11.7 Using Lists as Sets

A list can represent an unordered mathematical set—simply consider a value an element of a set if it appears in the list, and ignore the order of the list. To form the union of two sets, use append (as long as you don’t mind having duplicate elements). Other useful functions for sets include memq and delq, and their equal versions, member and delete.

Common Lisp note: Common Lisp has functions union (which avoids duplicate elements) and intersection for set operations, but SXEmacs Lisp does not have them. You can write them in Lisp if you wish.

Function: memq object list

This function tests to see whether object is a member of list. If it is, memq returns a list starting with the first occurrence of object. Otherwise, it returns nil. The letter ‘q’ in memq says that it uses eq to compare object against the elements of the list. For example:

(memq 'b '(a b c b a))
     ⇒ (b c b a)

(memq '(2) '((1) (2)))    ; (2) and (2) are not eq.
     ⇒ nil

Function: delq object list

This function destructively removes all elements eq to object from list. The letter ‘q’ in delq says that it uses eq to compare object against the elements of the list, like memq.

When delq deletes elements from the front of the list, it does so simply by advancing down the list and returning a sublist that starts after those elements:

(delq 'a '(a b c)) ≡ (cdr '(a b c))

When an element to be deleted appears in the middle of the list, removing it involves changing the CDRs (see Setcdr).

(setq sample-list '(a b c (4)))
     ⇒ (a b c (4))

(delq 'a sample-list)
     ⇒ (b c (4))

sample-list
     ⇒ (a b c (4))

(delq 'c sample-list)
     ⇒ (a b (4))

sample-list
     ⇒ (a b (4))

Note that (delq 'c sample-list) modifies sample-list to splice out the third element, but (delq 'a sample-list) does not splice anything—it just returns a shorter list. Don’t assume that a variable which formerly held the argument list now has fewer elements, or that it still holds the original list! Instead, save the result of delq and use that. Most often we store the result back into the variable that held the original list:

(setq flowers (delq 'rose flowers))

In the following example, the (4) that delq attempts to match and the (4) in the sample-list are not eq:

(delq '(4) sample-list)
     ⇒ (a c (4))

The following two functions are like memq and delq but use equal rather than eq to compare elements. They are new in Emacs 19.

Function: member object list

The function member tests to see whether object is a member of list, comparing members with object using equal. If object is a member, member returns a list starting with its first occurrence in list. Otherwise, it returns nil.

Compare this with memq:

(member '(2) '((1) (2)))  ; (2) and (2) are equal.
     ⇒ ((2))

(memq '(2) '((1) (2)))    ; (2) and (2) are not eq.
     ⇒ nil

;; Two strings with the same contents are equal.
(member "foo" '("foo" "bar"))
     ⇒ ("foo" "bar")

Function: delete object list

This function destructively removes all elements equal to object from list. It is to delq as member is to memq: it uses equal to compare elements with object, like member; when it finds an element that matches, it removes the element just as delq would. For example:

(delete '(2) '((2) (1) (2)))
     ⇒ '((1))

Common Lisp note: The functions member and delete in SXEmacs Lisp are derived from Maclisp, not Common Lisp. The Common Lisp versions do not use equal to compare elements.

See also the function add-to-list, in Setting Variables, for another way to add an element to a list stored in a variable.