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(SRFI 1) 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)) => 4Note that
find
has an ambiguity in its lookup semantics—iffind
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 usefind-tail
instead offind
—find-tail
has no such ambiguity:(cond ((find-tail pred lis) => (lambda (pair) ...)) ; Handle (CAR PAIR) (else ...)) ; Search failed.The non-standard
find-matching-item
procedure (and its aliaslist-search-positive
) works identically except that its argument order is reversed.list-search-negative
is similar tolist-search-positive
but the sense of the predicate is reversed.
(SRFI 1) 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
useseq?
to compare object with the elements of list, whilememv
useseqv?
andmember
usesequal?
.1(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 ofeq?
. This could be used to definememv
as follows:(define memv (member-procedure eqv?))
[1] 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
.