优雅的方式来计数物品 [英] elegant way to count items

问题描述

我有一个这样的列表：

  '(("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))

如何计算列表中每个元素的汽车中的条款总数？基本上我想要：

How can I count the total number of occurrences of the terms in the car of each element in the list? Basically I want:

(("Alpha" . 4)
 ("Beta" . 5)
 ("Gamma" . 3)
 ("Rho" . 1))

否，这不是功课。我还没有Lisp的想法。

No, this is not homework. I just don't have the "thinking in Lisp" thing quite yet.

在C＃中，我将使用LINQ来做到这一点。我也可以在lisp中使用while循环等，但是我在想这样做的方式似乎过于复杂。

In C#, I would use LINQ to do this. I can do it in lisp, too, using while loops and such but the way I am thinking of doing it seems overly complicated.

编辑

这是我所拥有的：

(defun count-uniq (list)
  "Returns an alist, each item is a cons cell where the car is
a unique element of LIST, and the cdr is the number of occurrences of that
unique element in the list. "
  (flet ((helper (list new)
                 (if (null list)
                     new
                   (let ((elt (assoc (car list) new)))
                     (helper (cdr list)
                             (if elt
                                 (progn (incf (cdr elt)) new)
                               (cons (cons (car list) 1) new)))))))
    (nreverse (helper list nil))))

推荐答案

我不知道这是最优雅的，但似乎是reas onable：

I dunno that this is the most elegant, but it seems reasonable:

(defun add-for-cheeso (data)
  (let (result)
    (dolist (elt data result)
      (let ((sofar (assoc (car elt) result)))
        (if sofar
            (setcdr sofar (1+ (cdr sofar)))
          (push (cons (car elt) 1) result))))))

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