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

Question

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

Your code is better rewritten to `(message "%s" (if (is-empty user-str) "." user-str))`. — Stefan, Oct 19 '18 at 01:21

score 55 · Accepted Answer · answered Jun 19 '09 at 20:08

55

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)

answered Jun 19 '09 at 20:08

mihi

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Are you *sure* `equal` is slower? Last time I looked at this (many years ago), it was somewhere between "identicall" and "a bit faster". – Stefan Oct 19 '18 at 01:19

score 9 · Answer 2 · edited Oct 01 '20 at 08:56

9

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).

edited Oct 01 '20 at 08:56

miguelmorin

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answered Oct 18 '18 at 16:17

Gina White

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I have Emacs 26.3 and both calls throw `Symbol's function definition is void.` – miguelmorin Nov 27 '19 at 20:50
1

@miguelmorin: Try putting `(require 'subr-x)` somewhere earlier in your file. That solved the problem for me. – Cadence Sep 28 '20 at 12:14

score 7 · Answer 3 · answered Mar 18 '14 at 04:28

7

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

answered Mar 18 '14 at 04:28

Mirzhan Irkegulov

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Joe · Answer 4 · 2019-11-27T22:16:59.807

6

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))

edited Nov 27 '19 at 22:16

answered May 14 '15 at 23:50

Joe

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score 3 · Answer 5 · edited Aug 08 '12 at 19:03

3

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.

edited Aug 08 '12 at 19:03

Brighid McDonnell

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answered Jun 19 '09 at 20:09

zimbu668

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

5 Answers5