April 2014
Beginner to intermediate
830 pages
15h 21m
Chinese
Content preview from Mathematica 錦囊妙計
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2.1 多引數函數映射（Mapping）
|
35
Apply
指令也可接受第三個引數，用來指定
作用層數
（level），此用法可以不用
Map
而
獲得相同的效用。[Page-034]
在此我們將
Plus
指令作用於層數
{1}
，這限制
Apply
指令僅作用於第一層元素。此處使
用了
預留位置序列
##
（slot sequence），用來帶入所有的引數。因為這個用法很常見，
Mathematica 提供一個算符
@@@
直接達到此功能，用於此範例中，可進一步省略
##
而獲
得更簡潔的程式碼。
當你想要於函數中使用其他引數時，還是必須使用
##
，如以下範例：
下式表示於每一項總和中，再加一次第二個元素，即第二個元素被加了兩次。
使用以上的語法，我們可以獲得簡化版的折扣訂單總和。
若串列具較深層的巢狀結構，你可以指定較大的作用層數獲得所需結果。例如前例訂單
被置放於
envelope
額外結構下時：
綜合使用
Map
與
Apply
指令也可獲得相同結果，因為
Ma
p 指令也接受作用層數指定。
ch02.indd 35 2014/4/2 上午 03:31:59
36
|
第二章
當然，你也可以刪除
envelope
的包覆，這可以透過取出指定元素
[[All,2]]
達到，這語
法代表將第一層各元素的第二個元素取出。[Page-035]
以下程式僅使用
Map ...
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