Longest decreasing sequence in Lisp

Question

I\'m working on some problems for my upcoming exam and I need some help with this Lisp function. I\'m working in CLISP. I have to find the longest decreasing sequence comprised

Answer 1

The algorithm as in Daniel's answer translated into Haskell, with some tweaks:

longest_sub xs = g2 xs [] 0 
  where
    g2 [] a _ = a
    g2 (x:t) a i  
         | even x    = g2 t a i 
         | otherwise = g4 t [x] 1
      where
        g4 [] b j = if j>i then reverse b else reverse a
        g4 xs@(x:t) b j            
             | even x || x >= head b
                         = if j>i then g2 xs b j 
                                  else g2 xs a i 
             | otherwise = g4 t (x:b) (j+1)

In Common Lisp:

(defun longest (xs)
  (prog  ((a nil) (i 0) b j x)                  ; var decls
      G2  (if (null xs) (return (reverse a)))   ; code
          (setf x (car xs) xs (cdr xs))
          (if (evenp x)
              (go G2))
      G3  (setf b (list x) j 1)
      G4  (if (null xs)
              (if (> j i)
                  (return (reverse b))
                  (return (reverse a))))
          (setf x (car xs) xs (cdr xs))
          (when (evenp x)
              (if (> j i) (setf a b i j))
              (go G2))
          (when (>= x (car b))
              (if (> j i) (setf a b i j))
              (go G3))
          (setf b (cons x b) j (+ j 1))
          (go G4)))

Function call is a glorified GOTO after all, isn't it?

see also: prog doc page.