7.2.1.2 Lists as sets

eqv? is used to test for membership by procedures which treat lists as sets.

Function: adjoin e l

adjoin returns the adjoint of the element e and the list l. That is, if e is in l, adjoin returns l, otherwise, it returns (cons e l).

Example:

(adjoin 'baz '(bar baz bang))
   ⇒ (bar baz bang)
(adjoin 'foo '(bar baz bang))
   ⇒ (foo bar baz bang)

Function: union l1 l2

union returns a list of all elements that are in l1 or l2. Duplicates between l1 and l2 are culled. Duplicates within l1 or within l2 may or may not be removed.

Example:

(union '(1 2 3 4) '(5 6 7 8))
   ⇒ (1 2 3 4 5 6 7 8)
(union '(0 1 2 3 4) '(3 4 5 6))
   ⇒ (5 6 0 1 2 3 4)

Function: intersection l1 l2

intersection returns a list of all elements that are in both l1 and l2.

Example:

(intersection '(1 2 3 4) '(3 4 5 6))
   ⇒ (3 4)
(intersection '(1 2 3 4) '(5 6 7 8))
   ⇒ ()

Function: set-difference l1 l2

set-difference returns a list of all elements that are in l1 but not in l2.

Example:

(set-difference '(1 2 3 4) '(3 4 5 6))
   ⇒ (1 2)
(set-difference '(1 2 3 4) '(1 2 3 4 5 6))
   ⇒ ()

Function: subset? list1 list2

Returns #t if every element of list1 is eqv? an element of list2; otherwise returns #f.

Example:

(subset? '(1 2 3 4) '(3 4 5 6))
   ⇒ #f
(subset? '(1 2 3 4) '(6 5 4 3 2 1 0))
   ⇒ #t

Function: member-if pred lst

member-if returns the list headed by the first element of lst to satisfy (pred element). Member-if returns #f if pred returns #f for every element in lst.

Example:

(member-if vector? '(a 2 b 4))
   ⇒ #f
(member-if number? '(a 2 b 4))
   ⇒ (2 b 4)

Function: some pred lst1 lst2 …

pred is a boolean function of as many arguments as there are list arguments to some i.e., lst plus any optional arguments. pred is applied to successive elements of the list arguments in order. some returns #t as soon as one of these applications returns #t, and is #f if none returns #t. All the lists should have the same length.

Example:

(some odd? '(1 2 3 4))
   ⇒ #t

(some odd? '(2 4 6 8))
   ⇒ #f

(some > '(1 3) '(2 4))
   ⇒ #f

Function: every pred lst1 lst2 …

every is analogous to some except it returns #t if every application of pred is #t and #f otherwise.

Example:

(every even? '(1 2 3 4))
   ⇒ #f

(every even? '(2 4 6 8))
   ⇒ #t

(every > '(2 3) '(1 4))
   ⇒ #f

Function: notany pred lst1 …

notany is analogous to some but returns #t if no application of pred returns #t or #f as soon as any one does.

Function: notevery pred lst1 …

notevery is analogous to some but returns #t as soon as an application of pred returns #f, and #f otherwise.

Example:

(notevery even? '(1 2 3 4))
   ⇒ #t

(notevery even? '(2 4 6 8))
   ⇒ #f

Function: list-of?? predicate

Returns a predicate which returns true if its argument is a list every element of which satisfies predicate.

Function: list-of?? predicate low-bound high-bound

low-bound and high-bound are non-negative integers. list-of?? returns a predicate which returns true if its argument is a list of length between low-bound and high-bound (inclusive); every element of which satisfies predicate.

Function: list-of?? predicate bound

bound is an integer. If bound is negative, list-of?? returns a predicate which returns true if its argument is a list of length greater than (- bound); every element of which satisfies predicate. Otherwise, list-of?? returns a predicate which returns true if its argument is a list of length less than or equal to bound; every element of which satisfies predicate.

Function: find-if pred lst

find-if searches for the first element in lst such that (pred element) returns #t. If it finds any such element in lst, element is returned. Otherwise, #f is returned.

Example:

(find-if number? '(foo 1 bar 2))
   ⇒ 1

(find-if number? '(foo bar baz bang))
   ⇒ #f

(find-if symbol? '(1 2 foo bar))
   ⇒ foo

Function: remove elt lst

remove removes all occurrences of elt from lst using eqv? to test for equality and returns everything that’s left. N.B.: other implementations (Chez, Scheme->C and T, at least) use equal? as the equality test.

Example:

(remove 1 '(1 2 1 3 1 4 1 5))
   ⇒ (2 3 4 5)

(remove 'foo '(bar baz bang))
   ⇒ (bar baz bang)

Function: remove-if pred lst

remove-if removes all elements from lst where (pred element) is #t and returns everything that’s left.

Example:

(remove-if number? '(1 2 3 4))
   ⇒ ()

(remove-if even? '(1 2 3 4 5 6 7 8))
   ⇒ (1 3 5 7)

Function: remove-if-not pred lst

remove-if-not removes all elements from lst for which (pred element) is #f and returns everything that’s left.

Example:

(remove-if-not number? '(foo bar baz))
   ⇒ ()
(remove-if-not odd? '(1 2 3 4 5 6 7 8))
   ⇒ (1 3 5 7)

Function: has-duplicates? lst

returns #t if 2 members of lst are equal?, #f otherwise.

Example:

(has-duplicates? '(1 2 3 4))
   ⇒ #f

(has-duplicates? '(2 4 3 4))
   ⇒ #t

The procedure remove-duplicates uses member (rather than memv).

Function: remove-duplicates lst

returns a copy of lst with its duplicate members removed. Elements are considered duplicate if they are equal?.

Example:

(remove-duplicates '(1 2 3 4))
   ⇒ (1 2 3 4)

(remove-duplicates '(2 4 3 4))
   ⇒ (2 4 3)