Ruby：for循环和每个循环之间有什么区别？ [英] Ruby: What is the difference between a for loop and an each loop?

问题描述

可能重复：

for Ruby in Ruby

假设我们有一个数组，像

sites =％w [stackoverflow stackexchange serverfault]

 对于网站中的x有什么区别do 
 puts x 
 end 
  

和

  sites.each do | x | 
 puts x 
 end 
  

？

对我来说，他们似乎做同样的事情，循环的的语法对我来说更清楚。有区别吗？在什么情况下这将是一个大问题？

解决方案

关于作用域有一个细微的差别，但我会建议理解它因为它揭示了Ruby的一些重要方面。


$ b 是一个语法结构，有点类似于如果。无论你在中为块定义什么，在之后仍然会定义：

  sites =％w [stackoverflow stackexchange serverfault] 
＃=> [stackoverflow，stackexchange，serverfault] 
 
在网站中做
放置x 
结束
 stackoverflow 
 stackexchange 
 serverfault 
＃=> [stackoverflow，stackexchange，serverfault] 
 x 
＃=> serverfault
  

另一方面，每个是接收块的方法。 Block引入了新的词法作用域，所以无论你在其中引入什么变量，在方法结束后都不会出现在这里：

  sites.each do | y | 
 puts y 
 end 
 stackoverflow 
 stackexchange 
 serverfault 
＃=> [stackoverflow，stackexchange，serverfault] 
 
 NameError：未定义局部变量或方法`y'for＃< Object：0x855f28 @ hhh =hello> （irb）
：/ usr / bin / irb：12：在< main> b 
 $ b 
我会建议完全忘记，因为使用每个都是惯用的用于遍历枚举的Ruby。它还通过减少副作用的机会来更好地回顾功能性程序设计的范例。

  
Possible Duplicate:

  for vs each in Ruby  
Let's say that we have an array, like


sites = %w[stackoverflow stackexchange serverfault]

What's the difference between
for x in sites do
  puts x
end
and
sites.each do |x|
  puts x
end
?

They seem to do the same exact thing, to me, and the syntax of the for loop is clearer to me. Is there a difference? In what situations would this be a big deal?
解决方案
There is a subtle difference regarding scoping, but I would recommend understanding it well as it reveals some of important aspects of Ruby.

for is a syntax construct, somewhat similar to if. Whatever you define in for block, will remain defined after for as well:
sites = %w[stackoverflow stackexchange serverfault]
#=> ["stackoverflow", "stackexchange", "serverfault"]

for x in sites do
  puts x
end
stackoverflow
stackexchange
serverfault
#=> ["stackoverflow", "stackexchange", "serverfault"]
x
#=> "serverfault"
On the other hand, each is a method which receives a block. Block introduces new lexical scope, so whatever variable you introduce in it, will not be there after the method finishes:
sites.each do |y|
  puts y
end
stackoverflow
stackexchange
serverfault
#=> ["stackoverflow", "stackexchange", "serverfault"]
y
NameError: undefined local variable or method `y' for #<Object:0x855f28 @hhh="hello">
    from (irb):9
    from /usr/bin/irb:12:in `<main>'
I would recommend forgetting about for completely, as using each is idiomatic in Ruby for traversing enumerables. It also recspects the paradigm of functional programming better, by decreasing chances of side-effects.

                        
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