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  ANSI Common Lisp   14 Conses   14.2 Dictionary of Conses

14.2.28 butlast, nbutlast Function

Syntax:
butlast list &optional n    result-list

nbutlast list &optional n    result-list

Arguments and Values:
list - a list, which might be a dotted list but must not be a circular list.

n - a non-negative integer.

result-list - a list.

Description:
butlast returns a copy of list from which the last n conses have been omitted. If n is not supplied, its value is 1. If there are fewer than n conses in list, nil is returned and, in the case of nbutlast, list is not modified.

nbutlast is like butlast, but nbutlast may modify list. It changes the cdr of the cons n+1 from the end of the list to nil.

Examples:
 (setq lst '(1 2 3 4 5 6 7 8 9))  (1 2 3 4 5 6 7 8 9)
 (butlast lst)  (1 2 3 4 5 6 7 8)
 (butlast lst 5)  (1 2 3 4)
 (butlast lst (+ 5 5))   NIL
 lst  (1 2 3 4 5 6 7 8 9)
 (nbutlast lst 3)  (1 2 3 4 5 6)
 lst  (1 2 3 4 5 6)
 (nbutlast lst 99)   NIL
 lst  (1 2 3 4 5 6)
 (butlast '(a b c d))  (A B C)
 (butlast '((a b) (c d)))  ((A B))
 (butlast '(a))   NIL
 (butlast nil)   NIL
 (setq foo (list 'a 'b 'c 'd))  (A B C D)
 (nbutlast foo)  (A B C)
 foo  (A B C)
 (nbutlast (list 'a))   NIL
 (nbutlast '())   NIL

Exceptional Situations:
Should signal an error of type type-error if list is not a proper list or a dotted list. Should signal an error of type type-error if n is not a non-negative integer.

Notes:
 (butlast list n) ==(ldiff list (last list n))

Allegro CL Implementation Details:
None.

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