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;<h1><join>Median of  a List of Numbers</join></h1>
;<p>General comments for those who tried the 'median' question
;<ol>
;<li> Remember to keep local variables local (use let)
;<li>Remember that the list has to be sorted before you look for  
;medians, etc
;</ol>
;<h2>Main Driver</h2>
;<p> There's two cases (odd and even numbered size lists): so let us do the common stuff in
;    the main function and handle the quirks in sub-routines.<pre>
(defun median (l)
   (let* ((size     (length l))
	   (index50 (floor (* size 0.5))))
	   (l1      (ordered l)))
     (if (oddp size)
	 (median-odd  l1 index50)
	 (median-even l1 index50))))
;</pre><h2>Two cases</h2><p>Case one:<pre>
(defun median-odd (l index50)
   (elt l index50))
;</pre><p>Case two:<pre>
(defun median-even (l index50)
   (avg (elt l (1- index50))
	(elt l index50)))
;</pre><h2>Helper Functions</h2>
;<p>The following function shows  my standard sort idioms: <ul><li>sort asscending numerically;<li> copy
;   the sorted list, so the sort does not much it up as a side effect;
;  <li>use stable sort; so input neighbors with the same score end up as
; neighbors in the output.</ul><pre>
(defun ordered (l &key (predicate #'<) (key #'identity))
   (stable-sort (copy-list l) predicate :key key))
;</pre><p>And the following  is almost too small to code as a sub-routine.<pre>
(defun avg (x y)
   (float (/ (+ x y) 2))
;</pre>