标记未使用的块变量 [英] Marking an unused block variable
问题描述
当存在不使用的块或局部变量时，有时人们用*标记，有时用_标记.
When there is a block or local variable that is not to be used, sometimes people mark it with *, and sometimes with _.
{[1, 2] => 3, [4, 5] => 6}.each{|(x, *), *| p x}
{[1, 2] => 3, [4, 5] => 6}.each{|(x, _), _| p x}
{[1, 2, 3], [4, 5, 6]}.each{|*, x, *| p x}
{[1, 2, 3], [4, 5, 6]}.each{|_, x, _| p x}
def (x, *), *; p x; end
def (x, _), _; p x; end
def *, x, *; p x; end
def _, x, _; p x; end
它们之间有什么区别，什么时候应该使用?如上例所示，当需要将多个变量标记为未使用时，还是更好?
What are the differences between them, and when should I use which? When there is need to mark multiple variables as unused as in the above examples, is either better?
推荐答案
*表示所有剩余参数". _只是另一个变量名，尽管有点特殊.因此它们是不同的，例如以下内容是没有道理的:
A * means "all remaining parameters". An _ is just another variable name, although it is a bit special. So they are different, for example the following does not make sense:
[[1, 2, 3], [4, 5, 6]].each{|*, x, *| p x} # Syntax error
实际上，Ruby应该如何知道第一颗星应该获得0、1或2个值(以及相反的值)?
Indeed, how is Ruby supposed to know if the first star should get 0, 1 or 2 of the values (and the reverse)?
在极少数情况下，您想使用星号来忽略参数.例如，如果您只想使用可变数量的参数中的最后一个:
There are very few cases where you want to use a star to ignore parameters. An example would be if you only want to use the last of a variable number of parameters:
[[1], [2, 3], [4, 5, 6]].each{|*, last| p last} # => prints 1, 3 and 6
Ruby允许您不给参数的其余部分"起个名字，但是您可以使用_:
Ruby allows you to not give a name to the "rest" of the parameters, but you can use _:
[[1], [2, 3], [4, 5, 6]].each{|*_, last| p last} # => prints 1, 3 and 6
通常，参数的数量是已知的，最好的选择是使用_:
Typically, the number of parameters is known and your best choice is to use a _:
[[1, 2, 3], [4, 5, 6]].each{|_, mid, _| p mid} # prints 2 and 5
请注意，您也可以不给最后一个参数命名(就像使用*时一样)，尽管它不太明显:
Note that you could leave the last paramater unnamed too (like you can when using a *), although it is less obvious:
[[1, 2, 3], [4, 5, 6]].each{|_, mid, | p mid} # prints 2 and 5
现在_是 指定变量名称，当您不想使用值时使用.它是一个特殊的变量名，原因有二:
Now _ is the designated variable name to use when you don't want to use a value. It is a special variable name for two reasons:
- 如果您不使用Ruby(如果警告已打开)，它将不会抱怨
- Ruby将允许您在参数列表中重复它.
第1点的示例
> ruby -w -e "def foo; x = 42; end; foo"
-e:1: warning: assigned but unused variable - x
> ruby -w -e "def foo; _ = 42; end; foo"
no warning
第2点示例:
[[1, 2, 3], [4, 5, 6]].each{|unused, mid, unused| p mid}
# => SyntaxError: (irb):23: duplicated argument name
[[1, 2, 3], [4, 5, 6]].each{|_, mid, _| p mid}
# => prints 2 and 5
最后，如@DigitalRoss所述，_将最后一个结果保存在irb
Finally, as @DigitalRoss notes, _ holds the last result in irb
更新:在Ruby 2.0中，您可以使用以_开头的任何变量表示未使用.这样，变量名可以更明确地说明被忽略的内容:
Update: In Ruby 2.0, you can use any variable starting with _ to signify it is unused. This way the variable name can be more explicit about what is being ignored:
_scheme, _domain, port, _url = parse_some_url
# ... do something with port
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