CLHS: Function NCONC

Syntax:

nconc &rest lists => concatenated-list

Arguments and Values:

list---each but the last must be a list (which might be a dotted list but must not be a circular list); the last list may be any object.

concatenated-list---a list.

Description:

Returns a list that is the concatenation of lists. If no lists are supplied, (nconc) returns nil. nconc is defined using the following recursive relationship:

 (nconc) =>  ()
 (nconc nil . lists) ==  (nconc . lists)
 (nconc list) =>  list
 (nconc list-1 list-2) ==  (progn (rplacd (last list-1) list-2) list-1)
 (nconc list-1 list-2 . lists) ==  (nconc (nconc list-1 list-2) . lists)

Examples:

 (nconc) =>  NIL
 (setq x '(a b c)) =>  (A B C)
 (setq y '(d e f)) =>  (D E F)
 (nconc x y) =>  (A B C D E F)
 x =>  (A B C D E F)

Note, in the example, that the value of x is now different, since its last cons has been rplacd'd to the value of y. If (nconc x y) were evaluated again, it would yield a piece of a circular list, whose printed representation would be (A B C D E F D E F D E F ...), repeating forever; if the *print-circle* switch were non-nil, it would be printed as (A B C . #1=(D E F . #1#)).

 (setq foo (list 'a 'b 'c 'd 'e)
       bar (list 'f 'g 'h 'i 'j)
       baz (list 'k 'l 'm)) =>  (K L M)
 (setq foo (nconc foo bar baz)) =>  (A B C D E F G H I J K L M)
 foo =>  (A B C D E F G H I J K L M)
 bar =>  (F G H I J K L M)
 baz =>  (K L M)

 (setq foo (list 'a 'b 'c 'd 'e)
       bar (list 'f 'g 'h 'i 'j)
       baz (list 'k 'l 'm)) =>  (K L M)
 (setq foo (nconc nil foo bar nil baz)) =>  (A B C D E F G H I J K L M) 
 foo =>  (A B C D E F G H I J K L M)
 bar =>  (F G H I J K L M)
 baz =>  (K L M)

Side Effects:

The lists are modified rather than copied.

Affected By: None.

Exceptional Situations: None.

See Also:

append, concatenate

Notes: None.

The following X3J13 cleanup issues, not part of the specification, apply to this section: