Question

Say I have a list with keywords:

'(("element1" :keyword1 "a" :keyword2 "b") 
  ("element2" :keyword3 "c" :keyword4 "d")
  ("element3" :keyword2 "e" :keyword4 "f"))

Which functions can I use to find which list elements contain :keyword2 and find its value in each list? I’m trying to do this in Emacs Lisp but I think with the cl package I could possibly adapt a Common Lisp solution? I’ve tried to use the find function as illustrated here but to no avail (of course, after changing a few syntax elements to adapt the examples to Emacs Lisp).

Answer 1

(require 'cl)

(defvar *data* '(("element1" :keyword1 "a" :keyword2 "b") 
                 ("element2" :keyword3 "c" :keyword4 "d")
                 ("element3" :keyword2 "e" :keyword4 "f")))

(find :keyword2 *data* :test #'find)
;;=> ("element1" :keyword1 "a" :keyword2 "b")

(getf (cdr (find :keyword2 *data* :test #'find)) :keyword2)
;;=> "b"

;; Above only finds the first match; to find all matches, 
;; use REMOVE* to remove elements that do not contain the keyword:

(remove* :keyword2 *data* :test-not #'find)
;;=> (("element1" :keyword1 "a" :keyword2 "b")
;;    ("element3" :keyword2 "e" :keyword4 "f"))

(mapcar (lambda (x) (getf (cdr x) :keyword2))
        (remove* :keyword2 *data* :test-not #'find))
;;=> ("b" "e")