Retrieve alist from list of alists by value of specific key inside of it

Question

I'm probably missing something simple here but... I've got a list of alists parsed from JSON using json-parse-string. The list of alists is like the following (details omitted for brevity):

( ((id . 1) (url . "http://gnu.org") (description . "GNU Operating System"))
  ((id . 2) (url . "http://fsf.org") (description . "Free Software Foundation")) ... )

I'm trying to retrieve an alist from the list by the value associated to id in the alist.

E.g. (function 2 list) would return ((id . 2) (url . "http://fsf.org") (description . "Free Software Foundation"))

Obviously I could while or cl-loop over the list to find this out, but what is more a Lispy way of doing this?

Answer 1

If the structure/sequence is very dependable then you might just do this:

(require 'cl-lib)

(cl-find 2 list :key #'cdar)
;; or
(car (cl-member 2 list :key #'cdar))

More robustly, you'd use one of the alist functions to obtain the correct value to test against.

(cl-find 2 list :key (apply-partially #'alist-get 'id))
=> ((id . 2) (url . "http://fsf.org") (description . "Free Software Foundation"))

(cl-find "http://fsf.org" list
         :key (apply-partially #'alist-get 'url)
         :test #'string=)
=> ((id . 2) (url . "http://fsf.org") (description . "Free Software Foundation"))

Answer 2

I'd probably do something like this:

(setq toto '(((id . 1)
              (url . "http://gnu.org")
              (description . "GNU Operating System"))
             ((id . 2)
              (url . "http://fsf.org")
              (description . "Free Software Foundation"))))

(defun foo (val lst)
  "..."
  (let ((al  (cl-member val lst :key `(lambda (xx) (cdr (rassoc ',val xx))))))
    (car al)))

(foo 2 toto) ; => ((id . 2) (url . "http://fsf.org") (description . "Free Software Foundation"))

Or if you have lexical-binding turned on (i.e., non-nil), then just use this for :key - no need for the backquoting to insert the value of val, which would otherwise be a free variable:

(lambda (xx) (cdr (rassoc val xx)))

Answer 3

To find a item from a list, I will probably use seq-find

(seq-find
 (lambda (alist)
   (= (alist-get 'id alist) 1))
 '(((id . 1) (url . "http://gnu.org") (description . "GNU Operating System")) 
   ((id . 2) (url . "http://fsf.org") (description . "Free Software Foundation"))))
;; => ((id . 1) (url . "http://gnu.org") (description . "GNU Operating System"))