Xah Lee, 2005-11-01
If you have no idea of elisp, goto: elisp basics.
For basic elisp examples, goto: Basic Elisp Examples.
The following example shows the definition of “fold”. A very useful function in functional programing, but doesn't exist in elisp. For a example fold in Mathematica, see: http://documents.wolfram.com/mathematica/functions/Fold
Also, we use fold to define replace-string-pairs, which does string replacement by a given list of find-replace pairs.
(defun fold (f x li) "Recursively applies (f x i) where i is the ith element in the list li. For example, (fold f x '(1 2)) returns (f (f x 1) 2)" (let ((li2 li) (ele) (x2 x)) (while (setq ele (pop li2)) (setq x2 (funcall f x2 ele)) ) x2 ) ) (defun replace-string-pairs (str pairs) "replace the string str repeatedy by the list pairs. Example: (replace-string-pairs \"abcdefg\" '( (\"a\" \"1\") (\"b\" \"2\") (\"c\" \"3\")) ) ⇒ 123defg" (fold (lambda (x y) "" (replace-regexp-in-string (nth 0 y) (nth 1 y) x) ) str pairs) )
This shows how to call a shell command and process its output.
For a detailed lesson, see: Linkify A Image Path.
(defun get-image-dimensions (img-file-path) "Returns a image file's width and height as a list. This function requires ImageMagick's “identity” shell command." (let (cmd-name sh-output width height) (setq cmd-name "identify") (setq sh-output (shell-command-to-string (concat cmd-name " " img-file-path)) ) ; sample output from “identify”: ; some.png PNG 520x429+0+0 DirectClass 8-bit 9.1k 0.0u 0:01 (string-match "^[^ ]+ [^ ]+ \\([0-9]+\\)x\\([0-9]+\\)" sh-output) (setq width (match-string 1 sh-output)) (setq height (match-string 2 sh-output)) (list (string-to-number width) (string-to-number height)) ))
(defun tag-image () "Replace a image file name with a HTML img tag. Example, if cursor is on the word “emacs_logo.png”, then it will became “<img src=\"emacs_logo.png\" alt=\"emacs logo\" width=\"123\" height=\"456\">”. This function requires the “identify” command from ImageMagick.com." (interactive) (let (img-file-path bounds img-dim width height altText myResult) (setq img-file-path (thing-at-point 'filename)) (setq bounds (bounds-of-thing-at-point 'filename)) (setq altText img-file-path) (setq altText (replace-regexp-in-string "\\.[A-Za-z]\\{3,4\\}$" "" altText t t)) (setq altText (replace-regexp-in-string "_" " " altText t t)) (setq img-dim (get-image-dimensions2 img-file-path)) (setq width (number-to-string (car img-dim))) (setq height (number-to-string (car (last img-dim)))) (setq myResult (concat "<img src=\"" img-file-path "\"" " " "alt=\"" altText "\"" " " "width=\"" width "\" " "height=\"" height "\">")) (save-excursion (delete-region (car bounds) (cdr bounds)) (insert myResult)) ))
; In Mac OS X, open current buffer in TextEdit, ; if it is dired, open it in Finder. (defun open-with-osx () "open the current dir in Finder or TextEdit" (interactive) (if (eq 'dired-mode major-mode) (shell-command "open .") (shell-command (concat "open -a /Applications/TextWrangler.app/ " buffer-file-name))))
The following example is the solution to this challenge:
Define a function f, such that (f "simple sexp") returns the argument as a lisp's list. Example: (f "(a (b) c)") returns “(a (b) c)”.
This problem is a simple example of a parser. The program reads a string that represents a tree, and turn this string into a actual tree structure.
The following code is given by Jorgen Schäfer↗, 2006.
(defun parse-string (s) (with-temp-buffer (insert s) (goto-char (point-min)) (parse))) (defun parse () (cond ((looking-at "\\s-*(\\s-*") (goto-char (match-end 0)) (let ((exprs nil) (this nil)) (while (and (not (looking-at "\\s-*)\\s-*")) (setq this (parse))) (setq exprs (cons this exprs))) (goto-char (match-end 0)) (apply 'list (reverse exprs)))) ((looking-at "\\s-*\\([a-z]+\\)\\s-*") (goto-char (match-end 0)) (intern (match-string 1))) (else (error "Syntax error")))) ; sample usage (parse-string "(a (b (h h)) (c d))")
See also: