robinganemccalla@gmail.com <robinganemccalla@gmail.com> wrote:
+---------------
| I have a list of data that I'm going to need to remove elements from,
| and select elements from randomly.  I am wondering the array functions
| in common lisp would be more effective (in terms of execution time)
| then using remove to remove and creating a loop that takes the car if
| iterations has reached the random number, and recursively calls itself
| on the cdr with a greater value of iterations otherwise.
+---------------

Probably *way* more efficient than the list-based algorithm you're
describing. Here's one quick & dirty way of doing it with arrays:

    > (defun randomize-list (list)
	(loop with vector = (coerce list 'vector)
	      for limit from (length vector) downto 1
	      for index = (random limit)
	      for item = (aref vector index)
	  do (setf (aref vector index) (aref vector (1- limit)))
	  collect item))
    RANDOMIZE-LIST
    cmu> (defvar *data* '(0 1 2 3 4 5 6 7 8 9 foo bar baz gorp quux))

    *DATA*
    cmu> (randomize-list *data*)

    (6 5 BAZ 1 8 BAR 0 2 3 9 FOO 4 QUUX 7 GORP)
    cmu> (randomize-list *data*)

    (FOO 4 2 5 BAZ 6 BAR 3 GORP 1 0 8 9 QUUX 7)
    cmu> (randomize-list *data*)

    (0 1 6 GORP 7 QUUX 9 FOO BAR BAZ 2 3 4 8 5)
    cmu> 
    > 

For extra credit...  ;-}

1. The above does unneeded work in some cases. Remove them.

2. Shuffle the vector in-place, and return the vector instead of a list.


-Rob

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Rob Warnock			<rpw3@rpw3.org>
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