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Returns the first element in list for which predicate is
true; returns #f if it doesn’t find such an element.
Predicate must be a procedure of one argument.
(find even? '(3 1 4 1 5 9)) => 4
Note that find has an ambiguity in its lookup semantics—if
find returns #f, you cannot tell (in general) if it
found a #f element that satisfied predicate, or if it did
not find any element at all. In many situations, this ambiguity
cannot arise—either the list being searched is known not to contain
any #f elements, or the list is guaranteed to have an element
satisfying predicate. However, in cases where this ambiguity
can arise, you should use find-tail instead of
find—find-tail has no such ambiguity:
(cond ((find-tail pred lis)
=> (lambda (pair) …)) ; Handle (CAR PAIR)
(else …)) ; Search failed.
Returns the first pair of list whose car satisfies
predicate; returns #f if there’s no such pair.
find-tail can be viewed as a general-predicate variant of
memv.
These procedures return the first pair of list whose car is
object; the returned pair is always one from which list is
composed. If object does not occur in list, #f
(n.b.: not the empty list) is returned. memq uses eq?
to compare object with the elements of list, while
memv uses eqv? and member uses compare, or
equal? if compare is not supplied.7
(memq 'a '(a b c)) ⇒ (a b c)
(memq 'b '(a b c)) ⇒ (b c)
(memq 'a '(b c d)) ⇒ #f
(memq (list 'a) '(b (a) c)) ⇒ #f
(member (list 'a) '(b (a) c)) ⇒ ((a) c)
(memq 101 '(100 101 102)) ⇒ unspecified
(memv 101 '(100 101 102)) ⇒ (101 102)
Returns a procedure similar to memq, except that predicate,
which must be an equivalence predicate, is used instead of eq?.
This could be used to define memv as follows:
(define memv (member-procedure eqv?))
Although they
are often used as predicates, memq, memv, and
member do not have question marks in their names because they
return useful values rather than just #t or #f.
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