Returns a newly allocated copy of list containing only the elements satisfying predicate. Predicate must be a procedure of one argument.
(filter odd? '(1 2 3 4 5)) ⇒ (1 3 5)
filter, except that the returned list contains only those
elements not satisfying predicate.
(remove odd? '(1 2 3 4 5)) ⇒ (2 4)
Partitions the elements of list with
predicate, and returns two values: the list of in-elements and
the list of out-elements. The list is not disordered—elements
occur in the result lists in the same order as they occur in the
argument list. The dynamic order in which the various
predicate are made is not specified. One of
the returned lists may share a common tail with the argument
(partition symbol? '(one 2 3 four five 6)) => (one four five) (2 3 6)
Linear-update variants of
partition. These procedures are allowed, but not required, to
alter the cons cells in the argument list to construct the
delete uses the comparison procedure compare, which
equal?, to find all elements of list that are
equal to x, and deletes them from list. The dynamic order
in which the various applications of compare are made is not
The list is not disordered—elements that appear in the result list occur in the same order as they occur in the argument list. The result may share a common tail with the argument list.
Note that fully general element deletion can be performed with the
remove! procedures, e.g.:
;; Delete all the even elements from LIS: (remove even? lis)
The comparison procedure is used in this way:
x ei). That is, x is always the first
argument, and a list element is always the second argument. The
comparison procedure will be used to compare each element of
list exactly once; the order in which it is applied to the
various ei is not specified. Thus, one can reliably
remove all the numbers greater than five from a list with
(delete 5 list <).
delete! is the linear-update variant of
delete. It is
allowed, but not required, to alter the cons cells in its argument
list to construct the result.
(delete x list eq?),
x list eq?),
(delete x list eqv?), and
(delete! x list eqv?), respectively.
Returns a deletion procedure similar to
Deletor should be one of the procedures
list-deletor!. Predicate must be an equivalence predicate.
The returned procedure accepts exactly two arguments: first, an object
to be deleted, and second, a list of objects from which it is to be
deleted. If deletor is
list-deletor, the procedure
returns a newly allocated copy of the given list in which all entries
equal to the given object have been removed. If deletor is
list-deletor!, the procedure returns a list consisting of the
top-level elements of the given list with all entries equal to the given
object removed; the given list is destructively modified to produce the
result. In either case predicate is used to compare the given
object to the elements of the given list.
Here are some examples that demonstrate how
delete-member-procedure could have been used to implement
(define delv (delete-member-procedure list-deletor eqv?)) (define delete! (delete-member-procedure list-deletor! equal?))
These procedures each return a procedure that deletes elements from lists. Predicate must be a procedure of one argument. The returned procedure accepts exactly one argument, which must be a proper list, and applies predicate to each of the elements of the argument, deleting those for which it is true.
The procedure returned by
list-deletor deletes elements
non-destructively, by returning a newly allocated copy of the argument
with the appropriate elements removed. The procedure returned by
list-deletor! performs a destructive deletion.