elegant way to count items

Question

I have a list shaped like this:

  \'((\"Alpha\" .  1538)
    (\"Beta\"  .  8036)
    (\"Gamma\" .  8990)
    (\"Beta\"  .  10052)
    (\"Alpha\" .  12837)
           


        

Answer 1

Here's what I think is an elegant functional solution using Emacs' alist functions, yielding a reusable frequencies function similar to Eli's answer:

(defun frequencies (vals)
  (reduce
   (lambda (freqs key)
     (cons (cons key (+ 1 (or (cdr (assoc key freqs)) 0)))
           (assq-delete-all-with-test key freqs 'equal)))
   vals
   :initial-value nil)))

(frequencies (mapcar 'car
                     '(("Alpha" .  1538)
                       ("Beta"  .  8036)
                       ("Gamma" .  8990)
                       ("Beta"  .  10052)
                       ("Alpha" .  12837)
                       ("Beta"  .  13634)
                       ("Beta"  .  14977)
                       ("Beta"  .  15719)
                       ("Alpha" .  17075)
                       ("Rho"   .  18949)
                       ("Gamma" .  21118)
                       ("Gamma" .  26923)
                       ("Alpha" .  31609))))
=> (("Alpha" . 4) ("Gamma" . 3) ("Rho" . 1) ("Beta" . 5))