Sets And Lists - GNU Emacs Lisp Reference Manual

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). You can remove equal duplicates using delete-dups. Other useful functions for sets include memq and delq, and their equal versions, member and delete.

— 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 and remq.

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:

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

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:

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

— Function: remq object list

This function returns a copy of list, with all elements removed which are eq to object. The letter `q' in remq says that it uses eq to compare object against the elements of list.
          (setq sample-list '(a b c a b c))
               ⇒ (a b c a b c)
          (remq 'a sample-list)
               ⇒ (b c b c)
          sample-list
               ⇒ (a b c a b c)
     
The function delq offers a way to perform this operation destructively. See Sets And Lists.

The following three functions are like memq, delq and remq, but use equal rather than eq to compare elements. See Equality Predicates.

— 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 sequence

If sequence is a list, this function destructively removes all elements equal to object from sequence. For lists, delete 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.
If sequence is a vector or string, delete returns a copy of sequence with all elements equal to object removed.
For example:
          (delete '(2) '((2) (1) (2)))
               ⇒ ((1))
          (delete '(2) [(2) (1) (2)])
               ⇒ [(1)]
     

— Function: remove object sequence

This function is the non-destructive counterpart of delete. If returns a copy of sequence, a list, vector, or string, with elements equal to object removed. For example:
          (remove '(2) '((2) (1) (2)))
               ⇒ ((1))
          (remove '(2) [(2) (1) (2)])
               ⇒ [(1)]
     

— Function: member-ignore-case object list

This function is like member, except that object should be a string and that it ignores differences in letter-case and text representation: upper-case and lower-case letters are treated as equal, and unibyte strings are converted to multibyte prior to comparison.

— Function: delete-dups list

This function destructively removes all equal duplicates from list, stores the result in list and returns it. Of several equal occurrences of an element in list, delete-dups keeps the first one.

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

5.7 Using Lists as Sets