10.3 Lists as Sets

These functions perform operations on lists that represent sets of elements.

Function: cl-member item list &key :test :test-not :key

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 by eql by default; you can use the :test, :test-not, and :key arguments to modify this behavior. See Sequences.

The standard Emacs lisp function member uses equal for comparisons; it is equivalent to (cl-member item list :test 'equal). With no keyword arguments, cl-member is equivalent to memq.

The cl-member-if and cl-member-if-not functions analogously search for elements that satisfy a given predicate.

Function: cl-tailp sublist list

This function returns t if sublist is a sublist of list, i.e., if sublist is eql to list or to any of its CDRs.

Function: cl-adjoin item list &key :test :test-not :key

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 by cl-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.

Function: cl-union list1 list2 &key :test :test-not :key

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.

Function: cl-nunion list1 list2 &key :test :test-not :key

This is a destructive version of cl-union; rather than copying, it tries to reuse the storage of the argument lists if possible.

Function: cl-intersection list1 list2 &key :test :test-not :key

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.

Function: cl-nintersection list1 list2 &key :test :test-not :key

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.

Function: cl-set-difference list1 list2 &key :test :test-not :key

This function computes the “set difference” of list1 and list2, i.e., the set of elements that appear in list1 but not in list2.

Function: cl-nset-difference list1 list2 &key :test :test-not :key

This is a destructive cl-set-difference, which will try to reuse list1 if possible.

Function: cl-set-exclusive-or list1 list2 &key :test :test-not :key

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.

Function: cl-nset-exclusive-or list1 list2 &key :test :test-not :key

This is a destructive cl-set-exclusive-or, which will try to reuse list1 and list2 if possible.

Function: cl-subsetp list1 list2 &key :test :test-not :key

This function checks whether list1 represents a subset of list2, i.e., whether every element of list1 also appears in list2.