SICP

1 (define x (list 'a 'b)) 2 3 (define y (list 'c 'd)) 4 5 (define z (append x y)) 6 7 (cdr x) 8 9 (define (append! x y) 10 (set-cdr! (last-pair x) y) 11 x) 12 13 (define (last-pair x) 14 (if (null? (cdr x)) 15 x 16 (last-pair (cdr x)))) 17 18 (define z1 (append! x y)) 19 20 (cdr z1)

 

x----->[ * ]----->[ * ]----->( )

            |            |

            v           v

            a           b

y----->[ * ]----->[ * ]----->( )

            |            |

            v           v

            c          d

z----->[ * ]----->[ * ]----->[ * ]----->[ * ]----->( )

            |            |             |            |

            v           v            v           v

            a          b            c            d           

 

z1------+

            |    

           v

x----->[ * ]----->[ * ]-----+

            |            |          |

            v           v          |

            a           b         |

                                    |

            +-----------------+

             |

            v

y----->[ * ]----->[ * ]----->( )

            |            |

            v           v

            c          d

1 (define (make-cycle x) 2 (set-cdr! (last-pair x) x) 3 x) 4 5 (define z2 (make-cycle (list 'a 'b 'c))) 6 ;z2 7 ;(last-pair z2)

               +---------------------------+                                                                       

                |                                  |

                v                                  |

z2----->[ * ]----->[ * ]----->[ * ]-----+

            |            |             |

            v           v            v

            a           b            c

 

如果求（last-pair z2）将会无限循环下去

 

1 ;;;;;;;;3.14 2 3 (define (mystery x) 4 (define (loop x y) 5 (if (null? x) 6 y 7 (let ((temp (cdr x))) 8 (set-cdr! x y) 9 (loop temp x)))) 10 (loop x '())) 11 12 (define v '(a b c d )) 13 14 (define w (mystery v)) 15 16 (cdr v)

 

可以发现这就是reverse的修改版

v-----------------------------------------+

                                                      |

                                                     v

w----->[ * ]----->[ * ]----->[ * ]----->[ * ]----->( )

            |            |             |            |

            v           v            v           v

            d          c             b           a        

 

转载于:https://www.cnblogs.com/tclan126/p/6546887.html