drew in a related thread: Dired / Wdired: How to test whether a filename is relative / absolute . [The renaming of directories (if applicable) will be dealt with by me on another day.]

  2.  dired-readin-insert does not work correctly to populate the buffer anew because the variable dired-directory was not updated with the new absolute path of the files that got renamed.

    /path/to/bin/gls: cannot access '/path/to/filename': No such file or directory

There will undoubtedly be additional hurdles to implement this new wdired feature, so I understand this will be one baby step at a time. To resolve this particular question, I am looking for something a little fancier than delete and push/add-to-list. Specifically, I would like to programmatically replace the old filename with the new filename and maintain the same list order.


EDIT:  Here is a link to the bug-report seeking an enhancement of wdired and dired: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=31601

  • You might want to retitle or rephrase your question, based on your answer. The answer is purely about replacing an element of a list of strings. It doesn't refer to Dired or Wdired or default-directory at all. – Drew May 25 '18 at 15:08
  • @Drew -- thank you for taking a look at this thread. I was looking for a couple of things with this question: (1) a quick solution to get me over over the hurdle, which the answer by @Andreas Röhler does well; and, (2) verification that dired-mode does not presently have the ability to update the dired-directory variable when a user renames a file in a buffer containing a recursive list of files in various subdirectories. I tried to rename a file in a dired-mode buffer also in Emacs 26 using the recursive listing of files as indicated in the question, and the variable is not updated. – lawlist May 25 '18 at 21:49
  • @Drew -- just a follow-up to let you know that adding (when (and (listp dired-directory) (member file dired-directory)) (setq dired-directory (ar-replace--in-list file newname dired-directory))) to dired-rename-file fixes the issue desribed in #2 of the question above. I would imagine that this will need to be added to other aspects of dired such as deleting a file; and, there may be an issue with speed/performance. Perhaps a hash-table system would be better suited to this type of job. I'll submit an enhancement request to the Emacs team. – lawlist May 26 '18 at 4:17
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(setq mylist '("apple" "pear" "peach" "nectarine" "watermelon"))

(defun ar-replace--in-list (elem replacement list)
  "Expects a LIST of strings.
ELEM: element to replace by arg REPLACEMENT"
  (let (newlist)
    (dolist (ele list)
      (push (if (string= ele elem) replacement ele) newlist))
    (nreverse newlist)))

So let's replace pear by cherry:

(ar-replace--in-list "pear" "cherry" mylist)

==>

("apple" "cherry" "peach" "nectarine" "watermelon")

Lists may contain other kind of elements than strings:

(defun ar-replace--in-list (pred elem replacement list)
  "Expects a LIST whose type must fit to arg PRED.
PRED: a function to select ELEM
ELEM: element to replace by arg REPLACEMENT"
  (let (newlist)
    (dolist (ele list)
      (push (if (funcall pred ele elem) replacement ele) newlist))
    (nreverse newlist)))

Call for this example:

(ar-replace--in-list 'string= "pear" "cherry" mylist)

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