Recursive Pattern: keep

A third recursive pattern is called the keep pattern. In 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:

• If a list be empty, return nil.
• Else, if the beginning of the list (the CAR of the list) passes a test
  • - act on that element and combine it, using cons with
  • - a recursive call by the function on the rest (the CDR) of the list.
• Otherwise, if the beginning of the list (the CAR of the list) fails the test
  • - skip on that element,
  • - and, recursively call the function on the rest (the CDR) of the list.

Here is an example that uses cond:

(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.