CODEWITHSUNDEEP
emacselisp
oneself
oneself

Reputation: 40321

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

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

Upvotes: 39

Views: 20170

Answers (5)

Gina White
Gina White

Reputation: 210

Starting in emacs 24.4, there are two different functions available for you to call, depending on what you mean by 'empty'.

(string-empty-p " ")
nil

(string-blank-p " ")
0

I'm having trouble finding docs to link to, but emacsredux.com has more information.

If you get the error Symbol's function definition is void., include (require 'subr-x) in the initialization file (it's a package for trimming and string manipulation).

Upvotes: 10

Joe
Joe

Reputation: 1045

I keep this one in my utils.lisp:

(defun empty-string-p (string)
  "Return true if the STRING is empty or nil. Expects string type."
  (or (null string)
      (zerop (length (string-trim string)))))

then I do:

(not (empty-string-p some-string))

Upvotes: 6

Mirzhan Irkegulov
Mirzhan Irkegulov

Reputation: 18075

If you work with strings heavily in your code, i strongly recommend using Magnar Sveen's s.el string manipulation library.

s-blank? checks if the string is empty:

(s-blank? "") ; => t

Upvotes: 7

zimbu668
zimbu668

Reputation: 1335

I'm not sure what the canonical way of testing this is, but you could use the length function and check to see if your string's length is greater than zero:

(length "abc")  
=> 3
(length "")  
=> 0

The EmacsWiki elisp cookbook has an example of a trim function if you want to remove whitespace before testing.

Upvotes: 3

mihi
mihi

Reputation: 6735

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)

Upvotes: 55

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