Something

(defun lowhigh-array(n)
  (let ((array (make-array n :element-type 'integer))
	(high n) (low 0))
    (loop for i from 0 to (1- n) do
	  (setf (aref array i) 
		(if (= (mod i 2) 0) 
		    (prog1
			low
		      (setf low (1+ low)))
		  (prog1
		      high
		    (setf high (1- high))))))
    array))