#### 14.2.47 set-exclusive-or, nset-exclusive-or [Function]

`set-exclusive-or` list-1 list-2 &key key test test-not ⇒ result-list

`nset-exclusive-or` list-1 list-2 &key key test test-not ⇒ result-list

#### Arguments and Values::

list-1—a proper list.

list-2—a proper list.

test—a designator for a function of two arguments that returns a generalized boolean.

test-not—a designator for a function of two arguments that returns a generalized boolean.

key—a designator for a function of one argument, or nil.

result-list—a list.

#### Description::

set-exclusive-or returns a list of elements that appear in exactly one of list-1 and list-2.

nset-exclusive-or is the destructive version of set-exclusive-or.

For all possible ordered pairs consisting of one element from list-1 and one element from list-2, the `:test` or `:test-not` function is used to determine whether they satisfy the test.

If `:key` is supplied, it is used to extract the part to be tested from the list-1 or list-2 element. The first argument to the `:test` or `:test-not` function is the part of an element of list-1 extracted by the `:key` function (if supplied); the second argument is the part of an element of list-2 extracted by the `:key` function (if supplied). If `:key` is not supplied or nil, the list-1 or list-2 element is used.

The result contains precisely those elements of list-1 and list-2 that appear in no matching pair.

The result list of set-exclusive-or might share storage with one of list-1 or list-2.

#### Examples::

     (setq lst1 (list 1 "a" "b")
           lst2 (list 1 "A" "b")) ⇒  (1 "A" "b")
     (set-exclusive-or lst1 lst2) ⇒  ("b" "A" "b" "a")
     (set-exclusive-or lst1 lst2 :test #'equal) ⇒  ("A" "a")
     (set-exclusive-or lst1 lst2 :test 'equalp) ⇒  NIL
     (nset-exclusive-or lst1 lst2) ⇒  ("a" "b" "A" "b")
     (setq lst1 (list (("a" . "b") ("c" . "d") ("e" . "f"))))
    ⇒  (("a" . "b") ("c" . "d") ("e" . "f"))
     (setq lst2 (list (("c" . "a") ("e" . "b") ("d" . "a"))))
    ⇒  (("c" . "a") ("e" . "b") ("d" . "a"))
     (nset-exclusive-or lst1 lst2 :test #'string= :key #'cdr)
    ⇒  (("c" . "d") ("e" . "f") ("c" . "a") ("d" . "a"))
     lst1 ⇒  (("a" . "b") ("c" . "d") ("e" . "f"))
     lst2 ⇒  (("c" . "a") ("d" . "a"))

#### Side Effects::

nset-exclusive-or is permitted to modify any part, car or cdr, of the list structure of list-1 or list-2.

#### Exceptional Situations::

Should be prepared to signal an error of type type-error if list-1 and list-2 are not proper lists.

#### See Also::

Compiler Terminology,

Traversal Rules and Side Effects

#### Notes::

The `:test-not` parameter is deprecated.

Since the nset-exclusive-or side effect is not required, it should not be used in for-effect-only positions in portable code.