Node:Keep, Previous:Accumulate, Up:Recursive Patterns

#### 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.