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*To*: CL-Cleanup@SAIL.Stanford.EDU*Subject*: Issue: LAST-N (Version 1)*From*: Kent M Pitman <KMP@STONY-BROOK.SCRC.Symbolics.COM>*Date*: Fri, 4 Dec 87 16:49 EST*Cc*: KMP@STONY-BROOK.SCRC.Symbolics.COM

Issue: LAST-N References: LAST (p267) Category: ENHANCEMENT Edit history: 04-Dec-87, Version 1 by Pitman Status: For Internal Discussion Problem Description: People always ask why LAST returns a cons and not an element. BUTLAST takes an argument N but LAST does not. Proposal (LAST-N:ALLOW-OPTIONAL-ARGUMENT): Allow LAST to take an optional argument N, saying how many cells to return. It is an error if N is negative or L is circular. If N is equal to the number of cons cells in the list L, then the atom which terminates the list L is returned. If N is greater than the number of cons cells in the list L, then it is an error if the list is not a proper list. If in this case the list is a proper list, then L is returned. Test Cases: (LAST '(A B C) 0) => () (BUTLAST '(A B C) 0) => (A B C) (LAST '(A B C) 1) => (C) (BUTLAST '(A B C) 1) => (A B) (LAST '(A B C) 2) => (B C) (BUTLAST '(A B C) 2) => (A) (LAST '(A B C) 3) => (A B C) (BUTLAST '(A B C) 3) => () (LAST '(A B C) 4) => (A B C) (BUTLAST '(A B C) 4) => () (LAST '(A B C)) => (C) (BUTLAST '(A B C)) => (A B) (LAST '(A . B) 0) => B (LAST '(A . B) 1) => (A . B) (LAST '(A . B) 2) is an error. Rationale: BUTLAST and LAST would select complementary parts of a list in general. That is (EQUAL L (APPEND (BUTLAST L N) (LAST L N))) would be T for N >= 0 and L being a proper list. This would make it more obvious why LAST should return a list and not an element. ie, it would return the "last N elements" where N=1 by default. Current Practice: Probably nobody does this. Adoption Cost: Very slight. The following code would suffice: (DEFUN LAST (LIST &OPTIONAL (N 1)) (CHECK-TYPE N (INTEGER 0)) (DO ((L LIST (CDR L)) (R LIST) (I 0 (+ I 1))) ((ATOM L) R) (IF (>= I N) (POP R)))) Some implementations might want to provide compiler optimizations for the N=1 case. Benefits: This utility is probably needed often enough to warrant its inclusion. It's present (as a separate function called NLEFT) in Symbolics Common Lisp and Interlisp. Conversion Cost: None. This is an upward compatible extension. Aesthetics: This would make things a little prettier. Discussion: This suggestion came up recently on a Symbolics design discussion list. Symbolics is contemplating offering it as an extension, but it seemed like the rest of the CL community might want to adopt it, too. Pitman thinks it's a nice idea.

**Follow-Ups**:**Issue: LAST-N (Version 1)***From:*David A. Moon <Moon@STONY-BROOK.SCRC.Symbolics.COM>

**Issue: LAST-N (Version 2)***From:*Kent M Pitman <KMP@STONY-BROOK.SCRC.Symbolics.COM>

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