Elisp: How to delete an element from an association list with string key

If you know there can only be a single matching entry in your list, you can also use the following form:

(setq al (delq (assoc <string> al) al)

Notice that the setq (which was missing from your sample code) is very important for `delete' operations on lists, otherwise the operation fails when the deleted element happens to be the first on the list.

The q in assq traditionally means eq equality is used for the objects.

In other words, assq is an eq flavored assoc.

Strings don't follow eq equality. Two strings which are equivalent character sequences might not be eq. The assoc in Emacs Lisp uses equal equality which works with strings.

So what you need here is an assoc-delete-all for your equal-based association list, but that function doesn't exist.

All I can find when I search for assoc-delete-all is this mailing list thread: http://lists.gnu.org/archive/html/emacs-devel/2005-07/msg00169.html

Roll your own. It's fairly trivial: you march down the list, and collect all those entries into a new list whose car does not match the given key under equal.

One useful thing to look at might be the Common Lisp compatibility library. http://www.gnu.org/software/emacs/manual/html_node/cl/index.html

There are some useful functions there, like remove*, with which you can delete from a list with a custom predicate function for testing the elements. With that you can do something like this:

;; remove "a" from al, using equal as the test, applied to the car of each element
(setq al (remove* "a" al :test 'equal :key 'car))

The destructive variant is delete*.