Instead of offering something useful from my Emacs init file, let me offer a tiny piece of code the like we’d waste time on while reading [EmacsChannel #emacs]. It is in fact so useless that I won’t even bother to post it on the EmacsWiki. I find these little examples are useful for people trying to learn EmacsLisp, however.
How to do this transformation:
I like it ti ekil I (the pony) → (ynop eht)
Note:
1. The characters in a line are reversed.
2. Matching parenthesis are mirrored.
3. The order of lines remains unchanged.
Here’s the code:
(defun reverse-chars-in-region (start end)
"Reverse the region character by character without reversing lines.
Use `reverse-region' to reverse the lines in the region."
(interactive "r")
(let ((str (buffer-substring start end)))
(delete-region start end)
(dolist (line (split-string str "\n"))
(let ((chars (mapcar (lambda (c)
(or (matching-paren c)
c))
(reverse (append line nil)))))
(when chars
(apply 'insert chars))
(newline)))))
Interesting stuff used:
1. (interactive “r”) to fill start and end
2. split-string to split the string into a list of substrings
3. (append line nil) to split the line into a list of characters
4. matching-paren to translate ( into ) and the other way around
#Emacs
(Please contact me if you want to remove your comment.)
⁂
hello alex,
in a more recursive style one could use something like :
(defun matching-char (char swap)
(or (cdr (assoc char swap))
(car (rassoc char swap))
char))
(defun recrev (string &optional swap)
(let* ((l (substring string -1 nil))
(c (matching-char l swap))
(s (substring string 0 -1)))
(concat c (if (eq (length s) 1) (matching-char s swap)
(recrev s swap)))))
then use it with
(recrev "(the pony)" '(("(" . ")"))) ---> "(ynop eht)"
– paul.r 2008-10-01 13:51 UTC