; ======================================================================
;
;          Structure and Interpretation of Computer Programs
;                  (trial answer to excercises)
;
;                  计算机程序的构造和解释(习题试解)
;
;                                             created: code17 07/14/05
;                                             modified:
; (保持内容完整不变前提下，可以任意转载)
; ======================================================================


;; SICP No.2.17

(define (last-pair items)
  (let ((len (length items)))
    (cond ((= len 0) (error "empty list"))
          ((= len 1) items)
          (else (last-pair (cdr items))))))


;; Test-it:
;; Welcome to MzScheme version 209, Copyright (c) 2004 PLT Scheme, Inc.
;; > (last-pair (list 23 72 149 34))
;; (34)
;; > (last-pair ())
;; empty list
;; > (last-pair (list 31))
;; (31)