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在索引和序列元素上并行循环

1.14 Looping in Parallel over Index and Sequence Items

Credit: Alex Martelli

问题 Problem

You need to loop on a sequence, but at each step you also need to know what index into the sequence you have reached.

Edit conflict! Other version:

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Edit conflict! Your version:

你需要在一个序列上循环，但是在每一步，你也需要知道什么索引进入到你已经到达的序列中。

解决 Solution

Together, the built-in functions xrange and zip make this easy. You need only this one instance of xrange, as it is fully reusable:

把内建函数xrange和zip放在一起将使它很容易。你只需要xrange的一个实例，好像它完全可重用。

indices = xrange(sys.maxint) 

Here's how you use the indices instance:

这儿是你怎么样使用index的实例

   1 for item, index in zip(sequence, indices):
   2     something(item, index)

This gives the same semantics as:

给出相同的语义：

   1 for index in range(len(sequence)):
   2     something(sequence[index], index)

but the change of emphasis allows greater clarity in many usage contexts.

但是改变的重点允许在更多使用环境中更加清楚。

Another alternative is to use class wrappers:

另一个可选择的是使用一个包装类：

   1 class Indexed:
   2     def _ _init_ _(self, seq):
   3         self.seq = seq
   4     def _ _getitem_ _(self, i):
   5         return self.seq[i], i

For example:

例如

   1 for item, index in Indexed(sequence):
   2     something(item, index)

In Python 2.2, with from _ _future_ _ import generators, you can also use:

在python2.2， 从_ _future_ _导入一个产生器， 你也可以使用：

   1 def Indexed(sequence):
   2     iterator = iter(sequence)
   3     for index in indices:
   4         yield iterator.next(  ), index
   5     # Note that we exit by propagating StopIteration when .next raises it!

However, the simplest roughly equivalent way remains the good old:

然而，最简单粗糙的等价方法仍然是老的：