ANSI Common Lisp   6 Iteration   6.1 The LOOP Facility   6.1.8 Examples of Miscellaneous Loop Features

6.1.8.1 Examples of clause grouping

;; Group conditional clauses.
 (loop for i in '(1 324 2345 323 2 4 235 252)
       when (oddp i)
         do (print i)
         and collect i into odd-numbers
         and do (terpri)
       else                              ; I is even.
         collect i into even-numbers
       finally
         (return (values odd-numbers even-numbers)))
1 

2345 

323 

235 
 (1 2345 323 235), (324 2 4 252)

;; Collect numbers larger than 3.
 (loop for i in '(1 2 3 4 5 6)
       when (and (> i 3) i)
       collect it)                      ; IT refers to (and (> i 3) i).
 (4 5 6)
 
;; Find a number in a list.
 (loop for i in '(1 2 3 4 5 6)
       when (and (> i 3) i)
       return it)
 4
     
;; The above example is similar to the following one.
 (loop for i in '(1 2 3 4 5 6)
       thereis (and (> i 3) i))
 4


;; Nest conditional clauses.
 (let ((list '(0 3.0 apple 4 5 9.8 orange banana)))
   (loop for i in list
         when (numberp i)
           when (floatp i)
             collect i into float-numbers
           else                                  ; Not (floatp i)
             collect i into other-numbers
         else                                    ; Not (numberp i)
           when (symbolp i) 
             collect i into symbol-list
           else                                  ; Not (symbolp i)
             do (error "found a funny value in list ~S, value ~S~%" list i)
         finally (return (values float-numbers other-numbers symbol-list))))
 (3.0 9.8), (0 4 5), (APPLE ORANGE BANANA)

;; Without the END preposition, the last AND would apply to the
;; inner IF rather than the outer one.
 (loop for x from 0 to 3 
       do (print x)
       if (zerop (mod x 2))
         do (princ " a")
          and if (zerop (floor x 2))
                do (princ " b")
                end
          and do (princ " c"))
0  a b c
1 
2  a c
3 
  NIL