Substitution of Expressions Lists Association Lists

11.3: Lists as Sets

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

Function: member item list

This MacLisp-compatible function searches list for an element which is equal to item. The member function is built-in to Emacs 19; this package defines it equivalently in Emacs 18. See the following function for a Common-Lisp compatible version.

Function: 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.

Note that this function's name is suffixed by `*' to avoid the incompatible member function defined in Emacs 19. (That function uses equal for comparisons; it is equivalent to (member* item list :test 'equal).)

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

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

This function combines two lists which represent sets of items, returning a list that represents the union of those two sets. The result list will contain all items which appear in list1 or list2, and no others. If an item appears in both list1 and list2 it will be 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: nunion list1 list2 &key :test :test-not :key

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

Function: 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 which appear in both list1 and list2.

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

This is a destructive version of intersection. It tries to reuse storage of list1 rather than copying. It does not reuse the storage of list2.

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

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

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

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

Function: 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.

Substitution of Expressions

Lists

Association Lists