Comments on: for循环与each的区别

By: rails guy小抄大全 - RubyEE.org - about Ruby,Rails,Sinatra,Mac OS & iPhone app

rails guy小抄大全 - RubyEE.org - about Ruby,Rails,Sinatra,Mac OS & iPhone app — Fri, 30 Jul 2010 02:34:04 +0000

[...] for循环与each的区别(–>認真看回復) http://www.letrails.cn/archives/difference-between-for-loop-and-each/ [...]

By: Yuanyi Zhang

Yuanyi Zhang — Mon, 18 Feb 2008 03:20:25 +0000

你说的对，for的确是通过each实现的：

>> a = "1\n2\n"
>> def a.each
>> yield(1)
>> end
>> for i in a
>> puts i
>> end
1
=> nil

只不过for在调用each之前先定义了一个局部变量：

for i in [1, 2]

等于：

i = nil
[1,2].each do |i| end

也就是说for的问题在于它在each的scope外面定义了一个同名变量。

帖子已更新

By: ShiningRay

ShiningRay — Sun, 17 Feb 2008 12:54:01 +0000

我认为应该还是是迭代器的问题，再例如Hash，each返回键值对

irb(main):011:0> for i in {‘a’=> 1, ‘b’ => 2, ‘c’ => 3}
irb(main):012:1> p i
irb(main):013:1> end
["a", 1]
["b", 2]
["c", 3]
=> {“a”=>1, “b”=>2, “c”=>3}
irb(main):014:0> {‘a’ => 1, ‘b’ => 2, ‘c’ => 3}.each do |i|
irb(main):015:1* p i
irb(main):016:1> end
["a", 1]
["b", 2]
["c", 3]
=> {“a”=>1, “b”=>2, “c”=>3}

String，则会自动以回车分割

irb(main):028:0> for i in “a\nb\nc”
irb(main):029:1> p i
irb(main):030:1> end
“a\n”
“b\n”
“c”
=> “a\nb\nc”
irb(main):031:0> “a\nb\nc”.each do |i|
irb(main):032:1* p i
irb(main):033:1> end
“a\n”
“b\n”
“c”
=> “a\nb\nc”

所以我认为for 绝对是内部调用迭代器each，这点应该和python类似，而不是for的intern_name是each

By: IceskYsl

IceskYsl — Sat, 16 Feb 2008 12:28:03 +0000

我一般使用 while 代替 for

By: admin

admin — Sat, 16 Feb 2008 05:47:02 +0000

each是Array类的一个方法，定义如下：

VALUE
rb_ary_each(ary)
VALUE ary;
{
long i;

for (i=0; ilen; i++) {
rb_yield(RARRAY(ary)->ptr[i]);
}
return ary;
}

for是Ruby关键字，由YACC生成的语法解析器直接执行，至于Backtrace，只是由于for的intern_name也是“each”，但for并没有调用each，它由语法分析器定义。

By: ShiningRay

ShiningRay — Sat, 16 Feb 2008 04:36:57 +0000

for in do
end
do可以省略
另外貌似for其实从根本上应该是调用each方法的
或者说，有个迭代器的概念在里面（我从Python的角度看）
因为出错的时候，for的块会显示为each?

irb(main):019:0> for i in [1,2,3]
irb(main):020:1> puts i / 0
irb(main):021:1> end
ZeroDivisionError: divided by 0
from (irb):20:in `/’
from (irb):20
from (irb):19:in `each’
from (irb):19
from :0