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Filtering Lists

procedure+: list-transform-positive list predicate

procedure+: list-transform-negative list predicate

These procedures return a newly allocated copy of list containing only the elements for which predicate is (respectively) true or false. Predicate must be a procedure of one argument.

(list-transform-positive '(1 2 3 4 5) odd?) => (1 3 5)
(list-transform-negative '(1 2 3 4 5) odd?) => (2 4)

procedure+: delq element list

procedure+: delv element list

procedure+: delete element list

Returns a newly allocated copy of list with all entries equal to element removed. delq uses eq? to compare element with the entries in list, delv uses eqv?, and delete uses equal?.

procedure+: delq! element list

procedure+: delv! element list

procedure+: delete! element list

Returns a list consisting of the top-level elements of list with all entries equal to element removed. These procedures are like delq, delv, and delete except that they destructively modify list. delq! uses eq? to compare element with the entries in list, delv! uses eqv?, and delete! uses equal?. Because the result may not be eq? to list, it is desirable to do something like (set! x (delete! x)).

(define x '(a b c b))
(delete 'b x)                           =>  (a c)
x                                       =>  (a b c b)

(define x '(a b c b))
(delete! 'b x)                          =>  (a c)
x                                       =>  (a c)
;; Returns correct result:
(delete! 'a x)                          =>  (c)

;; Didn't modify what x points to:
x                                       =>  (a c)

procedure+: delete-member-procedure deletor predicate

Returns a deletion procedure similar to delv or delete!. Deletor should be one of the procedures list-deletor or 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 delv and delete!:

(define delv (delete-member-procedure list-deletor eqv?))
(define delete! (delete-member-procedure list-deletor! equal?))

procedure+: list-deletor predicate

procedure+: list-deletor! predicate

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.

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