How to check if a string is empty in Emacs Lisp?

emacs elisp
oneself picture oneself · Jun 19, 2009 · Viewed 13.4k times · Source

I would like to write an if statement that will do something base on whether a string is empty. For example:

(defun prepend-dot-if-not-empty (user-str)
   (interactive "s")
   (if (is-empty user-str)
     (setq user-str (concat "." user-str)))
   (message user-str))

In this contrived example, I'm using (is-empty) in place of the real elisp method. What is the right way of doing this?

Thanks

Answer

mihi picture mihi · Jun 19, 2009

Since in elisp, a String is an int array, you can use

(= (length user-str) 0)

You can also use (string=) which is usually easier to read

(string= "" user-str)

Equal works as well, but a bit slower:

(equal "" user-str)

Related questions

How do I set the size of Emacs' window?

I'm trying to detect the size of the screen I'm starting emacs on, and adjust the size and position the window it is starting in (I guess that's the frame in emacs-speak) accordingly. I'm trying to set up my .emacs …

Read More
How do I create an empty file in emacs?

How can I create an empty file from emacs, ideally from within a dired buffer? For example, I've just opened a Python module in dired mode, created a new directory, opened that in dired, and now need to add an …

Read More
How to achieve code folding effects in Emacs?

Whats the best way to achieve something like code folding, or the type of cycling that org-mode uses. What would be the best solution in elisp to create this type of behavior? EDIT: I'm sorry I was not clear. I …

Read More