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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.
(keep-matching-items '(1 2 3 4 5) odd?) => (1 3 5) (delete-matching-items '(1 2 3 4 5) odd?) => (2 4)For compatibility, the procedure
list-transform-positive
is an alias forkeep-matching-items
, andlist-transform-negative
is an alias fordelete-matching-items
.
These procedures are exactly like
keep-matching-items
anddelete-matching-items
, respectively, except that they destructively modify the list argument rather than allocating a new result.
Returns a newly allocated copy of list with all entries equal to element removed.
delq
useseq?
to compare element with the entries in list,delv
useseqv?
, anddelete
usesequal?
.
Returns a list consisting of the top-level elements of list with all entries equal to element removed. These procedures are like
delq
,delv
, anddelete
except that they destructively modify list.delq!
useseq?
to compare element with the entries in list,delv!
useseqv?
, anddelete!
usesequal?
. Because the result may not beeq?
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)
Returns a deletion procedure similar to
delv
ordelete!
. Deletor should be one of the procedureslist-deletor
orlist-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 islist-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 islist-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 implementdelv
anddelete!
:(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 bylist-deletor!
performs a destructive deletion.