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In the `every`

recursive pattern, an action is performed on every
element of a list.

The basic pattern is:

- If a list be empty, return
`nil`

. - Else, act on the beginning of the list (the CAR of the list)
- - through a recursive call by the function on the rest (the CDR) of the list,
- - and, optionally, combine the acted-on element, using
`cons`

, with the results of acting on the rest.

Here is example:

(defun square-each (numbers-list) "Square each of a NUMBERS LIST, recursively." (if (not numbers-list) ; do-again-test nil (cons (* (car numbers-list) (car numbers-list)) (square-each (cdr numbers-list))))) ; next-step-expression

(square-each '(1 2 3)) ⇒ (1 4 9)

If `numbers-list`

is empty, do nothing. But if it has content,
construct a list combining the square of the first number in the list
with the result of the recursive call.

(The example follows the pattern exactly: `nil`

is returned if
the numbers’ list is empty. In practice, you would write the
conditional so it carries out the action when the numbers’ list is not
empty.)

The `print-elements-recursively`

function (see Recursion with a List) is another example of an `every`

pattern, except in this case, rather than bring the results together
using `cons`

, we print each element of output.

The `print-elements-recursively`

function looks like this:

(setq animals '(gazelle giraffe lion tiger))

(defun print-elements-recursively (list) "Print each element of LIST on a line of its own. Uses recursion." (when list ; do-again-test (print (car list)) ; body (print-elements-recursively ; recursive call (cdr list)))) ; next-step-expression (print-elements-recursively animals)

The pattern for `print-elements-recursively`

is:

- When the list is empty, do nothing.
- But when the list has at least one element,
- - act on the beginning of the list (the CAR of the list),
- - and make a recursive call on the rest (the CDR) of the list.

Next: Accumulate, Up: Recursive Patterns [Contents][Index]