Filtering in

About Filtering

Filtering is the process of creating a sequence from another sequence by copying all but certain elements from the original sequence. The choice of which items to include in the new sequence can be done by a predicate function (a function that evaluates to a boolean value), or by equality with a specific object.

Removing by a predicate

remove-if is a function that takes a predicate function and a sequence as arguments, and returns a sequence of only those elements for which the predicate evaluated to a false value. In other words, it "removes" all elements of the sequence for which the predicate evaluates to true. However, note that this function does not modify the original sequence! Instead, it returns a new sequence without the appropriate elements. If the predicate was true for all values then an empty sequence is returned.

(remove-if #'evenp '(1 2 3 4 5)) ; => (1 3 5)
(remove-if #'oddp '(1 3 5))      ; => ()

Keeping elements by a predicate

There is a function remove-if-not which is just like remove-if except that it keeps the element that the predicate evaluates to true for.

As an alternative one can use complement to change a predicate into its 'opposite'.

(remove-if #'evenp '(1 2 3 4 5 6))             ; => (1 3 5)
(remove-if-not #'oddp '(1 2 3 4 5 6))          ; => (1 3 5)
(remove-if (complement #'oddp) '(1 2 3 4 5 6)) ; => (1 3 5)

Removing a specific item

If one wants to remove a specific item from a sequence one can use remove which takes the item to remove and a sequence. All values in the sequence which are not equal to the item to remove are returned.

(remove 1 '(1 2 1 3 1 4)) ; => (2 3 4)
(remove #\l "hello")      ; => "heo"

The function to use for equality can be specified with the :test keyword parameter.

Note that remove could be implemented with remove-if by writing a predicate that tests an object for equality to a specific item.

Lots of options

Each of these functions also take many other keyword parameters which allow them to work on only subsequences, only apply a certain number of times and others. Looks to the Hyperspec documentation for full details

Destructive options

In addition to remove, remove-if and remove-if-not there are delete, delete-if and delete-if-not which have the equivalent affect except they may modify the sequence passed to them.