A third recursive pattern is called the
keep recursive pattern, each element of a list is tested;
the element is acted on and the results are kept only if the element
meets a criterion.
Again, this is very like the
every pattern, except the element is
skipped unless it meets a criterion.
The pattern has three parts:
Here is an example that uses
(defun keep-three-letter-words (word-list) "Keep three letter words in WORD-LIST." (cond ;; First do-again-test: stop-condition ((not word-list) nil) ;; Second do-again-test: when to act ((eq 3 (length (symbol-name (car word-list)))) ;; combine acted-on element with recursive call on shorter list (cons (car word-list) (keep-three-letter-words (cdr word-list)))) ;; Third do-again-test: when to skip element; ;; recursively call shorter list with next-step expression (t (keep-three-letter-words (cdr word-list))))) (keep-three-letter-words '(one two three four five six)) ⇒ (one two six)
It goes without saying that you need not use
nil as the test for
when to stop; and you can, of course, combine these patterns.