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These functions perform operations on lists that represent sets of elements.
This function searches list for an element matching item. If a match is found, it returns the cons cell whose car was the matching element. Otherwise, it returns
nil
. Elements are compared byeql
by default; you can use the:test
,:test-not
, and:key
arguments to modify this behavior. See Sequences.The standard Emacs lisp function
member
usesequal
for comparisons; it is equivalent to(cl-member
item list:test 'equal)
. With no keyword arguments,cl-member
is equivalent tomemq
.
The cl-member-if
and cl-member-if-not
functions
analogously search for elements that satisfy a given predicate.
This function returns
t
if sublist is a sublist of list, i.e., if sublist iseql
to list or to any of its cdrs.
This function conses item onto the front of list, like
(cons
item list)
, but only if item is not already present on the list (as determined bycl-member
). If a:key
argument is specified, it is applied to item as well as to the elements of list during the search, on the reasoning that item is “about” to become part of the list.
This function combines two lists that represent sets of items, returning a list that represents the union of those two sets. The resulting list contains all items that appear in list1 or list2, and no others. If an item appears in both list1 and list2 it is copied only once. If an item is duplicated in list1 or list2, it is undefined whether or not that duplication will survive in the result list. The order of elements in the result list is also undefined.
This is a destructive version of
cl-union
; rather than copying, it tries to reuse the storage of the argument lists if possible.
This function computes the intersection of the sets represented by list1 and list2. It returns the list of items that appear in both list1 and list2.
This is a destructive version of
cl-intersection
. It tries to reuse storage of list1 rather than copying. It does not reuse the storage of list2.
This function computes the “set difference” of list1 and list2, i.e., the set of elements that appear in list1 but not in list2.
This is a destructive
cl-set-difference
, which will try to reuse list1 if possible.
This function computes the “set exclusive or” of list1 and list2, i.e., the set of elements that appear in exactly one of list1 and list2.