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99 LISP problems: Problem #28

Solution to the 99 LISP Problems #28

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;;; Part a)
(defun lsort (alist)
  (sort
    alist
    #'<
    :key #'length))

;;; Part b)
(defun count-elem (alist elem)
  (apply
    #'+
    (mapcar
      (lambda (x)
        (if (equalp x elem) 1 0))
      alist)))

(defun lfsort (alist)
  (let ((lengths (mapcar #'length alist)))
    (sort
      alist #'<
      :key (lambda (x) (count-elem lengths (length x))))))

Lisp dialect: Steel Bank Common Lisp