Ruby 枚举器 - 为什么会出现异常？

发布于 2024-09-08 15:45:17 字数 523 浏览 7 评论 0原文

在 Ruby 中迭代一个块非常简单 - 它干净地完成并继续执行其余的代码。

另一方面，使用枚举器进行迭代则有点令人困惑。如果您在没有块的情况下调用 :each ，则会返回一个枚举器。然后可以在枚举器上调用 :next 来获取每个下一个迭代值。

然后奇怪的部分 - 当迭代完成时，枚举器不是返回 nil，而是抛出一个异常：“迭代到达结束”。结果是它甚至没有返回值。

例如：

test = [ 'test_value' ]
enumerator = test.each
enumerator.next
>> "test_value"
enumerator.next
>> StopIteration: iteration reached at end

这样做的原因仅仅是为了让枚举器可以返回 nil 值吗？只有当我发布这篇文章时，我才会想到答案（所以我仍然会发布它），但似乎情况一定如此。

如果是这样，这是处理此类问题的典型方式吗？使用异常来处理基本上按预期执行的代码似乎很奇怪。

原文

Iterating a block in Ruby is simple enough - it finishes cleanly and proceeds on to the rest of the code.

Iterating with an Enumerator, on the other hand, is a bit more confusing. If you call :each without a block, an Enumerator is returned instead. :next can then be called on the Enumerator to get each next iterative value.

And then the odd part- when iteration is complete, instead of the Enumerator returning nil, it throws an exception: "iteration reached at end". The result is that it doesn't even return a value.

For instance:

test = [ 'test_value' ]
enumerator = test.each
enumerator.next
>> "test_value"
enumerator.next
>> StopIteration: iteration reached at end

Is the reason for this simply so that nil values can be returned by the Enumerator? The answer occurs to me only as I post this (so I am going to post it still), but seems like it must be the case.

If that is so, is this a typical way of handling such issues? It seems odd to use an Exception to handle code that essentially performs as expected.

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反目相谮 2024-09-15 15:45:17

您是对的，原因是枚举器可以将 nil 作为有效值返回。为了回答您的问题：这是否典型，Python 使用也称为 StopIteration 的异常以相同的方式处理它。

>>> my_list = [1,2,3]
>>> i = iter(my_list)
>>> i.next()
1
>>> i.next()
2
>>> i.next()
3
>>> i.next()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration

当然，大多数时候 next 不会被直接调用（each 或使用 for 循环来代替），所以这个底层机制是'不经常暴露。

You are correct that the reason is so that nil can be returned as a valid value by the Enumerator. To answer your question of whether this is typical, Python handles it in the same way using an exception also called StopIteration.

>>> my_list = [1,2,3]
>>> i = iter(my_list)
>>> i.next()
1
>>> i.next()
2
>>> i.next()
3
>>> i.next()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
StopIteration

Of course, most of the time next isn't called directly (each or a for loop being used instead) so this underlying mechanism isn't exposed that often.

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