Filter

One of the core ideas in functional programming is to filter a set of items by some criterion. It may be somewhat suprising to learn that lisp does not have a built-in function named “filter” “select”, or “keep” that performs this operation. Instead, Common Lisp provides the “remove”, “remove-if”, and “remove-if-not” functions, which perform the complementary operation of removing items that satisfy or do not satisfy a given predicate.

The remove function, like similar sequence functions, takes an optional keyword :test-not argument that can be used to specify a test that must fail for an item to be considered for removal. Thus if you invert your logic for inclusion, you can use the remove function as a “filter” by specifying the predicate with :test-not.

> (defvar *nums* (map 'list (λ (n) (format nil "~r" n)) (iota 10)))
*NUMS*

;; Keep *nums* with four letters
> (remove 4 *nums* :key #'length :test-not #'=)
("zero" "four" "five" "nine")

;; Keep *nums* starting with the letter "t"
> (remove #\t *nums* :key (partial-apply-right #'elt 0) :test-not #'eql)
("two" "three")