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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:

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.