15.rst

练习 1.15

a)

先载入题目给定的程序：

.. literalinclude:: code/15.scm

然后使用 trace 追踪 p 的调用，计算它的运行次数：

1 ]=> (load "15.scm")

;Loading "15.scm"... done
;Value: sine

1 ]=> (trace-entry p)

;Unspecified return value

1 ]=> (sine 12.15)

[Entering #[compound-procedure 11 p]
    Args: 4.9999999999999996e-2]
[Entering #[compound-procedure 11 p]
    Args: .1495]
[Entering #[compound-procedure 11 p]
    Args: .4351345505]
[Entering #[compound-procedure 11 p]
    Args: .9758465331678772]
[Entering #[compound-procedure 11 p]
    Args: -.7895631144708228]
;Value: -.39980345741334

可以看到， p 共运行了 5 次。

b)

在求值 (sine a) 的时候， a 每次都被除以 3.0 ，而 sine 是一个递归程序，因此它的时间和空间复杂度都是 O(\log{a}) 。

如果以上预测是正确的话，那么每当 a 增大一倍(准确地说，是乘以因子 3)， p 的运行次数就会增加一次，以下是相应的测试：

1 ]=> (sine 10)                         ; 5 次 p 调用

[Entering #[compound-procedure 11 p]
    Args: .0411522633744856]
[Entering #[compound-procedure 11 p]
    Args: .12317802324595176]
[Entering #[compound-procedure 11 p]
    Args: .3620582351732319]
[Entering #[compound-procedure 11 p]
    Args: .8963314023464528]
[Entering #[compound-procedure 11 p]
    Args: -.19149217924571227]
;Value: -.5463890357524606

1 ]=> (sine 30)                         ; 6 次调用

[Entering #[compound-procedure 11 p]
    Args: .0411522633744856]
[Entering #[compound-procedure 11 p]
    Args: .12317802324595176]
[Entering #[compound-procedure 11 p]
    Args: .3620582351732319]
[Entering #[compound-procedure 11 p]
    Args: .8963314023464528]
[Entering #[compound-procedure 11 p]
    Args: -.19149217924571227]
[Entering #[compound-procedure 11 p]
    Args: -.5463890357524606]
;Value: -.9866890379958478

1 ]=> (sine 90)                         ; 7 次调用

[Entering #[compound-procedure 11 p]
    Args: .0411522633744856]
[Entering #[compound-procedure 11 p]
    Args: .12317802324595176]
[Entering #[compound-procedure 11 p]
    Args: .3620582351732319]
[Entering #[compound-procedure 11 p]
    Args: .8963314023464528]
[Entering #[compound-procedure 11 p]
    Args: -.19149217924571227]
[Entering #[compound-procedure 11 p]
    Args: -.5463890357524606]
[Entering #[compound-procedure 11 p]
    Args: -.9866890379958478]
;Value: .8823180886403317